Transcript Trophodynamics

Microbial Loop
Dr. Jason Turner
MARE 444
Food Web Dynamics
Trophic transfer is not a linear process,
but web-like
There are other organisms in addition to
traditionally-defined producers and
consumers
parasites
pathogens
cannibals
DOM and POM
Dissolved or particulate organic matter
Sloppy feeding, molting, waste generation
Detritus (detritivores)
Microbial loop
Nutrient regeneration
DOM
Transparent exo polymer (TEP) – secreted
from diatoms
Small organic molecules
Protists & Merozooplankton – release OM
ingested remains from food vacuoles
food punctured during capture – spines
soluble matter leached from fecal pellets
DOM
DOC – carbon
DON – nitrogen
Resource for heterotrophic pelagic
bacteria
The Microbial Loop
Sequence from
DOM→bacteria→protistans→
mesozooplankton
Web parallel to direct consumption of
phytoplankton by mesozooplankton
“classic food web”
The Microbial Loop
Microbial Loop – Azam et al. 1983
Being replaced by “Microbial Food Web”
Very recent addition to food web ecology
theory
New Sensation
Realization that heterotrophic bacteria are
numerous – recent (1974)
Pre-1974 – estimated abundance using
plate cultures
Low levels cultured compared to marshes
New Techniques
Counted with epifluorescent scope
acidine orange direct count (AO DC)
Average density in seawater (106)
Metabolism
uptake of labeled isotope
O2 consumption
Bacterial production – formation of RNA & DNA
Phytoplankton & Bacteria
Food Chain Transfer via DOM
Bacteria – 15-25% of primary production
Assimilate up to 80% of DOM
Most DOM & DOC sampled with 0.7μm
filters
Bacteriovores
Protozoan consumers of bacteria
Up to 3 trophic levels among organisms
< 8-20μm
Bacterial Match-Mismatch
Bacteria and Phytoplankton
Exudates produced by phytoplankton
Phytodetritus produced at the end of a
bloom
Release of DOM (phyto death, sloppy
feeding)
Pathogens (later)
Bacteria and Phytoplankton
A bacterial bloom often succeeds a
phytoplankton bloom
consequently, a food web may shift from one
that is based on high nutrients, diatoms,
and filter-feeding copepods to one that is
dominated by the microbial loop and
bactivorous zooplankton
what about dinos?
Bacteria and Phytoplankton
In oligotrophic waters, bacterial numbers
are independent of low phyto numbers
competition for nutrients, organic compounds
(FH)
protozoan grazing
Bacteria and Zooplankton
Nanoplanktonic grazers (mainly
zooflagellates)
Other zooplankton can be important
(larvaceans, ciliates, foraminiferans,
radiolarians)
Copepods cannot feed on bacteria
directly (too small)
Marine Viruses
1000 to one billion per milliliter
More viruses in surface waters versus
deeper - related to microbial loop?
Lyse bacteria and phyto cells  DOM
Can impact other organisms as well (e.g.,
sea urchins, catfish)
Marine Fungi
Some are infectious agents
Infect and lyse phytoplankton cells 
DOM
Chytrids infecting a diatom
Chytrids infecting a diatom
Prokaryotes
Dominant consumers of DOM
no other organisms compete effectively
bacterial respiration major loss of DOM
Bacteria
Most bacteria are free-living – subsist on
DOM
Usually small fraction of bacteria found on
particles
Microbial Communities
Include a suite of ecological interactions
organisms < 5μm
Primary producers, herbivores,
carnivores, mixotrophs, scavenging
remineralizers, parasites
Present and active at all scales
Microbial Food Webs
Principal affect mineral regeneration
Bacteria - limited reminerization potential
Incorporate organic matter then serve as
food for bacteriovores – regen. nutrients
Bacteriovores
Mixotrophs & Protozoans
important food sources for
metazooplankton – few strict herbivores