Primary producers

Primary consumers (herbivores)

Secondary consumers (carnivores)

Tertiary consumers

The way biomass is distributed among trophic levels in the food web provides clues to the efficiency of energy transfer through the ecosystem .

Note: this is a static depiction-it does not provide information on how fast biomass turns over within each trophic level.

Commonly used determinations of bacterial biomass

(mg C L-1)

• Cell abundance and cell volume (by microscopy and more recently by flow cytometry)

“Typical” bacterial cell densities in aquatic ecosystems

Habitat Cell density (cells ml-1)

Estuaries >5 x 106

Coastal (near shore) 1-5 x 106

Open Ocean 0.5-1 x 106

Deep Sea <0.01 x 106

Remember that cell abundance ≠carbon biomass

Fagerbakke et al. (1996)

Size matters

Determinations of cell volume

• Need to measure 300-400 cells per sample for statistical precision.

• Assumes spherical cells

• No standards; calibrated with microspheres that likely have different fluorescence and shape characteristics than bacterial cells

• Image analyses is highly dependent on edge determination

Bacterial cell volumes by depth in several ocean ecosystems

Location Cell Volume

(µm3)

Study

Escherichia coli

0.9-2.4 Watson et al. (1977)

Vibrio natriegens

0.9-3.5 Fagerbakke et al. (1996)

Sargasso Sea

0.03-0.06 Carlson et al. (1996)

North Sea 0.1-0.4 Fagerbakke et al. (1996)

Ross Sea,Antarctica

0.04-0.15 Ducklow et al. (2001)

Sargasso Sea

0.03-0.1 Gundersen et al. (2002)

Cell abundance (cells ml-1) x Cell volume (µm3 cell-1) = Biovolume (µm3 ml-1)

Cell Volume(µm3 cell-1)

0.020.03

0.040.05

0.060.07

0.08

Dep

th (m

)

0

200

400

600

800

1000

Polar FrontRoss SeaEQ-PAC (March)EQ-PAC (September)

Why are oceanic bacteria so small?

• Inactive or dormant – low growth rates?

• High S/V advantageous for growth in limiting nutrient media?

• Predation defense?

Consider a spherical microbe:

SA= 4πr2

V= 4/3 πr3

r = 0.50 µm

r = 1.0

SA = 3.1 µm2

V = 0.52 µm3

SA : V = 15.7

SA = 12.6 µm2

V = 4.2 µm3

SA : V = 3.0

The smaller the cell, the larger the SA : VGreater SA : V increases number of transport sites

(per unit biomass), and may allow smaller cells to out compete larger cells under limiting nutrient conditions.

To get from abundance and volume to biomass requires conversion factors

Location Density(fg C µm-3)

C content(fg C cell-1)

Method Study

E. coli 126-132 109-323 CHN analyses

Watson et al. (1977)

E. coli, P. putida, B. subtilius

160-930 CHN analyses

Bratbak et al. (1985)

Natural plankton

280 20 CHN Lee and Fuhrman

(1987)

Station ALOHA

3.5-8.8 Biomass constraint

Christian and Karl

(1994)

Southern Ocean

12 Direct Measureby TOC

Fukuda et al. (1998)

Ross Sea,Antarctica

77-165 7-13 C mass balance

Carlson et al. (1999)

Sargasso Sea

148 4-9 X-Ray micro.

Gunderson et al. (2002)

Most studies converge

~10 fg C cell-1

Buck et al. (1996)

25oN

5oS

45oN

Slope = 0.78 - 0.84 Slope = 0.52

Relationships between bacterial abundance and Chl a across diverse aquatic ecosystems.

1) At low concentrations of Chl a, bacterial abundance becomes increasingly important.

2) Along gradients in productivity, increases in Chl are proportionality greater than bacterial abundance.

Bird

and

Kal

ff (1

984)

Cole et al. (1988)

Generalized relationships between bacterial population size and chlorophyll

Bacterial and phytoplankton biomass in the upper ocean in various ecosystems

Location Bacterial Biomass(mg C m-2)

Phytoplankton Biomass

(mg C m-2)

Bact:Phyto

Sargasso Sea 659 573 1.2

North Atlantic 500 4500 0.11

Subarctic North Pacific

571 447 1.2

Station ALOHA 750 447 1.7

Arabian Sea 724 1248 0.58

AverageStand dev.

