A new perspective on ocean photosynthesis (N-POP)

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Sunlight penetrating into the ocean is the ultimate source of energy in oceanic ecosystems. This energy sustains fisheries and catalyses biogeochemical cycles that influence the carbon cycle and ultimately climate. Single-celled photosynthetic organisms termed phytoplankton are the gatekeepers of this energy flux, achieved via the absorption of light by photosynthetic pigments such as chlorophyll. However, the range of processes in the cell that determine how this light energy is subsequently used to support growth and survival of these crucial organisms are not well characterised. This 'knowledge gap' is a result of the current paradigm that mainly considers photosynthesis in terms of the amount of 'carbon fixed' rather than 'energy captured'. Here we propose that natural phytoplankton taxa are more adaptable in their energetic metabolism than currently assumed, using different 'molecular strategies' to specifically power a range of critical cellular processes beyond carbon fixation. These different strategies potentially represent more than half of photosynthetic energy flux in many circumstances. The corresponding diversity of metabolic strategies is thus likely to represent a fundamental process through which these crucial organisms adapt to thrive across different ocean ecosystems.We will obtain an improved understanding of how phytoplankton use light energy in ocean systems, which is critical to our ability to understand how oceanic ecosystems operate and thus predict how the ecosystem services they provide (including sustaining fisheries, sequestering atmospheric CO2 and producing other climate reactive gases) may change as the global system evolves. For example, we currently use satellites to measure the colour of the ocean and hence estimate phytoplankton abundance and infer rates of primary production without a full mechanistic understanding of how these phytoplankton use the available light energy. Such understanding of the coupling of light energy capture to cellular survival, growth rate and carbon fixation is crucial for facilitating better estimates of primary production and ultimately understanding the key role of the oceanic biota in the global carbon cycle. To achieve this, we propose an observational and experimental program that will define how diverse phytoplankton communities use light energy over natural gradients in nutrient and light availability. We will undertake a research cruise encompassing the South Atlantic, as well as the iron-limited regions of the Southern Ocean, to sample natural phytoplankton communities at sea. These will be analysed to determine their rates of photosynthesis and, through molecular functional and community structure analysis, to define the processes involved in harvesting and using photosynthetic energy. The information from the natural community experiments will be extrapolated to a larger scale via network analysis and mapping approaches to generate a global understanding of how cells use light energy.Our deliverables will be an observationally and experimentally derived integrated view of the environmental and physiological controls on how phytoplankton in ocean systems actually use light energy to power all cellular processes (beyond only considering carbon fixation) (OB1 and OB2), and a global-scale synoptic synthesis of where and when different photosynthetic strategies are used by phytoplankton (OB3). We expect this to move the paradigm away from the 'carbon-centric view' to more fully consider the potentially >50% of phytoplankton light energy usage that powers the metabolisms of the organisms underpinning the productivity of ocean ecosystems.