Microbial Pathways in the Sea

What is the relative importance of bacteria and viruses in regulating the flow of energy and the cycling of nutrients in marine ecosystems?

New and rapidly expanding field...

History is relevant to understanding how other marine ecological processes (e.g., fisheries yield models) are influenced by microbes.

• Traditional plate assays for counting bacteria indicated about 103 bacteria ml-1.

• Approximately equal (numerically) to phytoplankton

• Small size (100x smaller than phyto) suggested they were of minimal importance ecologically.

1950’s: Relatively large photosynthetic prokaryotes were recognized as important in Nitrogen cycling (e.g., Trichodesmium)

HISTORY

Late 1960s: Advent of new membrane filtration products allowed careful size-fractionation.

Pomeroy and Johannes (1968) Size-fractionated respiration (oxygen demand) greatest at < 5 um

Importance over-looked...Compare to earlier smallest ‘net’ sizes of 20 um!!

HISTORY

Subsequent fluorometry of bacterial cells in 1980s showed an incredible under-estimation of bacterial concentrations in the sea...

Not 103 ml-1, but 106-108 ml-1 !!

~5 orders of magnitude more abundant than phytoplankton!

Bacteria concentrations are relatively constant world-wide.

HISTORY

Why are bacteria so successful in the sea?

• High Carbon conversion (growth) efficiencies (around 80%)

• High production rates -- doubling times usually less than phytoplankton (up to several doublings per day)

Where does the Dissolved Organic Carbon (DOC) required by bacteria come from?

Estimated ~50% of phytoplankton production is required to fuel bacterial requirements

In the surface layer, phytoplankton DOC comes from:

• Exudation of organic material from cell during rapid growth

• ‘Autolysis’ -- self-rupturing of cell contents• ‘Sloppy feeding’ by metazoans

Constant supply of dissolved organic substrates from phytoplankton

At depth (below photic zone), DOC derived mostly from sinking detrital material.

Up to 80% of sinking organic materials can be solubilized and consumed by bacteria associated with ‘marine snow’

Highest concentration of bacteria in the sea is on ‘marine snow’

Other nutritional requirements of bacteria...

NitrogenPhosphorusSulfur

In other words, bacteria compete directly with phytoplankton for nutrients

Bacteria that reduce Nitrogen and Sulfur compounds derive oxygen from bound sources (ie, oxidized compounds like nitrate and sulfate).

These bacteria are obligate anaerobes since presence of oxygen will cause the spontaneous oxidation of reduced compounds.

Where would you expect to find ‘denitrifying’ bacteria in the sea?

Special cases...

Some bacteria derive energy to ‘fix’ CO2 from reduced compounds such as hydrogen sulfide (H2S).

Chemoautotrophy

Where would you expect to find chemoautotrophic bacteria?

Special cases...

Bacteria are competitive for substrates with phytoplankton.

• High growth rates -- bacteria respond rapidly, and are tightly coupled with supply of Dissolved nutrients

• Chemotaxis

Bacteria will out-compete phytoplankton for N and P, especially at low concentrations

UptakeRate

Concentration

• Multiple transport systems for dissolved substrates enhances uptake over wide range of concentrations

Bacterial advantages:

• Decomposer biomass > Producer biomass• Protozoans (flagellates and ciliates) graze

heavily on bacterial production

‘Microbial Loop’

Phytoplankton Zooplankton Fish

Bacteria(0.2-2 um) Flagellates

(1-5 um)

Ciliates(5-20 um)

• ‘Locks’ nutrients up in this recycling system• Prevents losses to deep sea (low f-ratio system)

In oligotrophic environments (mid-ocean gyres)

The Microbial Loop (Azam et al. 1983)

Infection of bacterial and phytoplankton by VIRUSES

Important source of cell lysing is by viral infection

50% (perhaps more?) of bacterial mortality due to viruses

Marine viruses (discovered in late 1980s):

• Non-living, non-cellular particles• Femtoplankton (0.2 um)• Require host for replication (infection)• About 1 order of magnitude more abundant than bacteria

Marine virus strategies:

Lytic

Lysogenic

Chronic

In eutrophic, coastal environments

• Producer biomass > Bacterial biomass• Metazoan grazers dominate the consumption of

primary production• N and P lost from the system through fecal pellets

(the fecal express!)

To summarize the relative importance of microbes in eutrophic and oligotrophic systems...

Nutrients are locked up in the microbial loop in oligotrophic systems (where they play a greater role)

Nutrients are exported by grazers in eutrophic systems (where they play a lesser role)

A revised view of the ‘microbial loop’: The ‘microbial web’

Class of newly discovered primary producers in open ocean < 5um

‘Small’ production unavailable to larger grazing metazoans

• Consumed by flagellate and ciliate grazers• Energy and material either recycled into

microbial loop or passed to larger ‘exporters’

Large phytoplankton > 5um responsible for passing energy/material along to the ‘exporters’

When and where do microbial processes dominate the flux of carbon?

Bacterial consumption of organic carbon exceeds carbon fixation

NET HETEROTRPHY

Primary production exceeds bacterial consumption

NET AUTOTROPHY

Primary Production vs. Bacterial respiration

Net Heterotrophy

Net Autotrophy

Expect spatially discontinuous patterns... but there are also temporally discontinuous regions

Expect spatially discontinuous patterns... but there are also temporally discontinuous regions

Estuaries and large river-dominated ecosystems have high fluxes of organic materials to fuel high bacterial production.

This can leads to one of the important symptoms of an unhealthy ecosystem: anoxia or hypoxia.

Archaea…the other domain

HalophilesThermophiles

‘Extremophiles’

The Microbial Web (Sherr and Sherr 1993?)