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Irradiance and the elemental stoichiometry of marine phytoplankton
Z. V. Finkel 1 and A. Quigg 2
Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901
J. A. RavenPlant Research Unit, University of Dundee at SCRI, Scottish Crop Research Institute, Invergowrie, Dundee,United Kingdom DD2 5DA
J. R. ReinfelderDepartment of Environmental Science, Rutgers University, New Brunswick, New Jersey 08901
O. E. SchofieldInstitute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901
P. G. FalkowskiInstitute of Marine and Coastal Sciences and Department of Geology, Rutgers University, New Brunswick,New Jersey 08901
Abstract
We analyzed the elemental composition (C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Mo, and Ni) of fivemarine phytoplankton species representing the four major marine phyla grown over a range of growthirradiances. We found substantial variability in the elemental composition between different species, which isconsistent with previously reported differences associated with evolutionary history. We found that manyelements (Fe, Mn, Zn, Cu, Co, and Mo) are enriched relative to phosphorus by about two to three orders ofmagnitude under irradiances that are limiting for growth, and net steady-state uptake of element : P is oftenelevated under lower irradiances. For most elements, the variability in element : P due to irradiance is comparableto the variability due to phylogenetic differences at any irradiance, but often the interaction between geneticdifferences and the phenotypic response to irradiance amplifies the differences in elemental composition betweenspecies. The fractionation of trace elements relative to phosphorus into phytoplankton biomass under low light isconsistent with depleted levels of Cu2+ and Mn2+ in deep chlorophyll maxima, suggesting that the export of low-light-acclimated phytoplankton is a major source of trace element flux to the deep ocean and an important factorin the biogeochemical cycles of many of the biologically limiting elements in the oceans.
In the first half of the 20th century Alfred Redfield andRichard Fleming established that marine plankton havea relatively constrained elemental ratio of 106C : 16N : 1Patoms (Redfield 1934, 1958; Fleming 1940) that is re-markably similar to the ratio of dissolved fixed inorganicnitrogen to phosphate in seawater. These observations ledto the paradigm that on average, phytoplankton will
consume inorganic nitrogen and phosphate in proportionto their availability in surface waters, converting them intocellular biomass that is eventually returned into theirinorganic forms through decomposition. The Redfield ratioconcept has been used extensively in biogeochemicalmodels to link phytoplankton production and the carboncycle to the nitrogen and phosphorus cycles. Associatedwith the increasing recognition that trace elements such asiron can limit primary production (Martin 1992) there hasbeen an effort to extend the Redfield ratio to Si and themicronutrients (Brzezinski 1985; Bruland et al. 1991; Ho etal. 2003) and to understand the physiological, ecological,biogeochemical, and evolutionary factors that contribute tothe elemental stoichiometry of marine organisms andseawater (Falkowski 2000; Quigg et al. 2003; Klausmeieret al. 2004).
A recent analysis of major nutrients and trace elementsin 29 algal species from the major phylogenetic groupsunder uniform culture conditions indicated that there aresystematic phylogenetic differences in the elemental com-position of phytoplankton (Quigg et al. 2003; Falkowski etal. 2004). On the basis of an analysis of available
1 Current address: Environmental Science Program, MountAllison University, Sackville, New Brunswick, Canada E4L 1A7.
2 Current address: Department of Marine Biology, Texas A&MUniversity, Galveston, Texas 77551.
AcknowledgmentsWe thank F. F. M. Morel, T-Y. Ho, and CEBIC for their role
in the early phase of this project, R. Sherrell and Paul Field withtheir assistance with ICPMS, C. Fuller and K Wyman for theirhelp with CHN, R. Chiampi for her help with sampling, and R.Armstrong, B. Sunda, A. J. Irwin, and three anonymous reviewersfor useful comments.
This work was supported by an NSF biocomplexity grant(EREUPT, OCE-0084032) to P.G.F. and O.E.S. and an EPASTAR Fellowship (F4E20409), NSERC, and NBIF funding toZ.V.F.
Limnol. Oceanogr., 51(6), 2006, 2690–2701
E 2006, by the American Society of Limnology and Oceanography, Inc.
2690
experimental data, Ho et al. (2003) deduced that pheno-typic differences in elemental composition due to environ-mental conditions such as irradiance, macronutrientconcentration, and micronutrient concentration rarelyresult in more than two- to fivefold variability in element(E) : P in response to an order-of-magnitude variation inirradiance or macronutrient concentration, or an order-of-magnitude variability for over three orders-of-magnitudevariation in trace metal concentrations. On this basis Quigget al. (2003) and Ho et al. (2003) hypothesize thatphenotypic variation in elemental stoichiometry is muchsmaller than genetic differences between phylogeneticgroups, leading to the hypothesis that differences inelemental composition among species primarily representphylogenetic differences in biochemical requirements andthe ability of the organisms to take up and store theseelements, not environmental or culture conditions.
Elemental stoichiometry can be strongly affected by themagnitude of photosynthetically active radiation andphotoperiod. Current theory predicts that a change ingrowth irradiance will affect the cellular concentration ofelements that are required for light harvesting and oxygenicphotosynthesis: N, Fe, and to a lesser extent Mn (Raven1990; Sunda and Huntsman 1997, 2004) and elementsassociated with a change in growth rate such as N and P(Sterner and Elser 2002). Experimental studies on diatoms,prymnesiophytes, and dinoflagellates indicate that a de-crease in growth irradiance can result in an increase in thecellular concentration of Fe and Mn, which have a criticalrole in light harvesting and photosynthesis (Sunda andHuntsman 1997, 1998a). Surprisingly few studies have beenconducted to look at the change in elements other than C,N, P, or Fe as a function of growth irradiance. RecentlySunda and Huntsman (2004) found that the cellularconcentration of Zn increased with decreasing light periodin the coastal diatom Thalassiosira pseudonana. Thissuggests that elements not intimately associated with lightharvesting may also be affected by growth irradiance. Nowork has simultaneously examined the effect of irradianceon a large range of major biologically required elements inphytoplankton.
We analyzed the elemental composition, C, N, P, S, K,Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Mo, and Ni, of fivemarine phytoplankton species representing four majormarine phyla over a light gradient to test two hypotheses:(1) growth irradiance is responsible for a relatively smallproportion of the total variability in the elementalstoichiometry of phytoplankton, and that (2) growthirradiance will preferentially alter the cellular concentra-tions of elements specifically associated with light harvest-ing and growth rate.
