Periodic Reporting for period 2 - SUMMIT (Novel roles of dimethylated sulphur in marine microbial interactions) Reporting period: 2021-03-01 to 2022-08-31 Summary of the context and overall objectives of the project Sulphur is an essential element for life that cycles rapidly in the pelagic ocean in the form of biogenic dimethylated compounds that arise from organism adaptation to saline and sunlit waters. There is functional and quantitative evidence that these organosulfur compounds are integral to the food web machinery, but their exact roles in the functioning of marine ecosystems remain largely unexplored. This is particularly important now that a renewed picture of marine food webs emerges, where classical functional roles (producer/consumer/decomposer) blur and concepts like multifunctional organisms and functional interdependence become the rule rather than the exception. Food webs are the core of marine ecosystem services like biodiversity and genetic resources, food provision, and CO2 sequestration. We can hardly understand (hence model and predict) the evolution of marine food webs and the ecosystem services they provide if we do not understand how they work differently from it was thought hitherto.The SUMMIT project aims to bridge microbial physiology, ecology and biogeochemistry investigate the roles of demethylated sulphur compounds in shaping organism-organism interactions that are fundamental for the functioning and evolution of marine ecosystems. More specifically, SUMMIT studies (1) phytoplankton-bacteria interactions through nitrogen fixation and vitamin exchange; (2) phytoplankton-phytoplankton interactions to overcome energy limitation to growth; (3) phytoplankton-herbivore interactions for chemically-oriented selective grazing. Overall, SUMMIT intends to assess if microbial food-web interactions through organosulphur compounds make microbial food-webs more robust and efficient. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far (1) Phytoplankton – bacterial interactions: we have combined measurements of organosulphur concentrations and cycling rates with sequencing of taxonomic and functional genes. The aim is to assess if target microbial activity rates can be predicted by quantifying expression of the relevant genes by metatranscriptomics. This has been conducted with a DNA & RNA sample collection from previous expeditions as well as with samples collected during the SUMMIT Mediterranean oceanographic expedition (June 2021).-In tropical coral reefs of Moorea, we have observed that an Acropora coral is a strong releaser of demethylated sulphur compounds and acrylate in response to sunlight-induced oxidative stress. Gene analysis has revealed that a remarkable proportion of this release occurs via zooxanthella expulsion. -Diel cycle studies in Antarctic, tropical and Mediterranean waters are revealing which are the microbes responsible for the catabolism of the phytoplankton osmolyte DMSP and the production of climate-active DMS. The role of sunlight in regulating these processes is being assessed.-During the SUMMIT-Med cruise in June 2021, we conducted incubation experiments at sea, aimed at investigating the links between organosulphur compound and nitrogen fixation. We are now in the sequencing and bioinformatics work to quantify the expression of the relevant genes.(2) Phytoplankton – phytoplankton interactions: we have identified the membrane transporter for the use of the algal osmolyte DMSP by eukaryotic phytoplankton. We are now investigating if this use allows cosmopolitan picoalgae like Micromonas to overcome their strict dependence on external vitamin B12 for protein synthesis.(3) Predator – phytoplankton interactions: we have conducted chemotaxis experiments to investigate if protist grazing on phytoplankton is modulated by chemical signals, and if this oriented grazing provides ecophysiological advantage to either the predator, the prey or both. These experiments include video cell tracking of predator swimming, cell counting of prey, and measurement of prey physiological stress. So far, we have seen that model herbivore protists are attracted by the algal osmolyte DMSP and its degradation product, DMS, which are released when model algal prey are exposed to sunlight stress. (4) We have developed a numerical 1D model of the organosulphur cycles across the microbial food web, with emphasis on phytotoplankton-grazer, phytoplankton-virus and phytoplankton-bacteria interactions. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) As aimed, the SUMMIT project goes beyond the state of the art in the knowledge of the involvement of organosulphur compounds in marine microbial interactions. Some of the landmarks of the current and expected progress are: (i) For the first time, we are linking experimentally measured rates of the production and consumption of organosulphur compounds to the expression of the genes encoding for these cycling process. Similar approaches are very rare in marine biogeochemistry and will help address the question if gene expression actually indicates the functioning and magnitude of the encoded activity. (ii) For the first time, we look at the diel patterns of organosulphur cycling and gene expression in the ocean. Diel cycles (i.e. through day and night) are extremely interesting as they inform about biological circadian rhythms, short-term coupling or decoupling of ecological interactions, the role of solar radiation (including UV) in triggering or inhibiting processes, the role of photochemistry.(iii) We have discovered the identity of the membrane transporter of the important osmolyte and substrate dimethylsulphoniopropionate (DMSP) in eukaryotic phytoplankton. Before the end of the project, we expect to reveal the main enzymatic reaction that leads to the assimilation of DMSP sulphur into methionine by microalgae.(iv) We expect to demonstrate how do phytoplankton that are strictly dependent on external vitamin B12 for protein synthesis overcome this dependence.(v) We expect to reveal whether or not a phytoplankton osmolyte like DMSP triggers heterotrophic N2 fixation.(vi) In chemotaxis assays with protistan predators and organosulphur compounds, we have gone beyond simple checks of positive (attraction) /negative (repulsion) / neutral response and have added behavioural analysis through cell swimming tracking. This is revealing, e.g. contrasting behaviours like exploratory vs feeding interest in a substrate.(vii) For the first time, we are studying the chemotaxis power of organosulphur compound on marine protists from natural communities, using an in-situ chemotaxis assay developed by colleagues in Sydney.(viii) For the first time, we are studying the potential role of protistan grazing in keeping phytoplankton prey populations healthier.(ix) We have improved our organosulphur cycling model, which will be instrumental to assess if microbial food-web interactions through organosulphur compounds make microbial food-webs more robust and efficient. Related documents simolab_April_2022_low.png
