A mechanistic microbial underpinning for the size-reactivity continuum of dissolved organic carbon degradation

Abstract

The reservoir of dissolved organic carbon (DOC) in the ocean is modified by multiple input and removal processes. Incubation experiments as well as measurements of oceanic DOC have demonstrated that the high molecular weight (HMW) fraction of DOC typically has a younger radiocarbon age and is more reactive biologically than the low molecular weight (LMW) fraction of DOC. These observations have been summarized as a ‘size-reactivity continuum’ of DOC reactivity, but mechanistic explanations for these observations have been lacking. Here we describe how our recent discovery of ‘selfish’ HMW organic matter uptake among bacteria in surface ocean waters may help explain the rapid removal of HMW DOC. ‘Selfish’ substrate uptake by bacteria encompasses rapid binding and partial hydrolysis of intact polysaccharides on the outer membrane of bacteria, seamlessly followed by the transport of large oligosaccharide fragments into the periplasm with little to no loss of LMW hydrolysis products. ‘Selfish’ bacteria therefore process HMW substrates in a manner distinct from bacteria that carry out extracellular hydrolysis that yields LMW hydrolysis products in the environment. Recognition of the presence and prevalence of selfish bacteria in the ocean has profound implications for carbon flow – the source and quantity of LMW substrates made available to non-extracellular-enzyme producing bacteria – as well as for efforts to model and measure bacterial interactions during organic matter degradation. This discovery also highlights the importance of targeted substrate binding and uptake as key (often understudied) factors in geochemical investigations of microbially driven carbon cycling in the ocean. We conclude with some speculative thoughts about the factors that may determine the prevalence of selfish substrate uptake in the environment.