Materials and methods
Culturing and sampling—Phytoplankton species investi-gated in this study included one nitrogen-fixing cyanobac-terium Cyanothece sp. (WH8904, provided by Dr. J.Waterbury), a prasinophyte, Pycnococcus provasolii(CCMP 1203), a dinoflagellate, Amphidinium carterae(CCMP 1314), and two members of the Bacillariophyceae:
Thalassiosira weissflogii (CCMP 1336) and Chaetoceroscalcitrans (CCMP 1315). The algae were grown at 19uC 61uC using a 12:12 light:dark cycle at five irradiances:quanta of 15, 30, 100, 250, and 500 mmol m22 s21. Allspecies were cultured in the synthetic seawater mediumAquil, which has a fixed nitrogen source, as described inHo et al. (2003). Each experimental species was maintainedin exponential growth through a minimum of six genera-tions before harvesting (Fogg and Thake 1987). For mostirradiance treatments three replicate bottles were main-tained for each species; in a few cases, due to experimentaldifficulties such as trace metal contamination or experi-mental error, there were only one or two replicate bottlesfor certain treatments, and in the case of 250 mmol m22 s21
treatment, some species had more than three replicatebottles, and no growth rate measurements were availablefor Cyanothece sp. (Table 1). Cell size and the number ofcells per unit volume of media were determined witha calibrated Beckman Multisizer Coulter Counter (II) andthese measurements were used to calculate growth rates.
Elemental analysis—All elements but carbon and nitro-gen were assayed with sector field high-resolution in-ductively coupled plasma mass spectrometry (HR-ICPMS,Element, ThermoFinnigan) fitted with a self-aspiratingmicroflow Teflon nebulizer (PFA-100, Elemental ScientificInc.) and a quartz Scott-type double pass spray chamber.Methodological details are provided in Ho et al. (2003) andQuigg et al. (2003). Carbon and nitrogen analysis wereobtained by filtering 25–50-mL culture samples ontoprecombusted 13-mm Gelman AE GF filters. These wereanalyzed for C and N content using a Carlo Erba elementalanalyzer.
All E : P ratios were averaged, then log-transformedto homogenize the variance. Observations from250 mmol m22 s21 quanta treatment have been previouslypublished in Ho et al. (2003) and Quigg et al. (2003) and arereported here for comparison to the other light treatments.Outliers were identified and removed if: one of the replicatesdiffered by more than one order of magnitude from theother (.2) replicates, and as a result log (E : P) at250 mmol m22 s21 for P. provasolii, A. carterae, and T.weissflogii differ slightly from those reported in Ho et al.(2003); if the filter or acid blank showed evidence ofcontamination; or if the element was below the detectionlimit of our method. Often Ni was below the detection limitof our method, and we were unable to get triplicatemeasurements for all irradiances and for all species, butbecause Ni measurements are so sparse in the literature wedecided to include the 36 successful measurements obtainedbut did not include Ni in any of the statistical analyses usedto determine the proportion of the variance in E : P due tophylogenetic effects and phenotypic response to irradiance.
Data analyses—To quantify and partition the magnitudeand variability in log (E : P) due to phylogeny andphenotype we use three main types of data analyses:(1) principal component analysis; (2) an examination of themaximum divided by the minimum E : P; (3) an analysis ofvariance (ANOVA). First, principal component analysis
Irradiance and elemental composition 2691
Table1.
Geo
met
ric
mea
nE
:Po
fm
ari
ne
ph
yto
pla
nk
ton
as
afu
nct
ion
of
gro
wth
irra
dia
nce
(I,
qu
an
ta,
mm
ol
m2
2s2
1).
Th
eu
nit
for
C,
N,
S,
K,
Mg
,a
nd
Ca
ism
ol
(mo
lP
)21,
an
dfo
rS
r,F
e,M
n,
Zn
,C
u,
Co
,M
o,
an
dN
iis
mm
ol
(mo
lP
)21
wit
h1
SE
(%)
inb
rack
ets.
Th
e2
50
mm
ol
m2
2s2
1q
ua
nta
trea
tmen
tis
fro
mQ
uig
get
al.
(20
03
).If
ther
ew
as
on
lyo
ne
rep
lica
ten
ost
an
da
rder
ror
isre
po
rted
;*
ind
ica
tes
two
rep
lica
tes,
an
d{
refe
rsto
mo
reth
an
thre
ere
pli
cate
s,re
spec
tiv
ely
.G
row
thra
te(1
SE
)is
rep
ort
eda
sd
ay
21,
an
dP
isce
llq
uo
tain
fem
tom
ole
so
fP
.
Taxa
Im
Pq
uo
taC
:PN
:PS
:PK
:PM
g:P
Ca
:PS
r:P
Fe
:PM
n:P
Zn
:PC
u:P
Co
:PM
o:P
Ni:
P
Thala
ssio
sira
wei
ssfl
ogii
15
0.1
3(1
.8)
96.7
25
(7.8
)201.5
74
(9.3
)41.0
27
(8.2
)0.7
68
(23.2
)2.6
27
(11.3
)1.5
48
(2.2
)0.1
38
(10.1
)1.2
78
(9.7
)14.7
27
(5.9
)4.2
91
(3.5
)1.6
17
(24.4
)0.4
85
(7.8
)0.1
42
(9.3
)0.0
58
(11.0
)
30
0.1
6(0
.2)
184.2
54
(22.5
)70.0
54
(37.2
)13.7
87
(42.6
)0.8
63
(12.3
)2.2
64
(13.9
)1.0
76
(4.7
)*0.1
19
(1.1
)*1.0
38
(15.0
)12.2
87
(1.3
)4.3
76
(4.0
)1.9
88
(32.6
)0.5
27
(13.6
)0.1
48
(14.7
)*0.0
24
(19.7
)
100
0.3
9(1
1.7
)102.1
62
(8.9
)79.5
77
(5.6
)14.8
57
(3.7
)1.0
32
(10.1
)1.3
37
(6.7
)*0.3
89
(1.3
)0.1
92
(13.4
)*1.0
01
(19.3
)5.2
14
(7.8
)*5.0
92
(17.2
)3.3
45
(33.4
)0.3
80
(8.7
)0.0
49
(47.5
)0.0
47
(21.9
)1.7
41
250
0.9
6130.2
65
(7.2
)83.2
32
(7.0
){13.1
