Date of Award

Fall 2014

Document Type


Degree Name

Master of Science (MS)


Ocean & Earth Sciences


Ocean and Earth Sciences

Committee Director

Alexander Bochdansky

Committee Member

Fred C. Dobbs

Committee Member

David Gauthier

Call Number for Print

Special Collections LD4331.O35 B345 2014


Prokaryotes (Bacteria and Archaea) comprise the largest component of biomass in the world's oceans. Their abundances are controlled by resource availability, viral infections and protist grazing. Many pico- and nano-eukaryotic predators grow almost as quickly as their prey, and greatly increase in numbers as soon as their prey do, leading in tum to depletion in prokaryotes. It is still unclear however, as to what extent microbial predators are able to feed in low prey environments, most prominently in the largest biome on Earth, the deep sea (below l 000 m depth). It has been hypothesized that in low prey environments, a lower threshold of prokaryote concentrations exists, at which it is no longer possible to obtain sufficient nutrition from predation. While lower feeding thresholds have been incorporated into ecosystem models for larger plankton, reports on the existence of a lower feeding threshold in microbial predator-prey systems have been conflicting. In functional feeding response experiments and in numerical (population) response experiments, I was able to demonstrate that lower thresholds indeed exist. The lower threshold below which feeding ceases (~ 121,000 prokaryotes mL-1), was lower than the threshold for the numerical response (~ 212,000 prokaryotes mL-1). By applying the lower feeding threshold and clearance rates to prokaryote and eukaryote abundances obtained from three different deep-sea expeditions throughout the Atlantic, I was able to predict potential environmental grazing coefficients. These data suggest that average prokaryote concentrations within the deep ocean are too low to accommodate the energy requirements needed for the survival of bacterivores. This result, however, contradicts findings that bacterivores exist in the deep sea, albeit at low numbers. These conflicting observations can be reconciled if the prey distribution in the deep sea is considered to be highly heterogeneous, and microbial predators are able to find and exploit highly concentrated prey patches on marine aggregates.


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