Transcriptomic and physiological responses of a model diatom (Phaeodactylum tricornutum) to heat shock and heat selection

Abstract

Diatoms are ecologically important phytoplankton as they contribute 40% of the marine primary production and play an essential role in the biogeochemical cycles of many elements. Ocean warming, as a pressing environmental stressor, can profoundly affect the population structure and ecological services of diatoms. However, the molecular mechanisms underpinning cellular and physiological changes of diatoms under heat wave and heat selection conditions remain underexplored. This study revealed the transcriptomic and physiological responses of a model diatom species P. tricornutum to ocean warming at an elevated temperature (25 °C in comparison with 20 °C) under scenarios of heat shock (∼7 days; ∼7 generations) and heat selection (∼175 days; ∼165 generations). While unchanged cell growth and reduced Fv/Fm were recorded in the heat shock treatment, significant reductions in the cell growth rate and Fv/Fm were evident in the heat selection treatment. Contents of Chl-a, carotenoid, total soluble protein, carbon (C), and nitrogen (N) increased significantly in both treatments. Transcriptomics analysis showed that elevated temperature suppressed signaling pathways related to photosynthesis but activated signaling pathways involving in fatty acid metabolism and nitrogen metabolism following the heat selection, associated with elevated substance storage and enlarged cell size. As a result, the increase of both C and N suggest that more energy was likely allocated to growth than division under ocean warming. Notably, P. tricornutum showed a higher resistance to nitrogen limitation condition after acclimating to ocean warming possibly by activation of nitrogen metabolism and uptake. P. tricornutum responded to the projected ocean warming through readjusting energy metabolism.