13
(7.1
){1.3
17
(17.4
)0.5
29
(5.9
)0.2
44
0.3
69
(38.5
)2.7
10
(13.7
)1.5
81
(14.4
){5.3
04
(1.4
){0.7
11
(3.3
){0.1
71
(2.6
){0.1
00
(0.9
){0.0
13
(14.5
){0.0
21
(91.2
)*
500
0.5
0(0
.4)
275.2
63
(10.3
)41.6
42
(40.2
)7.7
93
(40.6
)0.7
97
(4.0
)0.7
24
(9.5
)0.1
43
(7.4
)0.0
55
(7.3
)0.8
17
(10.8
)15.4
62
(1.8
)4.5
57
(2.6
)0.7
89
(25.5
)0.0
92
(5.7
)*0.0
72
(1.9
)0.0
17
(8.0
)
Pycn
oco
ccus
pro
vaso
lii
15
0.2
6(4
.0)
1.7
74
(74.2
)69.9
77
(80.2
)10.9
54
(74.2
)0.5
90
(8.3
)*0.6
81
(45.5
)1.2
28
(7.4
)*0.3
09
(12.6
)*20.3
89
(17.2
)*52.1
04
(87.0
)4.0
43
(29.1
)25.9
04
(50.5
)2.3
99
(31.6
)*0.1
16
0.1
18
(24.5
)*3.8
94
30
0.3
6(0
.6)
1.1
11
(20.6
)108.8
33
(18.9
)15.7
93
(20.7
)1.4
02
(10.5
)*1.5
03
(3.9
)*3.5
78
(57.2
)*0.5
17
2.4
59
(125)
43.7
22
(8.4
)2.4
07
(2.1
)8.3
74
(17.6
)2.3
69
(80.9
)*0.0
30
(10.3
)0.0
23
(72.5
)0.6
84
(32.2
)
100
0.5
6(0
.6)
1.5
72
(8.1
)112.1
36
(14.5
)12.2
94
(15.4
)1.8
84
(9.5
)*0.4
19
(0.9
)*1.3
99
(20.4
)*0.1
56
(34.7
)*0.6
16
(63.7
)*44.6
95
(0.9
)2.4
02
(2.0
)7.0
35
(15.7
)0.1
12
(82.7
)*0.0
27
(20.9
)*0.0
35
(40.4
)1.5
97
(23.7
)*
250
0.6
30.7
34
(1.6
)192.0
37
(2.9
){27.1
62
(3.2
){1.3
00
(1.2
){1.4
15
(9.4
){0.1
28
(2.5
){0.2
08
(11.8
){12.0
14
(7.5
){2.0
50
(2.1
){0.8
60
(8.3
){0.5
12
(8.5
){0.0
88
(2.9
){0.0
05
(21.4
){0.0
57
(11.1
){
500
0.8
0(1
6.3
)1.9
52
(15.2
)116.8
81
(19.4
)14.9
62
(21.4
)0.7
93
(1.3
)0.1
52
(3.2
)0.4
06
(14.5
)0.2
42
(12.3
)0.7
77
(16.0
)8.6
73
(0.7
)*1.3
31
(31.3
)*0.4
00
(30.7
)*0.1
73
(28.2
)*0.0
07
(117.7
)*
Cyanoth
ece
sp.
15
0.2
5(1
0.9
)1.0
83
(7.0
)135.0
08
(16.6
)21.9
57
(19.1
)0.8
86
(6.4
)1.3
96
(12.2
)0.7
53
(15.2
)0.3
68
(18.9
)3.8
44
(8.0
)*1053.4
2(8
.6)
57.1
79
(7.6
)72.0
40
(12.5
)1058.9
37
(8.0
)0.1
48
(14.2
)0.1
69
(0.5
)*243.7
97
(16.2
)
30
0.3
9(6
.1)
1.8
98
(30.0
)80.2
130
(38.9
)10.9
06
(31.7
)3.1
22
(5.2
)1.4
89
(12.2
)2.6
10
(11.2
)0.4
06
(11.3
)2.9
85
(11.6
)*48.2
09
(13.3
)18.3
32
(13.8
)9.2
64
(1.4
)1.7
18
(21.0
)*0.4
55
(11.1
)*0.0
37
(16.9
)*3.3
96
(6.8
)*
100
0.4
8(2
.0)
1.4
59
(6.3
)67.8
18
(6.6
)14.9
99
(3.2
)1.1
45
(4.7
)1.1
43
(6.2
)1.7
70
(49.4
)*0.8
93
(0.6
)*2.2
48
(14.4
)11.1
89
(25.6
)1.7
50
(29.1
)5.0
94
(32.5
)0.6
68
(7.1
)0.0
98
250
0.2
59
(5.5
)44.5
10
(10.5
){13.3
20
(5.5
){7.1
17
(9.6
){0.3
01
(6.5
){1.0
57
(13.8
){0.2
30
(6.4
){0.0
10
(2.9
){0.0
35
(4.1
){0.1
15
(5.1
){
500
0.7
4(0
.9)
15.2
72
(10.8
)14.0
32
(10.8
)1.4
62
(10.8
)1.0
17
(2.5
)0.6
01
(1.9
)0.5
51
(5.7
)0.9
29
(2.7
)0.8
58
(3.4
)7.6
76
(5.3
)0.7
94
(19.9
)*1.1
40
(15.0
)0.2
77
(0.8
)0.0
32
0.0
14
(5.7
)0.0
51
(41.2
)*
Am
phid
iniu
m
cart
erae
15
0.1
4(1
.2)
48.1
15
(3.6
)196.0
60
(8.7
)26.0
18
(9.7
)0.3
95
(29.0
)0.5
19
(6.1
)*1.3
34
(24.6
)359.2
65
(27.5
)175.6
21
(11.3
)137.9
98
(18.1
)*331.9
21
(12.3
)2.6
14
(4.7
)*0.1
39
(22.0
)5.2
67
(22.3
)*
30
0.2
7(2
.2)
66.7
91
(5.5
)114.2
46
(22.5
)14.3
31
(28.4
)1.6
89
(21.2
)0.5
95
(16.4
)1.0
22
(14.3
)*0.1
61
(26.5
)1.1
42
(15.9
)20.5
47
(29.4
)4.4
90
(37.3
)4.1
20
(31.2
)0.5
69
(8.7
)*0.2
01
(83.9
)*0.0
14
(61.4
)*
100
0.4
6(3
.6)
65.2
12
(34.8
)105.1
35
(37.4
)14.0
92
(37.2
)1.3
28
(4.4
)*0.6
02
(11.3
)*0.2
68
0.4
77
(29.2
)*2.1
79
(25.0
)29.9
23
(6.7
)*3.5
60
(14.1
)6.6
25
(36.2
)*1.6
09
(16.9
)*0.1
82
(58.0
)*0.0
19
(65.2
)*
250
0.5
255.7
51
(5.3
)125.8
19
(5.8
){18.5
10
(6.1
)1.4
16
(6.4
){0.1
03
(21.4
)*0.3
27
(8.1
)0.2
55
(11.5
)1.1
28
(5.4
){18.7
75
(33.3
)5.1
33
(4.0
)1.1
77
(33.4
)0.5
10
(15.2
)0.3
64
(3.3
)0.1
79
(2.6
)0.0
49
(86.0
)*
500
0.5
9(1
.6)
34.2
28
(18.5
)161.2
99
(18.6
)20.9
34
(18.2
)1.5
58
(3.0
)*0.1
80
(18.5
)*0.3
26
(27.8
)0.0
91
(9.8
)1.1
64
(18.5
)39.0
09
(3.0
)7.1
47
(0.9
)0.6
45
(40.8
)*0.2
03
(31.3
)0.2
58
(14.4
)0.0
36
(8.6
)
Chaet
oce
ros
calc
itra
ns
15
0.2
1(7
.3)
1.4
40
(46.7
)148.6
70
(46.7
)21.5
89
(49.7
)0.0
55
(32.1
)*6.5
28
(2.6
)*520.7
05
(0.5
)238.9
79
(4.0
)*39.9
00
(10.3
)*209.6
16
(6.5
)0.2
20
(35.9
)0.5
98
(60.4
)76.5
90
(22.8
)
30
0.3
0(0
.8)
2.7
54
(4.1
2)
93.2
30
(9.8
)10.9
30
(7.7
)0.5
33
(5.7
)0.8
13
(3.9
)0.1
73
(9.1
)2.1
04
(4.1
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)8.8
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(2.7
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17
(21.7
)0.3
24
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100
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3.4
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(44.4
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83
(37.9
)8.0
29
(37.9
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(46.8
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84
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(42.3
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55
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(15.8
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(20.3
)5.8
22
(70.9
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30
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)*0.0
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)*0.3
08
(19.0
)
