The direct implications of warming on the phenotype and underlying functional traits of marine phytoplankton

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Marine phytoplankton mediate oceanic biogeochemical cycling by linking cellular metabolism with many elemental fluxes including C, N, P and Si. These elemental transformations are physiologically regulated processes that are influenced by phytoplankton phenotypes that change over different spatial and temporal scales. It is expected ocean warming (increasing sea surface temperatures; SST) will alter these patterns because temperature is the primary environmental control governing metabolism and growth in many phytoplankton groups. Whilst research on the effects of warming SST on biogeographical range shifts is advancing, it remains unknown how phytoplankton mediated biogeochemical transformations may be altered. Focusing on patterns in species functional traits (FTs), this thesis applied trait-based approaches in laboratory and field studies to explore how biogeochemically-related FTs vary over environmental gradients. I quantified the thermal performance curves (TPCs) of FTs in representative species from two laboratory-cultured phytoplankton functional types to understand trade-offs associated with thermal acclimation and adaptation. To assess whether these laboratory-based patterns of FT trade-offs and expression were consistent in the field; I replicated a similar TPC experiment with a natural phytoplankton community. Finally, to understand how multiple environmental gradients interact to influence phytoplankton FT expression, I tracked a diatom-specific FT over northern Australia to spatially map the diatom phenotypes present to deduce the likely biogeochemical roles of the species in the region. This thesis demonstrates the importance of understanding the relationship between the duration of thermal exposure, FT expression and trade-offs in regulating the phytoplankton phenotype, as all of these factors differentially affect species’ growth rates (and therefore fitness and biogeography) but also the acquisition of C, N, P and Si, and therefore the marine cycling of these elements. Furthermore, ocean mapping of FTs proves insightful for understanding variability of biogeochemical transformations between different ocean regions by providing a link between cellular and community level processes.
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