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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.
HISTORY
1950’s: Relatively large photosynthetic prokaryotes
were recognized as important in Nitrogen cycling (e.g.,
Trichodesmium)
• 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.
HISTORY
Later… Advent of new membrane filtration
products allowed careful size-fractionation.
Pomeroy and Johannes (1968) Size-fractionated
respiration (oxygen demand) was 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.
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?
Constant supply of dissolved organic substrates from
phytoplankton
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
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...
Nitrogen
Phosphorus
Sulfur
In other words, bacteria compete directly with
phytoplankton for nutrients
Special cases...
Chemoautotrophy
Some bacteria derive energy to ‘fix’ CO2 from
reduced compounds such as hydrogen sulfide
(H2S).
Where would we expect to find chemoautotrophic bacteria?
Bacteria compete for substrates with phytoplankton.
Uptake
Rate
Bacterial advantages:
Concentration
• High growth rates -- bacteria respond rapidly, and are
tightly coupled with supply of dissolved nutrients
• Chemotaxis-- can direct their movements toward the
highest concentration of nutrients
• Multiple transport systems for dissolved substrates enhances
uptake over wide range of concentrations
Bacteria will out-compete phytoplankton for N and
P, especially at low concentrations
In oligotrophic environments (mid-ocean gyres)
• Decomposer biomass > Producer biomass
• Protozoans (flagellates and ciliates) graze
heavily on bacterial production
• ‘Locks’ nutrients up in this recycling system
• Prevents losses to deep sea, but not much
available for larger consumers. Why?
Phytoplankton
Fish
Zooplankton
(DOM)
Ciliates
Bacteria
(5-20 um)
(0.2-2 um)
‘Microbial Loop’
Flagellates
(1-5 um)
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
Role of viruses in host metabolism may be more
important than its role as an agent of mortality
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; their biomas is that of ~75,000,000 blue whales
A phage can either kill the cell (lysis), or transfer genetic material from a prior host
(horizontal gene transfer) or from its own genome (accessory gene expression). The
transferred genes can allow a cell to expand into different niches. Small viral-like
particles known as GTAs (Gene transfer agents) can also transfer genes between
organisms. Two scenarios can explain viral effects. In the 'Red Queen' effect, the virus
and cell are in an evolutionary 'arms race', in which they evolve resistance to each other
until the virus ultimately kills the host. In the 'Cheshire Cat' hypothesis, the host moves
from a diploid, non-mobile stage to a motile, haploid stage, thereby evading the virus.
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 revision the ‘microbial loop’: The ‘microbial web’
Class of newly discovered primary producers in open ocean < 5um
‘Small’ production unavailable to larger grazing
metazoans; is consumed by flagellates and ciliates
• 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’
The Microbial Web (Sherr and Sherr 1993)
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 Autotrophy
Net Heterotrophy
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 allocthonous organic materials to fuel
high bacterial production.
This can leads to one of the important symptoms of
an unhealthy ecosystem: anoxia or hypoxia.
Marine virus strategies:
Lytic
Chronic
Lysogenic
Special cases...
Bacteria that reduce Nitrogen and Sulfur compounds
derive oxygen from bound sources (i.e., 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?