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Abstract. Lipid remodeling, the modification of cell membrane chemistry via structural rearrangements within the lipid pool of an organism, is a common physiological response amongst all domains of life to alleviate environmental stress and maintain cellular homeostasis. Whereas culture experiments and environmental studies of phytoplankton have demonstrated the plasticity of lipids in response to specific abiotic stressors, few analyses have explored the impacts of multi-environmental stressors at the community-level scale. Here, we study changes in the pool of intact polar lipids (IPLs) of a phytoplanktonic community exposed to multi-environmental stressors during a ∼ 2-month-long mesocosm experiment deployed in the eastern tropical South Pacific off the coast of Callao, Peru. We investigate lipid remodeling of IPLs in response to changing nutrient stoichiometries, temperature, pH, and light availability in surface and subsurface water masses with contrasting redox potentials, using multiple linear regressions, classification and regression trees, and random forest analyses. We observe proportional increases in certain glycolipids (namely mono- and diglycosyldiacylglycerol – MGDG and DGDG, respectively) associated with higher temperatures and oxic conditions, consistent with previous observations of their utility to compensate for thermal stress and their degradation under oxygen stress. N-bearing (i.e., betaine lipids and phosphatidylethanolamine – BLs and PE) and non-N-bearing (i.e., MGDG; phosphatidylglycerol, PG; and sulfoquinovosyldiacylglycerol, SQDG) IPLs are anti-correlated and have strong positive correlations with nitrogen-replete and nitrogen-depleted conditions, respectively, which suggests a substitution mechanism for N-bearing IPLs under nitrogen limitation. Reduced CO2(aq) availability and increased pH levels are associated with greater proportions of DGDG and SQDG IPLs, possibly in response to the lower concentration of CO2(aq) and the overall lower availability of inorganic carbon for fixation. A higher production of MGDG in surface waters corresponds well with its established photoprotective and antioxidant mechanisms in thylakoid membranes. The observed statistical relationships between IPL distributions, physicochemical parameters, and the composition of the phytoplankton community suggest evidence of lipid remodeling in response to environmental stressors. These physiological responses may allow phytoplankton to reallocate resources from structural or extrachloroplastic membrane lipids (i.e., phospholipids and betaine lipids) under high-growth conditions to thylakoid and/or plastid membrane lipids (i.e., glycolipids and certain phosphatidylglycerols) under growth-limiting conditions. Further investigation of the exact mechanisms controlling the observed trends in lipid distributions is necessary to better understand how membrane reorganization under multi-environmental stressors can affect the pools of cellular C, N, P, and S, as well as their fluxes to higher trophic levels in marine environments subjected to increasing environmental pressure. Our results suggest that future studies addressing the biogeochemical consequences of climate change in the eastern tropical South Pacific Ocean must take into consideration the impacts of lipid remodeling in phytoplankton. 

Abstract Oceanic trenches are an important sink for organic matter (OM). However, little is known about how much of the OM reaching the hadal region derives from the sunlit surface ocean and other sources. We provide new insight into the OM sources in the Atacama Trench by examining the elemental and stable isotope composition of carbon and nitrogen in bulk OM throughout the entire water column and down to bathyal and hadal sediments. Moreover, we estimated the particulate organic carbon (POC) concentration and downward carbon flux. Our results, based on two‐way variance analysis, showed statistical differences in δ¹⁵N_PONbetween the epipelagic zone and the deep zones. However, no statistical differences in δ¹³C_POCand C:N ratio between hadalpelagic and shallower pelagic zones were found, except for δ¹³C_POCin the oxygen‐deficient zone. On the contrary, whereas the isotopic signatures of hadal sediments were distinct from those over the entire water column, they were similar to the values in bathyal sediments. Thus, our results suggest that bathyal sediments could contribute more OM to hadal sediments than the different zones of the water column. Indeed, whereas POC flux estimates derived from remote sensing data indicate that ∼16%–27% of POC could evade surface remineralization within the top 200 m and potentially be exported to depths beyond the mesopelagic region, model estimates suggest that ∼3.3% of it could reach hadal depths. Our results provide a quantitative baseline of pelagic‐benthic coupling which can aid in assessment of carbon cycling changes in future climate scenarios. 

Phytoplankton, a diverse group of small photosynthetic algae inhabiting the sunlit region near the ocean surface, form the base of marine trophic webs ( 1 ). Whereas phytoplankton have evolved in tandem with the climate system for hundreds of millions of years ( 2 ), cumulative greenhouse gas emissions are causing rising ocean temperature, acidification, and oxygen loss at increasing rates ( 3 ). How phytoplankton will respond and adapt to these multistressors in the future ( 4 ), and how this will in turn threaten marine trophic webs and food supply for humans, remain important questions in oceanography. On page 1487 of this issue, Holm et al. ( 5 ) show that a physiological adaptation of phytoplankton to a warming world will lead to a reduction in their nutritional value, with negative consequences for marine ecosystems. 

Abstract. Elevated organic matter (OM) concentrations are found in hadalsurface sediments relative to the surrounding abyssal seabed. However, theorigin of this biological material remains elusive. Here, we report on thecomposition and distribution of cellular membrane intact polar lipids (IPLs)extracted from surface sediments around the deepest points of the AtacamaTrench and adjacent bathyal margin to assess and constrain the sources oflabile OM in the hadal seabed. Multiscale bootstrap resampling of IPLs'structural diversity and abundance indicates distinct lipid signatures inthe sediments of the Atacama Trench that are more closely related to thosefound in bathyal sediments than to those previously reported for the upperocean water column in the region. Whereas the overall number of unique IPLstructures in hadal sediments contributes a small fraction of the total IPLpool, we also report a high contribution of phospholipids with mono- anddi-unsaturated fatty acids that are not associated with photoautotrophicsources and that resemble traits of physiological adaptation to highpressure and low temperature. Our results indicate that IPLs in hadalsediments of the Atacama Trench predominantly derive from in situ microbialproduction and biomass, whereas the export of the most labile lipidcomponent of the OM pool from the euphotic zone and the overlying oxygenminimum zone is neglectable. While other OM sources such as the downslopeand/or lateral transport of labile OM cannot be ruled out and remain to bestudied, they are likely less important in view of the lability ofester-bond IPLs. Our results contribute to the understanding of themechanisms that control the delivery of labile OM to this extreme deep-seaecosystem. Furthermore, they provide insights into some potentialphysiological adaptation of the in situ microbial community to high pressure andlow temperature through lipid remodeling.