Download Marine Microbial Processes Outline

Marine Microbial Processes
Outline
• size-structured food webs
• brief history of the development of our
current understanding of microbially
dominated food webs
• carbon cycling in marine food webs
• evolving concepts
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Definitions
Autotroph: carbon and energy for growth comes from
non-organic sources. For example, phytoplankton are
autotrophs because they use CO2 for their carbon source
and use sunlight for their energy source
Heterotroph: carbon and energy for growth comes from
pre-formed organic material. For example, herbivorous
zooplankton are heterotrophs because they consume
phytoplankton for their carbon and energy needs.
Oligotrophic: Refers to low nutrient and low productivity
environments. For example the subtropical gyres are
oligotrophic regions
Eutrophic: Refers to high nutrient and high productivity
environments. For example the coastal upwelling areas are
eutrophic regions
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Optimal Prey Size of Pelagic Animals
Marine Food Webs are Size-Structured
Our conceptualization of marine food webs is built
on the general rule that preferred prey size is
approximately 1/10 consumer size
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Traditional
Food Chain
Concept
(early1970’s)
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Traditional Bacterial Concentrations
Estimated from Transmission Light
Microscopy and Culture-Plate Colony Counts
Use of Epifluorescent Microscopy and Fluorescent DNA
Stains Became Widespread Between 1975 and 1985
•
dramatically increased
estimates of bacterial
concentrations in the
ocean
•
Allowed easy distinction
between autotrophic and
heterotrophic cells (i.e.,
chlorophyll containing
or chlorophyll lacking)
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Bacterial Concentrations Before (Red Fill) and After
(Blue Fill) the Introduction of Epifluorescent Microscopy
New view of marine food webs that recognizes the
importance of high bacterial biomass and a large
fraction of nanoflagellates (2-20-micron diameter cells)
that are heterotrophic
What is the carbon and
energy source for all
this newly discovered
heterotrophic bacteria?
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Heterotrophic Bacteria are growing on
dissolved organic matter released from
phytoplankton by steady leakage,sudden cell
senescence or sloppy feeding by zooplankton
The term Microbial Loop is coined by Azam et
al. (1983) to describe the role that microbes
play in the marine ecosystem carbon cycle
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Most carbon entering the heterotrophic
bacteria is eventually respired back to carbon
dioxide
Summary:
Early 1970’s versus Early 1980’s
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Discovery of an Important New
Bacteria-Sized Autotroph
In 1988 Sally Chisholm and Others Published a
Paper Describing the Presence of a New Type of
Very Small Autotroph that is Present in High
Abundance - Especially in Oligotrophic Regions
The Discovery was Made using a New Technique
called Analytical Flow Cytometry
This Important New Autotroph Came to be
Known as Prochlorococcus
Simple Diagram of Flow
Cytometeric Method
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Prochlorococcus abundance is similar in
magnitude to that of heterotrophic bacteria
New View (1990’s) of Marine Food Webs
that Recognizes the Importance of
Prochlorococcus
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Relative Importance of Prochlorococcus and
Heterotrophic Bacteria in Oligotrophic Systems
The Role of Microbes in Material
Flow Through Marine Ecosystems…
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The Changing Role of Marine Microbes
Along a Nutrient Gradient
Microbes are Recyclers ----------------------------------------> Microbes are Direct Trophic Link
The role of marine microbes as
recyclers in eutrophic waters versus a
direct trophic link in oligotrophic waters
derives solely from the concept that
the dominant cell size in the
phytoplankton community shifts to
smaller forms as nutrient concentration
is reduced.
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Role of Microbes in Carbon Cycling
in the Ocean…
Carbon Cycling
When the dominant phytoplankton cells are large, the dominant grazers are
large and the large fecal material easily sinks to the deep ocean taking organic
carbon with it - this forms an efficient biological carbon pump. The opposite is
true when the dominant phytoplankton is small and the biological pump is more
inefficient.
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Seasonal Variation in the Global Biosphere
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Magnitude of CO2 flux between Land and Ocean
Reservoirs
Conclusions
• Heterotrophic and autotrophic bacteria make up a significant
percentage of the total community biomass in the ocean
• In eutrophic systems the microbial community acts as a sink
for organic carbon - i.e. most microbial carbon is respired back
to CO2
• In oligotrophic systems, Prochlorococcus is an extremely
important component of the phytoplankton
– the microbial community forms a direct trophic link between primary
production and higher trophic levels
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Conclusions
• As nutrient concentration is reduced the competitive growth
advantage shifts to small phytoplankton cells
• Small phytoplankton cells enhance the importance of microbial
grazers and increases the level of nitrogen recycling in the upper
ocean
• Small phytoplankton cells also enhance the percentage of organic
carbon that is respired back to carbon dioxide and consequently
is NOT pumped to the deep ocean
Evolving Concepts of Microbial Food
Webs…
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High Nutrient Low Chlorophyll Regions (HNLC)
Iron Cycling in HNLC Regions
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Station Aloha - Subtropical North Pacific
Station
Aloha
Time Series of N:P Ratio for Total
Dissolved, Suspended Particulates in the
North Pacific Subtropical Gyre (from Karl
1999)
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Conclusion
• Iron limits primary production in high nutrient low
chlorophyll (HNLC) regions of the Subarctic
Pacific, Equatorial Pacific and Southern Ocean.
• North Pacific subtropical gyre seems to be moving
toward phosphorus limitation due to added inputs of
nitrogen to the system via nitrogen fixation. This is
probably a climate change response
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Good Luck to Those Students Who Are
Going out on Seneca Lake Today!
will be like this?…
…or this?
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