CV (%)

64110516%

14431740120%

0.960.6264%

Bacterial biomass constitutes a large pool of living carbon in marine ecosystems. Note greater variation between ecosystems in phytoplankton biomass relative to bacterial biomass.Bacterial biomass calculated assuming 10 fg C cell-1; phytoplankton biomass calculated assuming C:Chl of 50:1.

• Over wide range of aquatic ecosystems, bacterial biomass appears correlated with phytoplankton biomass.

• Bacterial biomass is generally less variable than phytoplankton biomass across large gradients in productivity.

• With increasing oligotrophy, bacterial biomass becomes a proportionality larger fraction of total plankton biomass.

Plankton biomass relationships

An inverted food web in low productivity ecosystems?

Total carbon consumed by bacteria = (biomass x turnover rate) ⁄ growth efficiency

We need information on turnover and efficiency

Primary producers

Primary consumers (Bacteria)

Secondary consumers (microzooplankton)

Tertiary consumers (mesozooplankton)

To determine the importance of microbes to ocean food webs we need to:

1. Quantify population size and mass• Biomass, abundance, cell sizes

2. Quantify growth rates and production• Biomass production, respiration,

cell division, and turnover

3. Understand factors limiting microbial growth• Metabolic flexibility, physiology

What is it we want to know?

• Biomass (biogenic carbon, trophic linkages)

• Carbon fluxes (production, respiration, DOC utilization rates)

• Rates of growth (cell physiology, nutritional status)

The Microbial Loop : A central theme in marine microplankton ecology

Classic Food web

Phytoplankton

Herbivores

Higher trophic levels (zooplankton, fish, etc.)

A simplified depiction of the microbial loop

Dissolved organic matter

Heterotrophic bacteria

Protozoa

Inorganic Nutrients

Growth in a closed system

Closed system; variable growth rate – cells are inoculated into media and grow until resources are depleted (logistic growth model).

Time (hours or days)0 1 2 3 4 5 6 7 8

Cel

l abu

ndan

ce(c

ells

ml-1

)

050

100150200250300

Calculating growth rates from a population dividing exponentially

• From an exponentially growing population the specific growth rate (µ) can be derived from:dN/dt = µNNt = Noeµt

or alternatively: µ = (ln Nt-ln No) / tµ has units of time-1

µ= 0.693 d-1

d = 1 day

Doubling time (d) is the time required for the population to increase by 100%; it is related to µ by:

Nt = Noeµd

Nt/No = eµd = 2d = ln 2/µd has units of days or hours.

T(hours or

days)

Nt(cells or biomass)

0 1

1 2

2 4

3 8

4 16

5 32

6 64

7 128

8 256

Exponential growth phase

Time (hours or days)0 1 2 3 4 5 6 7 8

ln c

ell a

bund

ance

0123456

Growth in a chemostatOpen system: constant

supply of limiting nutrients;

growth rate is determined by the rate

that a limiting nutrient is added or removed from

the system. Typically use an exponential

model to model growth dynamics.

Is the ocean more like a batch culture

or a chemostat?

SeawaterMedia

Air Inlet

Overflow

Sampling port

Growth chamber

Measuring the rate of growth by natural assemblages of plankton

is complicated…

• Most direct method would be to measure changes in biomass over time.

Why is that difficult for naturally occuring plankton?

In natural seawater sample….

•Mixed assemblage of microbes with variable growth rates.

•Growth is often balanced by loss (predation or disease), this no net change in biomass with time.

Biomass Production• Production is defined as the rate that new biomass is

synthesized.

• Production is mathematically related to biomass and growth as:

P = µBµ = specific growth rate (time-1)B = biomass (mg C L-1)

• **Note that µ = P/B

• Thus, P has units of mg C L-1 d-1

• Two main forms of biomass production:

– Primary production: is the rate of biomass synthesis via reduction of CO2; in the ocean mostly controlled by the growth and biomass of photosynthetic organisms.

