Dimensions of microbial biodiversity in the North Pacific subtropical Gyre

Abstract

Microorganisms support all life in the oceans and are fundamental to maintaining a habitable biosphere on Earth. However an understanding of their taxonomic and functional distributions across space and time are just beginning to emerge and numerous niches within the marine environment are still awaiting exploration. The motivation for this thesis is to improve our understanding of distributions of microbes and their metabolic potential at Station ALOHA, a long-term study site representative of the North Pacific Subtropical Gyre (NPSG). We observed changes in diversity and community composition at Station ALOHA across time, ocean depth and on plastic debris, a new anthropogenically derived niche in the NPSG. Despite surface waters only experiencing mild seasonal variation in the abiotic environment, using near monthly picoplankton samples collected across a 2-year period at 25m depth, we observed that microbial community composition correlated with solar irradiance, thereby demonstrating seasonal trends. Ocean surface microbes are known to differ fundamentally from those found in the ocean's interior, yet the nature of the transitions from shallow to deep surface water communities is not well understood. Using a high resolution depth series across twelve time points, we observed that microbial communities partitioned into four groups that consisted of all samples above the deep chlorophyll maximum (DCM), 125m samples below the DCM, all 200 m samples and all 500, 770 and 1000m samples. Our data also revealed a sharp discontinuity in genomic traits including GC%, genome size and proteome elemental composition spanning the DCM, suggesting that nitrogen limitation was key to shaping this sharp genomic transition zone across disparate clades. In contrast, we observed that plastic debris in the NPSG forms a habitat for complex microbial assemblages that have organisms, lifestyles and metabolic pathways that are distinct and potentially less nutrient limited than picoplankton in the surrounding water column. Taken together this work helps expand our understanding of spatial and temporal distributions of microorganisms at Station ALOHA and can help direct future microbial oceanography surveys, highlighting new directions for future research.