2692 Finkel et al.
(PCA) is used to determine the proportion of total variancein the data that can be described by a small number oflinear combinations of predictor variables. Each principalcomponent is a linear combination of log (E1 : P), log(E2 : P), …, log (En : P), ordered so that each componentexplains the maximum amount of residual variance. Therepresentation of the data on the two principal componentaxes permits a visualization of the maximum amount ofvariability possible from a projection of the data ontoa plane. The PCA biplot presents a vector along each log(E : P) axis projected onto the plane. The composition ofeach principal component can be recovered by projectingeach vector onto the respective axis. In addition, thetaxonomic affiliation and light treatment are indicatedthrough the use of color and shape of the data points on theplot. Second, the maximum variability in E : P was assessedby calculating the maximum 3 minimum21 for each log(E : P) over all species and irradiance treatments (total ormaximum variability in log [E : P]). Third, a two-wayANOVA was used to quantify and evaluate the statisticalsignificance of phylogeny, phenotypic response to irradi-ance, and the interaction of phylogeny and phenotype asfactors predicting variation in each log (E : P) and log netsteady-state uptake (mol E [mol P h]21), separately(Tables 3 and 4). Most species–treatment combinationshad three replicates, but because of experimental andanalytical difficulties, the design was not completelybalanced (orthogonal). As a result, the sums of squares
for each of the main effects depended on the order in whichthey were included in the model (Sokal and Rohlf 1994).For our data this amounted to only a small (,5%)variation; analyses reported are an average of the twopossible models. The PCA and ANOVA analyses wereperformed with R (R Development Core Team 2005).
Results
The antilog of the mean of the log-transformed E : P(geometric mean) measured for the five different species ofmarine phytoplankton as a function of growth irradianceare presented in Table 1 and all replicates are reported inWeb Appendix 1 (http://www.aslo.org/lo/toc/vol_51/issue_6/2690a1.pdf). Elemental ratios are only available for threeirradiances, 15, 30, and 100 mmol m22 s21 quanta for C.calcitrans because this species did not grow at 250 and500 mmol m22 s21 under our growth conditions. In thisstudy, cellular P, N, C, and cell volume are all highlycorrelated, indicating that most trends in E : P will alsoapply to E normalized to cell volume or carbon (Table 2).Normalization to P is preferred since it was measuredsimultaneously with the majority of the elements usingICPMS. Recent results on Trichodesmium spp. and twodiatom cultures indicate that phosphorus can accumulateon the outside of phytoplankton cells (Sanudo-Wilhelmy etal. 2004), although in T. pseudonana grown in f/2, less than40% of total P was associated with the cell wall,plasmalemma, and nuclei (Reinfelder and Fisher 1991). Ifphosphorus accumulated on the surface of the phytoplank-ton in this study, then E : P may be underestimated, but thecorrelation of cellular P with C, N, and cell volume indicatethat all the trends among species and with irradiance arerobust (Table 2).
Genetic differences versus phenotypic response in log(E : P) over a range of growth irradiance—A PCA using thecorrelations between log (E : P) was used to visualize therelative effect of genetic differences and phenotypic re-sponse to growth irradiance on two groupings of elements:
Table 2. Correlations between cell size (cell volume V [mm3])and cellular C, N, and P (mol), all are highly significant (n 5 94, p, 0.0001).
Elements R
P C 0.80P N 0.79P V 0.86V C 0.89V N 0.87N C 0.98
Table 3. Two-way analysis of variance of log (E : P) as a function of taxonomic differences (phylogenetic effect), irradiance(phenotypic effect), and the interaction (phenotypic 3 phylogenetic effect) between these two effects (reported as a percentage of totalsum of squares [SS]). Total SS is the total sum of squared deviation from the mean for the data, p % 0.001 for all E : P, and n is the totalnumber of observations.
Phylogenetic effect % Phenotypic effect % Interaction % Residual error % Total SS Total n
log (C : P) 34 16 31 20 7.28 95log (N : P) 15 26 39 20 7.04 95log (S : P) 26 34 30 11 1.78 59log (K : P) 36 30 31 4 7.63 56log (Mg : P) 14 58 23 4 9.76 58log (Ca : P) 47 7 41 6 6.15 58log (Sr : P) 17 29 45 9 6.38 60log (Fe : P) 20 55 23 3 30.69 74log (Mn : P) 15 43 40 1 32.36 75log (Zn : P) 13 67 16 4 30.41 72log (Cu : P) 11 63 26 1 81.71 70log (Co : P) 40 23 32 4 20.37 64log (Mo : P) 19 43 29 9 17.87 68
Irradiance and elemental composition 2693
the trace metals Fe : P, Mn : P, Zn : P, Cu : P, Co : P, Mo : P;and the macronutrients and alkali and alkaline earthmetals: C : P, N : P, S : P, K : P, Mg : P, Ca : P, Sr : P(Fig. 1; the component loadings are provided in the figurelegend). These two sets of elements were consideredseparately because observations of the two sets of elementswere frequently not paired and an aggregate considerationwould precipitously decrease the sample size. The first twoprincipal components account for 84.8% of the variance intransition metal data and 61.1% of the variance in themacronutrient and alkali and alkaline earth metal data.
