Exploring Microbial Food Web Dynamics Across Transition Zones in Aquatic Systems

Microbial communities play an essential role in nutrient cycling in aquatic systems. Transition zones within these systems, which are characterized by a steep chemical gradient, are highly dynamic areas of activity with distinct microbial populations residing on either side. Mechanisms by which energy transfer occurs in proximity to these transition zones from chemoautotrophic prokaryotes up to higher trophic level organisms are not well characterized. To better understand food web dynamics occurring within these unique environments, grazing experiments were conducted using Fluorescently-Labeled Prey (FLP) analogs to track and quantify the disappearance of prey over time. The sites sampled for this study were the Gorda Ridge hydrothermal vent system in the Pacific Ocean and Siders Pond, a meromictic lake in Falmouth, MA. By tracking FLP disappearance in samples collected across the transition zones of these sites, we were able to quantify grazing performed by heterotrophic eukaryotes. At Gorda Ridge, grazing rates were compared between the vent, plume, and background seawater of the vent system; the highest grazing rates among the sites sampled were those of the background seawater and at the vent of Sir Ventsalot. At Siders Pond, grazing rates were compared between the oxygenated surface water, the mixolimnion, and the anoxic bottom layer of the pond, the monimolimnion. Experiments showed that grazing occurred at a higher rate in the monimolimnion than in the mixolimnion, consistent with preliminary prokaryote counts that showed more overall abundance of biomass in the monimolimnion. Samples were also collected for metagenomic analysis from both sites in order to catalog phylogenetic diversity as well as determine gene expression patterns for these unique microbial communities.