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Microbial Processes and Material
Flow in Ocean Ecosystems
Outline
• Ecological terms to be used in lecture
• Concept of size-structured food webs
• Brief history of the development of our
current understanding of microbial processes
in marine systems
• Role of microbes in nitrogen and carbon
cycling
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
Epifluorescent
Microscopy, in
combination with
fluorescent DNA
stains, dramatically
increased our estimates
of bacterial
concentrations in the
ocean and also enabled
us to distinguish large
autotrophic cells
(chlorophyll
containing) from
heterotrophic cells
(lacking chlorophyll)
(circa 1975-1985)
Comparison of
Bacterial
Concentrations
Before and
After 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
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 communiy 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 bacteria make up a large 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 carbon is respired
• In oligotrophic systems, Prochlorococcus is an
extremely important component of the phytoplankton
• In oligotrophic systems the microbial community forms
a direct trophic link between primary production and
the highest 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