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Microbial Food Webs
Bruce Monger ([email protected])
Department of Earth and Atmospheric Science, 4134 Snee Hall
Outline
• size-structured food webs
• brief history of the development of our
current understanding of microbially
dominated food webs
• nitrogen and carbon cycling in marine food
webs
• evolving concepts
Definitions
Autotroph: Grows on non-organic forms of carbon and
energy. For example, phytoplankton are autotrophs - they
use CO2 for their carbon and use sun light for their energy
Heterotroph: Uses carbon and energy contained in preformed organic carbon for growth. For example,
herbivorous zooplankton 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
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
Traditional
Food Chain
Concept
(early1970’s)
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)
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
um diam.) that are
heterotrophic
Cycling of
Organic
Carbon from
Phytoplankton
via Exudates
& Cell
Senescence to
Heterotrophic
Bacteria
The Term
Microbial Loop
is Coined by
Azam et al.
(1983) to
Describe the
Role Microbes
play in Marine
Ecosystems
Sink Versus Link
Controversy ends
with the
Recognition that
Most Carbon
Entering the
Heterotrophic
Bacteria is
Eventually
Respired Back to
Carbon Dioxide
Summary:
Early 1970’s versus Early 1980’s
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
Relative Abundance of Prochlorococcus
and Heterotrophic Bacteria
New View
(1990’s) of
Marine Food
Webs that
Recognizes the
Importance of
Prochlorococcus
Relative Importance of Prochlorococcus and
Heterotrophic Bacteria in Oligotrophic Systems
The Role of Microbes in Material
Flow Through Marine
Ecosystems…
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
Role of Microbes in Nitrogen and
Carbon Cycling in the Ocean…
Nitrogen Cycling
Primary
Production
fueled mostly
by Nitrate
from the deep
ocean
Primary
Production
fueled mostly
from
Recycled
Ammonia
Upwelled nitrate from the deep ocean is the dominant source of nitrogen for
phytoplankton growth in eutrophic waters. Recycled ammonia is the dominant
source nitrogen in oligotrophic waters.
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.
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
• 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
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…
High Nutrient Low Chlorophyll Regions (HNLC)
Dust (Iron) Induced Phytoplankton Blooms
Downwind of the Galapagos Islands
Iron Cycling in HNLC Regions
Station Aloha - Subtropical North Pacific
Station
Aloha
Time Series of
N:P Ratio for
Total Dissolved,
Suspended
Particulates and
Exported
Particulates in
the North Pacific
Subtropical Gyre
(from Karl 1999)
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