attenuation of remineralization. This results in the deposition of organic carbon-rich sediments. Carbon drawdown is estimated by biogeochemical models; however, a major process is ignored: carbon fixation in the mid- and lower water column. Here, we show that chemoautotrophic carbon fixation is important in the Arabian 13 13 Sea OMZ; and manifests in a C-depleted signature of sedimentary organic carbon. We determined the δ C 13 values of C deposited in close spatial proximity but over a steep bottom-water oxygen gradient, and the δ C org composition of biomarkers of chemoautotrophic bacteria capable of anaerobic ammonia oxidation (anammox). Isotope mixing models show that detritus from anammox bacteria or other chemoautotrophs likely forms a substantial part of the organic matter deposited within the Arabian Sea OMZ(~17%), implying that the contribution of chemoautotrophs to settling organic matter is exported to the sediment. This has implications for the evaluation of past, and future, OMZs: biogeochemical models that operate on the assumption that all sinking organic matter is photosynthetically derived, without new addition of carbon, could significantly underestimate the extent of remineralization. Oxygen demand in oxygen minimum zones could thus be higher than projections suggest, leading to a more intense expansion of OMZs than expected. Plain Language Summary: Oxygen minimum zones are areas in the ocean in which algae produce large amounts of organic material. When this sinks towards the seafloor, all oxygen at depth is used up. This results in vast 'dead zones' where almost no oxygen is available to sustain life. With global warming, and increased nutrients from rivers, dead zones are forecast to expand. Computer models can calculate this, by considering algal production, and the amount of material delivered to the seafloor. However, these models often ignore a major process: anaerobic bacteria in the deeper water column, that can live at the edge or in the middle of these dead zones, which can also produce organic material from the dissolved CO . In this study, we used the fact that 2 these bacteria add a distinct signature to the organic material, to show that one fifth of the organic matter on the seafloor could stem from bacteria living in these dead zones. Thus, models that have missed out on considering this contribution could have underestimated the extent of oxygen depletion we are to expect in a future, warming world. A more intense expansion of dead zones than expected could have severe ecological, economical (fisheries), and climatic consequences.

Levin, S.R., Gandon, S., West, S.A., 2020. The social coevolution hypothesis for the origin of enzymatic cooperation. Nature Ecology &Evolution 4, 132-137.

https://doi.org/10.1038/s41559-019-1039-3 At the start of life, the origin of a primitive genome required individual replicators, or genes, to act like enzymes and cooperatively copy each other. The evolutionary stability of such enzymatic cooperation poses a problem, because it would have been susceptible to parasitic replicators that did not act like enzymes but could still benefit from the enzymatic behaviour of other replicators. Existing hypotheses to solve this problem require restrictive assumptions that may not be justified, such as the evolution of a cell membrane before the evolution of enzymatic cooperation. We show theoretically that, instead, selection itself can lead to replicators grouping themselves together in a way that favours cooperation. We show that the tendency to physically associate with others and cooperative enzymatic activity can coevolve, leading to the evolution of physically linked cooperative replicators. Our results shift the empirical problem from a search for special environmental conditions to questions about what types of phenotypes can be produced by simple replicators.

Lewis, C.B., Walker, B.D., Druffel, E.R.M., 2020. Isotopic and optical heterogeneity of solid phase extracted marine dissolved organic carbon. Marine Chemistry 219, 103752.

http://www.sciencedirect.com/science/article/pii/S0304420320300049 Marine dissolved organic carbon (DOC) is the ocean's largest exchangeable reservoir of organic carbon. The biogeochemical cycling of DOC plays an important role in ocean carbon storage on various timescales. Solid- phase extraction (SPE) is a process used to isolate DOC from seawater for biogeochemical analysis. This study 14 13 examines how DOC isotopic (Δ C, δ C) and optical (absorbance) properties of SPE-DOM change as a function of eluent volume (and hydrophobicity). These properties were measured in 28 SPE-DOC fractions incrementally eluted from Bond Elut PPL (styrene-divinylbenzene polymer) cartridges, totaling 32 mL of 14 13 methanol. We show that the early eluted SPE- DOC has distinctly different ∆ C and δ C values than those