Abstract
The pool of dissolved organic matter (DOM) in the deep ocean represents one of the largest carbon sinks on the planet. In recent years, studies have shown that most of this pool is recalcitrant, because individual compounds are present at low concentrations and because certain compounds seem resistant to microbial degradation. The formation of the diverse and recalcitrant deep ocean DOM pool has been attributed to repeated and successive processing of DOM by microorganisms over time scales of weeks to years. Little is known however, about the transformation and cycling that labile DOM undergoes in the first hours upon its release from phytoplankton. Here we provide direct experimental evidence showing that within hours of labile DOM release, its breakdown and recombination with ambient DOM leads to the formation of a diverse array of new molecules in oligotrophic North Atlantic surface waters. Furthermore, our results reveal a preferential breakdown of N and P containing molecules versus those containing only carbon. Hence, we show the preferential breakdown and molecular diversification are the crucial first steps in the eventual formation of carbon rich DOM that is resistant to microbial remineralization.
Highlights
The pool of dissolved organic matter (DOM) in the deep ocean represents one of the largest carbon sinks on the planet
Most labile DOM is remineralized by microorganisms to C O2, but a small proportion is transformed into more recalcitrant DOM which is eventually exported to the deep ocean where it is stored for millennia[3,4]
Around 2 μM was added to the water, which is equivalent to the natural concentration of DOM that is expected to be released on a daily basis
Summary
The pool of dissolved organic matter (DOM) in the deep ocean represents one of the largest carbon sinks on the planet. This is largely because the transformation and cycling of labile DOM is difficult to study due to it short residence time, low micro- to nano-molar concentrations, which are dwarfed by the high micromolar concentration of recalcitrant DOM, and the high molecular diversity of the DOM p ool[13,15]. 13C and 15N isotope labelling experiments combined with nanoSIMS revealed that viral infection cause algae to leak organic matter prior to viral lysis[16], while DNA stable isotope probing (SIP) can track the incorporation of 13C-labelled DOC substrates into bacterioplankton c ommunities[17] Such approaches provide little insight into changes in the molecular composition of the labile DOM pool
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