Microbial Pathways in the Sea

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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?
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New and rapidly expanding field...
History is relevant to understanding how other
marine ecological processes (e.g., fisheries
yield models) are influenced by microbes.
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HISTORY
1950s 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.

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HISTORY
Later Advent of new membrane filtration products
allowed careful size-fractionation.
Pomeroy and Johannes (1968) Size-fractionated
respiration (oxygen demand) was greatest at lt 5 um
Importance over-looked... Compare to earlier
smallest net sizes of 20 um!!
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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.
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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)

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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

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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
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Other nutritional requirements of bacteria...
Nitrogen Phosphorus Sulfur
In other words, bacteria compete directly with
phytoplankton for nutrients
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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?
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Bacteria compete for substrates with
phytoplankton.
Bacterial advantages

• 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
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In oligotrophic environments (mid-ocean gyres)

• Decomposer biomass gt 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?

(DOM)
Microbial Loop
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The Microbial Loop (Azam et al. 1983)
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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

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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.
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In eutrophic, coastal environments

• Producer biomass gt Bacterial biomass
• Metazoan grazers dominate the consumption of
  primary production
• N and P lost from the system through fecal
  pellets (the fecal express!)

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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)
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A revision the microbial loop The microbial
web
Class of newly discovered primary producers in
open ocean lt 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 gt 5um responsible for passing
energy/material along to the exporters
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The Microbial Web (Sherr and Sherr 1993)
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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
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Primary Production vs. Bacterial respiration
Net Autotrophy
Net Heterotrophy
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Expect spatially discontinuous patterns... but
there are also temporally discontinuous regions
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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.
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Marine virus strategies
Lytic
Chronic
Lysogenic
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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?