Microbial Community Structure and Activities during EXPORTS as Revealed by Quantitative ‘Omics

Bacteria and archaea have substantial roles in shaping phytoplankton activities and the fate of primary production and dissolved organic carbon in low nutrient environments. Up to half of primary production is routed through the microbial loop, substantially reducing the carbon available for export, and there is increasing evidence that the types of microbes in a community and the regulation of their functional activities can substantially alter primary producer physiology and DOC cycling. Here, we examined shifts in bacterial community composition and functional activities using quantitative metagenomics and metatranscriptomics during the 2018 EXPORTS cruise at Ocean Station Papa in a HNLC region of the North Pacific to understand the roles bacteria have in determining carbon fate. Samples were collected throughout the water column over the 24 day cruise and extracted with internal standard spike-ins to calculate per liter genome equivalents and transcript abundances. During the cruise, there was a substantial shift in primary production followed by increased bacterial production and abundance. The metagenomes revealed that the mixed layer microbial communities experienced three substantial transitional periods in coordination with these shifts, in which cyanobacteria increased and decrease together with several major heterotrophic taxa. The communities underneath the mixed layer, while relatively more stable (consistently enriched in Thaumarchaeota, Actinobacteria, Thioglobus, etc) also showed shifts, suggesting dynamic interactions with carbon transport from above. An analysis of Metagenome Assembled Genomes (MAGs) revealed the fine scale changes in population structure during these transitions. Lastly, mapping of the transcriptomes onto MAGS revealed organism-specific activities suggestive of different sources and types of sustaining DOC. These quantitative measurements of gene and transcriptional shifts during EXPORTS are providing critical data for incorporating bacterial metabolism into models of carbon export.