The first principal component for the trace metalsseparates the observations by growth irradiance andaccounts for 71% of the total variance in the data. Mostmetals are enriched relative to phosphorus, especially Fe,Mn, Cu, and Zn under low light, 15 and 30 mmol m22 s21
quanta, and become increasingly depleted relative tophosphorus under irradiances saturating for growth, 250and 500 mmol m22 s21. The species T. weissflogii and P.provasolii both have much lower metal enrichments at lowlight than the other species. The second principal compo-nent for the trace metals, dominated by Co : P, separatesout subtle species-specific differences in metal composition.For the macronutrients and alkali and alkaline earth metalsthe first principal axis (,35% of the variance) alsoseparates out different light treatments where many of theelements are relatively more enriched under low light,especially C : P, N : P, Mg : P, K : P, and Sr : P. The secondprincipal component, especially C : P, N : P, and Ca : P,again highlights some of the differences between taxa.
The maximum variability in E : P (maximum minimum21
in E : P over all species and growth irradiances) is stronglycorrelated with its contribution to total cell mass (Fig. 2).C, N, and S all exhibit less than eightfold range inmaximum minimum21 in E : P, the alkali and alkaline earthmetals have about one order of magnitude range, and thetrace metals tend to have a two to three order of magnituderange. A two-way ANOVA indicates that irradiance,taxonomic differences, and their interaction are all signif-icant sources of variation in log (E : P), p , 0.001 (Table 3).Phylogeny, irradiance, and the interaction between the two
in aggregate typically accounts for .80%, and often .90%,of the variation in the data. Among the macronutrients it isnotable that a large proportion of the total variance in log(Mg : P), an element known to be affected by photoaccli-mation, is associated with the phenotypic response toirradiance. Similarly a large proportion of the totalvariance in many of the trace metals, especially log(Fe : P), log (Mn : P), log (Zn : P), log (Cu : P), and log(Mo : P), is associated with irradiance. In contrast, more ofthe variation in log (C : P), log (K : P), log (Ca : P), and log(Co : P) is explained by phylogenetic effects. The multipli-cative interaction between phylogenetic effects and pheno-typic response to irradiance was significant for all log (E : P)examined, tending to account for .30% of the total sum ofsquares.
Functional response in E : P as a function of growth irra-diance—Most of the trace elements, Fe, Mn, Sr, Zn, Cu,Co, Mo, and the macronutrients, K and Mg, are enrichedrelative to phosphorus under irradiances that are limitingfor growth. For some species, a select number of elementsare also enriched relative to phosphorus at saturatingirradiances, including Fe, Mn, and Zn in T. weissfloggi, andCo in Cyanothece sp. and P. provasolii, suggestive of anincreased species-specific requirement for these elementsunder saturating irradiance and high growth rates (Figs. 3,4). In general, macronutrient : P did not change consistentlywith irradiance across the different species examined: C : Pwas high under saturating irradiance for A. carterae andhigh under low growth irradiance for T. weissflogii andCyanothece sp.; N : P tended to decrease with irradiance forT. weissflogii, Cyanothece sp., and A. carterae but did notsignificantly change for C. calcitrans or P. provasolii. Incontrast, log S : P and Ca : P have a maximum E : P atintermediate irradiance, ,100 mmol m22 s21 quanta(Fig. 3).
Functional response in the net rate of steady-state influx asa function of growth irradiance—Previous studies havefound that steady-state trace element uptake is a functionof free metal availability in the media; therefore increases in
Table 4. Two-way analysis of variance of log steady-state uptake rate (mol E [mol P h]21) as a function of irradiance (phenotypicresponse) and taxonomic differences (phylogenetic effect), and the interaction between these two effects (%). Total SS is the total sum ofsquared deviation from the mean for the data, p % 0.001 for all E : P, and n is the total number of observations.
Phylogenetic effect % Phenotypic effect % Interaction % Residual error % Total SS Total n
C 19 40 25 16 9.28 88N 14 41 28 17 9.10 88S 18 55 23 5 5.69 59K 47 15 32 6 5.50 56Mg 29 32 33 6 8.11 58Ca 38 29 29 4 11.21 58Sr 26 14 52 8 8.25 60Fe 37 26 33 4 20.22 66Mn 14 28 56 2 17.15 68Zn 29 36 30 5 20.11 65Cu 21 49 30 1 62.25 63Co 44 26 23 7 12.81 57Mo 18 26 44 12 12.75 61
2694 Finkel et al.
growth rate with an increase in irradiance without a changein nutrient concentrations will result in a decrease in E : Por E : C, termed the growth dilution hypothesis (Sunda andHuntsman 1997). The steady-state net influx (m 3 E : P) forall the alkali and alkaline earth (except Ca) and transitionelements is a function of irradiance (Table 4); this indicatesthat decreases in E : P are not only due to changes in growthrate, but active regulation in net steady-state nutrient in-flux as a function of growth and irradiance (Figs. 5, 6). Asa result, Mn : P, Fe : P, Zn : P, Mo : P, and Co : P canincrease about one order of magnitude and Cu : P morethan three orders of magnitude under low versus highgrowth irradiance. For all species except T. weissflogii, thenet steady-state rate of trace element influx for Fe, Mn, Cu,Co, Mo, and Mg is much higher under low irradiance (15and 30 mmol m22 s21 quanta). In T. weissflogii the netsteady-state rate of influx for Fe, Mn, Zn, Cu, and Co allincrease relative to P at 500 mmol m22 s21 relative to250 mmol m22 s21. The net rate of steady-state influx of Caand S tends to be elevated at intermediate irradiance, themaximum tending to occur ,100 mmol m22 s21, but notfor all species (Fig. 5).
Discussion
Differences in elemental stoichiometry characterizedifferent taxonomic groups grown under comparableenvironmental conditions (Ho et al. 2003; Quigg et al.2003; Falkowski et al. 2004). Ho et al. (2003) defined anaverage elemental stoichiometry of nutrient-saturatedmarine phytoplankton grown at 250 mmol m22 s21 as:
C124N16P1S1:3K1:7Mg0:56Ca0:5ð Þ1000
Sr5:0Fe7:5Mn3:8Zn0:80Cu0:38Co0:19 Mo0:03:
Fig. 1. Principal component analysis on correlations for: (A)the transition metals log (Fe : P), (Mn : P), (Zn : P), (Cu : P),(Co : P), (Mo : P). The first two components account for 84.8%of the data; principal component I is 0.44 log (Fe : P) + 0.44 log(Mn : P) + 0.42 log (Zn : P) + 0.45 log (Cu : P) + 0.33 log (Co : P) +0.34 log (Mo : P) and principal component II is 20.20 log (Fe : P)+ 0.36 log (Mn : P) 2 0.35 log (Zn : P) 2 0.17 log (Cu : P) + 0.77 log(Co : P) 2 0.30 log (Mo : P); (B) the macronutrients and alkali andalkaline earth metals, log (C : P), (N : P), (S : P), (K : P), (Mg : P),(Ca : P), (Sr : P). The first two principal components account for61.1% of the variance; principal component I is 0.56 log (C : P) +0.57 log (N : P) + 0.02 log (S : P) + 0.31 log (K : P) + 0.38 log(Mg : P) 2 0.08 log (Ca : P) + 0.34 log (Sr : P) and principalcomponent II is 20.29 log (C : P) 2 0.30 log (N : P) + 0.21 log(S : P) + 0.30 log (K : P) + 0.49 log (Mg : P) + 0.62 log (Ca : P) + 0.27log (Sr : P). The color of the data points is used to represent light
rtreatments: 500 (black), 250 (blue), 100 (green), 30 (orange), and15 mmol m22 s21 quanta (red), and shapes represent species:triangle T. weissflogii, inverted triangle C. calcitrans, circles P.provasolii, diamonds A. carterae, and squares Cyanothece sp.
