Abstract
Heterotrophic microbial communities play a central role in the marine carbon cycle. They are active in nearly all known environments, from the surface to the deep ocean, in the sediments, and from the equator to the Poles. In order to process complex organic matter, these communities produce extracellular enzymes of the correct structural specificity to hydrolyze substrates to sizes sufficiently small for uptake. Extracellular enzymatic hydrolysis thus initiates heterotrophic carbon cycling. Our knowledge of the enzymatic capabilities of microbial communities in the ocean is still underdeveloped. Recent studies, however, suggest that there may be large-scale patterns of enzymatic function in the ocean, patterns of community function that may be connected to emerging patterns of microbial community composition. Here I review some of these large-scale contrasts in microbial enzyme activities, between high-latitude and temperate surface ocean waters, contrasts between inshore and offshore waters, changes with depth gradients in the ocean, and contrasts between the water column and underlying sediments. These contrasting patterns are set in the context of recent studies of microbial communities and patterns of microbial biogeography. Focusing on microbial community function as well as composition and potential should yield clearer understanding of the factors driving carbon cycling in the ocean.
Highlights
Organic matter remineralization by heterotrophic microbial communities is a central component of the marine carbon cycle
These communities process approximately half of all CO2 initially fixed into organic carbon by phytoplankton [1], transforming, repackaging, and respiring dissolved and particulate organic carbon (DOC and POC) and simultaneously regenerating nutrients
Heterotrophic microbes act as the final filter through which organic matter passes before burial, a process that removes CO2 from the atmosphere on geologic timescales [2]
Summary
Organic matter remineralization by heterotrophic microbial communities is a central component of the marine carbon cycle. Most have used oligo- and polymers (peptides as well as polysaccharides) of varying structure in order to investigate enzyme structural specificity and activity in the field [38, 39] (see [40] for a recent review) These investigations have demonstrated that size alone does not necessarily limit the rate of organic matter turnover in marine systems; larger substrates can be metabolized as rapidly as their smaller components [41, 42]. Recent investigations of microbial enzyme activities in marine waters and sediments suggest that there are broadscale patterns to the microbial processing of high molecular weight organic matter Finding these patterns and pinpointing their underlying causes—opening the microbial “black box”—will require deeper understanding of the complex interactions between microbial communities and their substrates. Some of the recent work suggesting the existence of these broad patterns is reviewed, followed by a discussion of factors and actors that may be driving these patterns; an outlook for the future concludes this review
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