– Secondary production: formation of biomass via assimilation of organic matter; controlled by growth and biomass of chemoheterotrophs (heterotrophic bacteria, zooplankton, etc.)

• Absorption of light energy by pigments or photoproteins (light antenna). Energy excites e- in the antenna and this energy is passed to the photosynthetic reaction centers via the flow of electrons. This process creates reducing power (NADPH) and chemical energy (ATP).

• Energy and reducing power gained from light harvesting are used to reduce CO2 to organic matter (dark reactions).

Photosynthesis

Photosynthesis

CO2 + 2H2O CH2O + O2 + H2O + heat

Sunlight

ATP, NADPH

H2O O2

3 mol ATP and 2 mol NADPH are consumed for every 1 mol CO2 fixed.

CO2

Inorganic carbon pools in the sea:

CO2 : carbon dioxideH2CO3 : carbonic acid HCO3

- : bicarbonateCO3

2- : carbonate

CO2 ↔ H2CO3 ↔ HCO3- ↔ CO3

2-

0.5% 89% 10.5%

Proportion of carbon species in seawater

2 Key enzymes for phytoplankton photosynthesis

• RUBISCO (1,5-bisphosphate carboxylase/oxygenase)-key enzyme in the Calvin-Benson cycle, incorporates CO2 into 3-phosphoglycerate. Most abundant protein on Earth.

• Carbonic anhydrase: converts bicarbonate to CO2, and vice versa. Most marine phytoplankton transport bicarbonate and carbonic anhydrase dehydrates to CO2 intracellularly near RUBISCO.

Net and Gross Photosynthesis

• Net photosynthesis (PN): net organic carbon production in the light. Rate of photosynthesis excluding material lost to respiration in the light (RL).PN = PG - RL

• Gross photosynthesis (PG): The total amount of light energy converted into biochemical energy. PG = PN + RL

• Relatively straightforward to measure net or gross photosynthesis in pure cultures, but the oceans contain many organisms that contribute to respiration (e.g., bacteria, zooplankton).

CO2 + 2H2O CH2O + O2 + H2O + heat

Sunlight

Primary Production• Gross Primary Production: The rate of organic

carbon production via the reduction of CO2; in the ocean predominately due to photosynthesis.

• Net Primary Production: Gross primary production, less photoautotrophic respiration (RA), integrated over time and depth.NPP = PN – RA

• Net Community Production: Gross primary production, less all autotrophic and heterotrophic losses due to respiration (RA+H).NCP = PG – RA+H

• Units for all measures of production: mg C m-2 d-1

14C-assimialtion rate (µg C L-1 d-1)

0 1 2 3 4 5 6 7

Dep

th (m

)

0

50

100

150

200

Trapezoidal integrationDepth (m) Production

(µg C L-1 d-1)5 6.5

25 6.4

45 5.0

75 3.0

5-75 m Int. 363 mg C m-2 d-1

Area of trapezoid = Height * avg. base

[(25 m -5 m) * (6.5 mg C m-3 d-1 + 6.4 mg C m-3 d-1)/2] = 129 mg C m-2 d-1

[(45 m -25 m) * (6.4 mg C m-3 d-1 + 5.0 mg C m-3 d-1)/2] = 114 mg C m-2 d-1

[(75 m -45 m) * (5.0 mg C m-3 d-1 + 3.0 mg C m-3 d-1)/2] = 120 mg C m-2 d-1

Sum 5-75 m = 363 mg C m-2 d-1

Primary productivity in the seaD

epth

(m)

50

100

150

200

250

Compensation depth (Pcell = Rcell)

Critical depth (Pwater = Rwater)

∫ 1/p dP/dt >0Z

0

∫ 1/p dP/dt <0Z

0

Net Photosynthesis

Res

pira

tion

Winter mixing introduces nutrients to the upper ocean;

seasonal increases in irradiance results in

deepening of the critical depth and

shoaling of the mixed layer. The result: net

accumulation of biomass.

Critical depth

The Spring Bloom

Mixed layer

Sverdrup (1953)

Phytoplankton biomass can only accumulate (production exceeds respiration) when the depth of the

mixed layer shoals above the critical depth.