Fig. 2. Variability in (maximum 3 minimum21) in E : P asa function of total contribution to cellular biomass (median E : P).Macronutrients are reported as mol mol21, micronutrientsmmol mol21.
Irradiance and elemental composition 2695
Note that Ni : P was not measured by Ho et al. (2003), butat 250 mmol m22 s21 we found it ranged between speciesfrom 0.02 to 0.12 with an average of 0.06 mmol mol21. Onthe basis of the five species examined, over 15 to500 mmol m22 s21 quanta, phenotypic responses to irradi-ance is responsible for less than one to about three ordersof magnitude in E : P within species, comparable tophylogenetic differences (Quigg et al. 2003), contrary toour original hypothesis, although phylogenetic effects arestill clearly visible (Fig. 1). A change in irradiance alters thestoichiometry of several of these elements (Figs. 1, 3, 4;Table 1). In particular, many of the trace metals are highestat very low irradiance, Fe : P, Mn : P, Zn : P, Cu : P Co : P,Mo : P, and Ni : P, whereas S : P, K : P, Mg : P, and Ca : Ptend to be highest at intermediate irradiances. Irradiancethus has different effects on E : P in different species, oftenmagnifying between-species differences in elemental stoi-chiometry. For example, the elemental stoichiometry ofCyanothece sp. at 15 mmol m22 s21 is:
C135N22P1 K1:4S0:9Mg0:56Ca0:37 Cu0:11 Fe0:10ð Þ1000
Zn72:04Mn57:18Sr3:84Mo0:17Co0:15:
This example and the data in Table 1 indicate that theelemental stoichiometry of phytoplankton biomass maydiffer significantly between light-limited and light-saturated
regions of the ocean because of both the phenotypicresponse to light and corresponding changes in thetaxonomic composition of the communities.
The role of photoacclimation in variation in E : P—Phytoplankton regulate their metabolic rates in responseto changes in incident irradiance through a variety ofplastic physiological responses broadly referred to asphotoacclimation. Although there are taxonomic differ-ences in acclimation strategies, there are some generalphenotypic physiological responses that are common to allspecies; for example cellular pigment concentration tends toincrease with decreases in irradiance (Richardson et al.1983; Falkowski and LaRoche 1991; MacIntyre et al.2001). Many cellular components associated with photo-acclimation have unique elemental stoichiometries that canalter the overall elemental stoichiometry of cells over a light
Fig. 4. Log (E : P) for the trace metals (61 SE) as a functionof growth irradiance. The different species symbols as in Fig. 1,units as in Table 1.
Fig. 3. Log (E : P) for the macronutrients and the alkali andalkaline earth metals (61 SE) as a function of growth irradiance.The different species are denoted by symbols as in Fig. 1, units asin Table 1.
2696 Finkel et al.
gradient (Raven 1988, 1990). For example, each chloro-phyll a molecule has a Mg atom, so the change in pigmentwith irradiance results in a shift in Mg : P (Table 1),although, in contrast with trace elements, biophysicalgradients across the cell membrane likely play a major rolein controlling the intracellular concentrations of Mg, Ca,and K (on a fresh weight basis). On the basis of thedecreasing efficiency of exciton transfer with the size of thelight-harvesting antennae (at large antennae sizes), Raven(1988, 1990) estimated that photoacclimation to lowirradiance can trigger an increased need for cellular Fe : Pand Mn : P because of their requirement in the photosyn-thetic reaction centers, photosynthetic electron transport,and water splitting capacity, in agreement with field andlaboratory measurements (Sunda and Huntsman 1997,1998a). Increases in nitrogen (N : P) have also beenobserved and attributed to increased protein associatedwith increased cellular pigment (Sciandra et al. 1997; Flynnet al. 2001). In other studies, especially those where theexperimental species were well acclimated to the growthirradiance, C : N : P did not change significantly withirradiance (Rhee and Gotham 1981; Goldman 1986;Leonardos and Geider 2004), in agreement with ourobservations. We found that a large number of elements,especially the trace metals, but not only those associated
with light harvesting, became enriched relative to phos-phorus under low growth irradiance. For a few speciesa select number of elements became enriched relative tophosphorus at saturating irradiances including Fe, Mn, andZn in the coastal diatom T. weissfloggi, and Co in thediazotroph Cyanothece sp. and the prasinophyte, P.provasolii, perhaps indicating an increased requirementfor these elements under saturating irradiance and highgrowth rates (Fig. 4). The large increases in Zn : P, Mo : P,Co : P, and Cu : P under low growth irradiance questionsthe interpretation that observed increases in total cellularFe : P and Mn : P under low growth irradiance are due tometabolic requirements.
Mechanistic models for E : P—The growth rate hypoth-esis of Sterner and Elser (2002) predicts a decrease in N : Pwith increased growth rate because of an increased cellular
Fig. 5. Log steady-state net rate of influx for each macro-nutrient and the alkali and alkaline earth metal relative to P (logmol E 3 [mol P 3 h]21) as a function of growth irradiance.Species symbols as in Fig. 1, units as in Table 1.
Fig. 6. Log steady-state net rate of influx for the trace metalsrelative to P (log mmol E 3 [mol P 3 h]21) as a function ofgrowth irradiance. Species symbols as in Fig. 1, units as in Table 1.
