Algal blooms produce large amounts of dimethyl sulfide (DMS), a volatile with a diverse signaling role in marine food webs that is emitted to the atmosphere, where it can affect cloud formation. The algal enzymes responsible for forming DMS from dimethylsulfoniopropionate (DMSP) remain unidentified despite their critical role in the global sulfur cycle. We identified and characterized Alma1, a DMSP lyase from the bloom-forming algae Emiliania huxleyi. Alma1 is a tetrameric, redox-sensitive enzyme of the aspartate racemase superfamily. Recombinant Alma1 exhibits biochemical features identical to the DMSP lyase in E. huxleyi, and DMS released by various E. huxleyi isolates correlates with their Alma1 levels. Sequence homology searches suggest that Alma1 represents a gene family present in major, globally distributed phytoplankton taxa and in other marine organisms.

Dimethyl sulfide (DMS) is produced in oceans in vast amounts (>10(7) tons/year) and mediates a wide range of processes from regulating marine life forms to cloud formation. Nonetheless, none of the enzymes that produce DMS from dimethylsulfoniopropionate (DMSP) has been adequately characterized. We describe the expression and purification of DddD from the marine bacterium Marinomonas sp. MWYL1 and its biochemical characterization. We identified DMSP and acetyl-coenzyme A to be DddD's native substrates and Asp602 as the active site residue mediating the CoA-transferase prior to lyase activity. These findings shed light on the biochemical utilization of DMSP in the marine environment.

The sinking of organic particles in the ocean and their degradation by marine microorganisms is one of the main drivers of one of the most conspicuous carbon fluxes on Earth, the biological pump [1][2][3][4][5][6][7] . Yet, the mechanisms determining the magnitude of the pump remain poorly understood, limiting our ability to predict this carbon flux in future ocean scenarios. Current ocean models assume that the biological pump is governed by the competition between sinking speed and degradation rate, with the two processes independent from one another [8][9][10][11] . Contrary to this paradigm, we show that sinking itself is a primary determinant of the rate at which bacteria degrade particles. Heterotrophic bacterial degradation rates were obtained from a laboratory study on model surfacecolonized particles at atmospheric pressure under a range of flow speeds to mimic different sinking velocities. We find that even modest sinking speeds of 8 m/day enhance degradation rates more than 10-fold compared to degradation rates of non-sinking particles. We discovered that the molecular mechanism underlying this sinking-enhanced degradation is the flow-induced removal of the oligomeric breakdown products from the particles, which otherwise compete for enzymatic activity. This mechanism applies across several substrates and bacterial strains, suggesting it could potentially occur more broadly under natural marine conditions. Integrating our findings into a mathematical model of vertical particulate carbon flux, we show that the coupling of sinking and degradation may contribute, in conjunction with other processes, to determine the magnitude of the vertical carbon flux in the ocean.The biological pump is the process by which CO2 from the atmosphere is converted by marine photosynthetic organisms into biomass and inorganic carbonate shells, which undergo aggregation when those cells die to form 'marine snow' particles that sink to the ocean depth [3][4][5] .Several processes that vary in magnitude with site, depth and season concurrently affect the sinking of particles and the vertical export of the carbon present in marine snow. These processes

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