Understanding the Microbial Loop in Food Web Dynamics

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