Date of Award

Spring 4-27-2023

Document Type


First Advisor

Caroline Solomon

Second Advisor

Jacob Cram


The ocean plays a critical role in the carbon cycle, with phytoplankton serving as a source of energy and nutrients (such as nitrogen and phosphorus) to larger organisms in upper trophic levels. Additionally, marine snow, a cluster of components including dead zooplankton, organic carbon debris, and polymers as carbohydrates, proteins, and DNA, transport these nutrients and associated microorganisms to the deeper parts of the ocean. However, the relationship between microbial abundance and particle sizes of marine snow across the water column is not well understood. Previous studies suggest that microbial abundance is lower in the bathypelagic zone (1,000m - 4,000m) compared to the epipelagic zone (ocean surface - 200m). This study aimed to investigate and quantify the relationship between microbial abundance and marine snow at different depths in the open ocean. Samples were collected above the East Pacific Rise, a hydrothermal feature in the Eastern Tropical North Pacific Ocean, during the AT42-09 cruise in the Spring of 2019 and analyzed through epifluorescence microscopy. We showed that microbial abundances varied with depth and that free-living abundances were higher than particle-attached abundances. We found similar microbial abundance on all particle size classes, which, when considering the logarithmic spacing of our particle size classes, suggest a higher abundance of microorganisms on small than on larger particles. Our approach faced some problems with reagent contamination, evidenced by microbial abundances in our negative controls that were about one-fourth the abundances of microorganisms in our particle samples, and which were higher than the abundances in a few samples in the deep water. This contamination shows that for microscopy studies of particles, which harbor presumably few microbes, additional attention to reagent contamination, above and beyond that needed for free-living microbe studies, is required. Furthermore, the challenges in quantifying microbial abundance in oligotrophic ocean environments and the lack of research on particle-associated microbial communities in the open ocean highlight the need for further investigation with future studies. This study provides a valuable first methodological step towards quantifying organisms on marine snow, and by extension understanding the interactions between microbial communities and marine snow and their role in the carbon cycle of deep ocean ecosystems.



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