Patterns of depth-dependent variation in microbial abundances, activities and single-cell characteristics across the tropical and subtropical global ocean

Abstract

Global ocean expeditions allow comparing variables that have been collected with standardized methodologies and treated with the same protocols in the same instruments. We looked at the global depth patterns of distribution of microbial abundances and heterotrophic prokaryote cell size and activity (as measured with flow cytometry and leucine incorporation) in the tropical and subtropical Atlantic, Pacific and Indian Oceans by inspecting 126 stations in at least 13 Longhurst provinces from the surface down to 4,000 m during the Malaspina 2010 expedition. We looked at the log-log slope of the relationship between depth and abundance and activity of heterotrophic prokaryotes, abundance of viruses and heterotrophic flagellates, the relative contribution prokaryotes to high (HNA) and low (LNA) nucleic acid content cells and the proportion of viruses with low, medium and high nucleic acid content. In addition, we determined the depth variation of intact-membrane (“live”) and damaged-membrane (“dead”) prokaryotes, and those of actively respiring cells (i.e. reducing CTC, 5-Cyano-2,3-ditolyl tetrazolium chloride). As general patterns, HNA decreased less strongly than LNA bacteria (so the %HNA increased with depth, as also did cell size), and leucine incorporation decreased less than prokaryote abundance (so the specific growth rate also decreased with depth, but less than abundance or production). The abundance of all heterotrophic groups decreased vertically but heterotrophic protists did so at much higher rate, so the ratio prokaryotes to small eukaryotes increased with depth, while viral abundance decreased at about the same rate than prokaryote abundance. Large viruses decreased more than medium-sized viruses and those more than the small viruses. The amount of CTC+ cells decreased less than total bacterial abundances so the %CTC+ increased with depth. Cells with intact membranes varied more with depth than the cells with damaged membranes. We observed significant variation around these average values in different oceans and Longhurst provinces, and significant correlations between the slopes and the surface values of some variables