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. 1. Introduction 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. In benthic environments, 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]. The activities of heterotrophic microbial communities therefore affect marine environments on spatial scales from local to global and on timescales from minutes to millennia. Despite the importance of microbially driven carbon cycling, the specific factors that determine the extent, rate, and location of organic matter remineralization in the ocean are poorly understood. For example, DOC, one of the largest actively cycling organic carbon reservoirs on earth [3], is operationally defined as being labile, semilabile, semirefractory, or refractory [4], based on timescales of removal in
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