Temperature and light regulated patterns of physiology, morphology and elemental stoichiometry in geographically distinct isolates of a cosmopolitan diatom

Abstract. Anthropogenic influence on climate change has profound and diverse consequences on marine ecosystems. At the base of the food web, phytoplankton, are experiencing altered temperature regimes. In south-east Australia, the southward extension of warm waters, driven in part by the East Australian Current (EAC), is rapidly warming regional ocean temperatures, leading to the intensification of marine heat waves (MHWs). In this study, we investigated thermally adapted Leptocylindrus danicus strains isolated from four distinct latitudes to determine how silica production rates vary with temperature and irradiance. We also explored how the intra-specific phenotypic variability affects physiology and silica production. We found strong latitudinal effects on strain-specific cell volume (ranging from 313 ± 22 µm² to 2070 ± 105 µm³) and pigment quotas (chl a 1.04 ± 0.21 to 3.70 ± 1.17 pg cell^-1; chl c 0.26 ± 0.07 to 2.09 ± 1.50 pg cell^-1), both increasing with increasing temperature. There was also a significant effect of temperature on silicification rates, which varied depending on growth irradiance and cell normalisation. By identifying temperature and light regulated shifts in growth, morphology and silicification in a cosmopolitan diatom, we can gain an improved understanding of the range in intraspecific phenotypic variability of this key phytoplankton group. This study provides an assessment on how key diatom traits vary along a latitudinal gradient, providing unique insight into how ocean warming may influence resilience and adaptation potential of L. danicus, and how shift in physiology may impact diatom-regulated carbon and silicon cycling.