Dynamics of Planktonic and Sinking Particle-Associated Microbes in the North Pacific Subtropical Gyre

Abstract

While it has been long appreciated that microbes can influence global processes such as climate, how microbial dynamics and interactions impact specific ecosystem processes still remains to be fully characterized. Since the North Pacific Subtropical Gyre (NPSG) is such a vast ecosystem and is considered to be the world’s largest contiguous biome, it is under sampled and understudied with respect to biological processes. Seasonality is minimal in the NPSG compared to other temperate and polar habitats, yet microbial communities are central to this ecosystem and small temporal shifts may cause large-scale changes. Microbial processes that result in the formation of sinking particles, their sinking, and subsequent deep-sea export are key elements in the global carbon cycle, carbon export, and nutrient delivery to the deep-sea. Additionally, the effects of mesoscale eddies common to the NPSG on microbial community structure and function are also not well constrained. In this dissertation, I examined the temporal and spatial microbial community dynamics over time, space and habitat (free-living or particle attached) at Station ALOHA. With respect to particles reaching the deep-sea at 4000m, I found that particle-attached bacteria sampled during summer elevated carbon flux (ECF) events encoded metabolic pathways reflecting their surface water origins. At other times over the three-year study, mid- and deep-water particle colonization, predation, degradation and re-packaging appeared to shape the biotic composition of particles reaching the abyss. In terms of eddy effects on free-living microbes, I observed the enrichment of a specific high light I (HLI) Prochlorococcus sequence in a cyclonic eddy along with a number of specific Prochlorococcus genes, including some involved in nitrogen acquisition and metabolism, suggesting that nutrient adaptations, in addition to lower temperature growth range, may provide a competitive edge to cyclone-enriched Prochlorococcus variants. Finally, with respect to eddy effects on particle attached microbial communities, I found a cyclone-correlating strain of Pseudoalteromonas, a copiotrophic genus often found associating with key microbial taxa in surface waters. In total, my results reveal key microbial players and biological processes involved in production, particle formation, export, and consumption that may influence the ocean’s biological pump and help sustain ecosystems throughout the water column in the NPSG.