1. Microbial Loop
Dr. Jason Turner
MARE 444

2. 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

3. DOM and POM Dissolved or particulate organic matter
Sloppy feeding, molting, waste generation
Detritus (detritivores)
Microbial loop
Nutrient regeneration

4. 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

5. DOM DOC – carbon DON – nitrogen
Resource for heterotrophic pelagic bacteria

7. The Microbial Loop Sequence from DOM→bacteria→protistans→
mesozooplankton
Web parallel to direct consumption of phytoplankton by mesozooplankton
“classic food web”

8. The Microbial Loop Microbial Loop – Azam et al. 1983
Being replaced by “Microbial Food Web”
Very recent addition to food web ecology theory

9. New Sensation
Realization that heterotrophic bacteria are numerous – recent (1974)
Pre-1974 – estimated abundance using plate cultures
Low levels cultured compared to marshes

10. 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

11. Phytoplankton & Bacteria

13. 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

14. Bacteriovores Protozoan consumers of bacteria
Up to 3 trophic levels among organisms
< 8-20μm

15. Bacterial Match-Mismatch

16. Bacteria and Phytoplankton
Exudates produced by phytoplankton
Phytodetritus produced at the end of a bloom
Release of DOM (phyto death, sloppy feeding)
Pathogens (later)

17. 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?

18. Bacteria and Phytoplankton
In oligotrophic waters, bacterial numbers are independent of low phyto numbers
competition for nutrients, organic compounds (FH)
protozoan grazing

19. Bacteria and Zooplankton
Nanoplanktonic grazers (mainly zooflagellates)
Other zooplankton can be important (larvaceans, ciliates, foraminiferans, radiolarians)
Copepods cannot feed on bacteria directly (too small)

20. 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)

23. Marine Fungi Some are infectious agents
Infect and lyse phytoplankton cells  DOM

25. Chytrids infecting a diatom

26. Chytrids infecting a diatom

27. Procaryotes Dominant consumers of DOM
no other organisms compete effectively
bacterial respiration major loss of DOM

28. Bacteria Most bacteria are free-living – subsist on DOM
Usually small fraction of bacteria found on particles

29. Microbial Communities
Include a suite of ecological interactions
organisms < 5μm
Primary producers, herbivores, carnivores, mixotrophs, scavenging remineralizers, parasites
Present and active at all scales

30. Microbial Food Webs Principal affect mineral regeneration
Bacteria - limited reminerization potential
Incorporate organic matter then serve as food for bacteriovores – regen. nutrients

31. Bacteriovores Mixotrophs & Protozoans
important food sources for metazooplankton – few strict herbivores