Irradiance and elemental composition 2697
allocation of P to RNA synthesis to meet protein synthesisdemand (Elser et al. 2000; Sterner and Elser 2002). There isaccumulating evidence against the generality of the growthrate hypothesis, especially when phosphorus is not thelimiting element (Elser et al. 2003; Agren 2004). The growthrate hypothesis is based on the inference that an increase ingrowth requires an increase in the pool of cellularphosphorus; this differs from the theoretical calculationsof Raven (1988, 1990), which explicitly consider the molesof element required to make a reaction run at a specific rate(but see Agren 2004; Klausmeier et al. 2004). In this studythe interspecific relationship between N : P and growth rateis weak (Table 1). Sunda and Huntsman (1997, 1998a,b,2004) propose that
E : P ~ V=m ð1Þ
where V is the steady-state nutrient uptake or net influx ofthe element E. Sunda and Huntsman (1998) suggest thatE : P is generally controlled by a feedback mechanism, andV increases with nutrient availability, resulting in increasedcellular quota, which then leads to increased growth (m) andeventually a decrease in E : P. There is an inherentassumption that the elements that compose a large pro-portion of cellular biomass, such as C and P, have cellularconcentrations that are linearly related to growth rate. Asa result this model predicts that under nutrient-saturatingconditions an increase in growth rate due to irradiancewill result in a decrease in cellular E : C or E : P. ForT. pseudonana and Prorocentrum minimum, Sunda andHuntsman (1997, 1998a,b) found that Mn : C and Fe : Cwere independent of irradiance between 50 and500 mmol m22 s21 quanta, and that changes in E : C wereprimarily due to biodilution due to increased growth rate.Our observations contradict the result of Sunda andHuntsman under growth irradiances lower than those theytested.
Re-evaluating the effect of irradiance on the mechanisticmodel for E : P—For five different species we find thatbelow 50 mmol m22 s21 quanta the steady-state net nutri-ent influx (Figs. 5, 6) for Fe, Mn, Zn, Cu, Co, Mo, Sr, andMg are strongly affected by irradiance, resulting in a largeaccumulation of these E relative to P (and C) under lowirradiance (Fig. 3). This accumulation does not occurproportionally over the range of irradiances; instead thereis a radical increase in the above-mentioned E : P betweenthe lowest irradiances, 15 and 30 mmol m22 s21, examined.This is not likely due to the effect of irradiance on theavailability of free metal because an increase in irradiancewould only increase the availability of Fe and the steady-state uptake, resulting in an underestimation of the steady-state uptake rate under low light (Sunda and Huntsman1997, 1998a,b). This rapid accumulation of E : P under lowirradiance has been previously reported for Fe : C in T.weissflogii and furthermore it was demonstrated that theobserved accumulation of Fe : C under low light(,37 mmol m22 s21) was inconsistent with any reasonableaccumulation of photosystems II and I, indicating that thelarge accumulation may be due to accumulation of the
trace element into a nonmetabolic pool (Strzepek and Price2000).
Previously, increases in cellular Fe : P and Mn : P havebeen attributed to an increase in metabolic requirementsfor these elements under low light (Raven 1988, 1990;Sunda and Huntsman 2004). Although increased require-ments for Fe : P and Mn : P are predicted on the basis ofincreased cellular concentrations of Fe and Mn-richphotosynthetic machinery, total cellular Fe : P and Mn : Pdo not necessarily reflect biochemical need. For most of thetrace elements only a small proportion of their total cellularconcentration has been identified in known metabolicprocesses (Hewitt 1983). Substantial cellular accumulation(for example, storage) could make it difficult to detect anypatterns in elemental stoichiometry from whole cells.Storage vacuoles, such as the acidocalcisomes (polyphos-phate vacuoles), are known to have associated quantities ofa number of elements including Na, Mg, S, Cl, K, Ca, Zn,and Cd (Hall 2002; Docampo et al. 2005). On the basis ofexperimental data on storage of C, N, and P, Fogg andThake (1987) hypothesized that the accumulation abilityfor a given nutrient will increase as its proportion of totalcell mass decreases. For the species examined, themultiplicative range in E : P is correlated with log medianE : P in biomass, suggesting an increased accumulationcapacity for nutrients that make up a small proportion ofcell biomass (Fig. 2). A large storage capacity for anelement will make it extremely difficult to detect changes inits stoichiometry with other elements associated withmetabolic requirements from measurements of whole cells.The large accumulation of trace metals relative tophosphorus under low growth irradiance is consistent withStrzepek and Price’s (2000) hypothesis that there may be anincrease in the storage of trace elements in nonmetabolicpools under very low growth irradiance. The storage ofmicronutrients may provide the cells and their progenywith a pool of micronutrients that can be exploited once thecells are exposed to saturating growth irradiance, or may bea competitive strategy used by phytoplankton to starveheterotrophs with overlapping nutrient requirements (Gro-ver 2000).
The elemental stoichiometry of phytoplankton cells isthe sum of E associated with metabolic requirements andcellular accumulation normalized by the sum of P or othernormalizing element associated with metabolic require-ments and cellular accumulation, or equivalently therelative rates of net steady-state influx (influx 2 efflux) ofdifferent nutrients into the cell:
phytoplankton E : P
~ requirementE z accumulationEð Þ
| requirementP z accumulationPð Þ{1
~ influxE { effuxEð Þ influxP { effluxPð Þ{1:
ð2Þ
To understand why the elemental stoichiometry of phyto-plankton changes with irradiance, we need to evaluate howeach of the quantities in Eq. 2 change with irradiance.There is very little information on how growth irradiance
2698 Finkel et al.
affects the influx and efflux rates for different elements.Experimental studies of nutrient uptake suggest that anincrease in growth irradiance increases the maximum rateof uptake, Vmax (Reshkin and Knauer 1979; Anderson andRoels 1981), which is often interpreted as an estimate of themaximum rate of net influx of a given nutrient. There isconsiderable species- and nutrient-specific variability in theirradiance dependence of Vmax (Eppley et al. 1971; Reshkinand Knauer 1979; Litchman et al. 2004), but experimentaldata support theoretical predictions that the light de-pendence of Vmax is most pronounced for nutrients thatrequire relatively more energy to be assimilated (Litchmanet al. 2004).
On the basis of the information available, the observedincrease in E : P under low growth irradiance is as likely dueto a decrease in efflux of E relative to P as to an increase ininflux of E relative to P. On the basis of bacterial influx andefflux systems it has been hypothesized that it may be moreenergetically costly to restrict and select entry of differentions than induce efflux (Silver 1996). Observations of effluxin phytoplankton tend to be highly variable and related tofactors that cause physiological stress including highirradiance (Zlotnik and Dubinsky 1989), supporting thehypothesis that efflux may be a quantitatively importantprocess in determining the steady-state elemental stoichi-ometry of phytoplankton. Analyses of the relative costs andbenefits of influx, efflux, and storage of different elementsunder different growth rates and growth irradiances wouldaid in interpreting the rather large accumulation ofmicronutrients relative to C, N, and P under low light.Alternatively, there is some evidence that some trace metaltransporters are not very specific for a single metal, leadingto complex antagonistic interactions. For example, Zn andCu can be taken up incidentally by transporters for Fe andMn (Bruland et al. 1991, and references therein), and mayaccount for coordinated changes in many of the traceelements in response to irradiance.
The cycling and transport of Mn, Zn, Cd, Cu, and Fecan be greatly affected by phytoplankton in the upperwater column (Sunda and Huntsman 1998a; Morel andPrice 2003), but these effects are complicated by photo-chemical effects on redox cycling, hydrographic conditions,and the complex biological and chemical controls on theformation and destruction of organic complexes that bindto many of the trace metals (Knauer and Martin 1973;Morel and Price 2003). Knauer and Martin (1973) foundincreases in total dissolved Cd, Cu, Mn, and Zn associatedwith periods of upwelling that were, in some instances,followed by moderate decreases in Cd and Zn in the surfacewaters of Monterey Bay, California. Kudo and Matsunaga(1998) also found after the spring bloom in Funka Bay thatalthough Cd and the macronutrients were removed fromthe euphotic zone, Ni and Cu exhibited very little change,suggesting that very large blooms may be required tomeasurably detect an effect on the total concentration ofthe soluble metals with a biological role in the phytoplank-ton (Knauer and Martin 1973; Kudo and Matsunaga1998). Over the course of the spring bloom in South SanFrancisco Bay, California, large depletions observed intotal dissolved Ni, Cd, and Zn, but not Cu, were associated
with decreases in the macronutrients Si and P (Luoma et al.1998), indicating that environmental conditions associatedwith the bloom (such as high vertical mixing and exposureto low average irradiance) may account for high metalincorporation into phytoplankton biomass. Luoma et al.(1998) suggest that the differences in the inferred elementalstoichiometry of the phytoplankton from South SanFrancisco Bay versus open ocean plankton (Bruland et al.1991) are likely due to differences in trace elementavailability in the different systems and the taxonomiccomposition of the plankton. Our results support theassertion that both differences in environmental conditions,irradiance, and nutrient concentration and stoichiometry(Bruland et al. 1991), and the taxonomic composition ofthe phytoplankton community can significantly affect theelemental stoichiometry of phytoplankton-derived particu-late organic material and the corresponding depletion ofdissolved nutrients.
The large phenotypic shift in the elemental compositionof phytoplankton in response to growth irradianceindicates that phytoplankton may contribute to significantdifferences in the elemental composition of particulatematter between light-limited and light-saturated regionsof the ocean. Significant proportions of phytoplanktonbiomass and export production may be light-limited andnutrient-saturated. It has been estimated that mesoscaleeddies are responsible for 35–50% of new production,,0.5 mol m22 yr21 N (McGillicuddy et al. 1998). Theseeddies lift micro- and macronutrients into the lowerportions of the euphotic zone, often stimulating thegrowth and export of light-limited phytoplankton, espe-cially diatoms (Goldman and McGillicuddy 2003; Sweeneyet al. 2003). Our observations of extremely high accumu-lation of Fe, Mn, Zn, Cu, Co, and Mo relative to P undervery low irradiance suggest that areas characterized byeddies, intense vertical mixing, or a pronounced deepchlorophyll maxima should be examined for higher thantypical Fe, Mn, Zn, Cu, Co, and Mo relative to P inparticulate matter and corresponding depletion in theconcentration of bioavailable dissolved concentrations ofthese elements.
The effects of low irradiance and taxonomic compositionon elemental stoichiometry, especially Fe, Mn, Zn, Co, Cu,and Mo relative to P, are large and likely contribute todifferences in the elemental stoichiometry of plankton andcycling in the ocean, but several factors may make itdifficult to observe in a field setting. Changes in irradianceare often causally correlated with changes in macro- andmicronutrient stoichiometry and concentration andchanges in the taxonomic composition of the phytoplank-ton community, making it extremely difficult to identifyany particular cause with a change in the elementalcomposition of particulate organic matter. In addition,the bioavailability of Fe, Co, Cd, Cu, and Zn is regulated inpart by their reaction with organic complexing agents withmostly uncharacterized low molecular mass ligands that arelikely associated with metal sequestration, detoxification,and in some cases transport into phytoplankton cells, andphotochemically affected redox cycling contributes to thebiological availability and distribution of Fe, Mn, Cu, and
Irradiance and elemental composition 2699
Co (Sunda and Huntsman 1998a; Morel and Price 2003).For example, depletion of Cu2+ and Mn2+ reported in deepchlorophyll maxima (Moffett 1995; Sunda and Huntsman1998a) is consistent with our observations, but has beenattributed primarily to these other factors. Using adsorp-tive cathodic stripping voltammetry, Moffett (1995) foundvery low free cupric ion concentration in samples collectedfrom the Bermuda Atlantic Time Series Station followingperiods of intense vertical mixing and associated with thedeep chlorophyll maximum in the Sargasso Sea. Thebioavailability of cupric ion is controlled by ligand (L1)concentrations that likely act to prevent Cu toxicity,especially for sensitive phytoplankton species. Cu accumu-lation in phytoplankton under low light is consistent withthe variable relationship between cell density of Synecho-coccus and Prochlorococcus and ligand (L1) concentrations(Moffett 1995), especially after intense vertical mixing andin the deep chlorophyll maximum. Mn2+ tends to decreasewith depth in the surface; this profile has been attributed toa combination of photodissociation associated with Mnredox cycling and atmospheric inputs (Sunda and Hunts-man 1998a and references therein), although low-light-enhanced accumulation in phytoplankton biomass mayalso contribute to this characteristic vertical distribution.Ideally, evidence for low-light trace element enrichmentrelative to P in phytoplankton biomass should come fromsimultaneous measurements of the free dissolved elementalcomposition of the water column, the elemental stochio-metry of the particulate matter in different size fractionsand size-fractionated high-performance liquid chromato-graphy determination of total chlorophyll, and taxonomiccomposition complemented by microscopic identificationof taxa with depth in stratified and well-mixed watercolumns. If high phytoplankton biomass is responsiblefor, even in part, the observed depletion of trace ele-ments such as Cu2+ and Mn2+ in deep chlorophyllmaxima, we would expect similar patterns of depletion inother trace metal profiles, most likely bioavailable Fe,Zn, and Co. Since the trends in trace element enrichmentunder low light are to some degree taxon specific, wehypothesize that there may be differences in the stoichi-ometry of trace element depletion in regimes where thedeep chlorophyll maximum is composed of differentalgal groups. Overall these results suggest that thefractionation of trace metals relative to phosphorus andcarbon in light-limited, nutrient-saturated phytoplanktonmay be a very important and previously unrecognizedcomponent of the biogeochemical cycling of trace elementsin the ocean.
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Received: 3 June 2005Accepted: 17 July 2006Amended: 27 June 2006
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