Phaeodactylum tricornutum Raw sequence reads
Marine heatwaves (MHWs) are intense climatic events posing significant threats to marine ecosystems. Despite known ecological impacts of MHWs, biological mechanisms on how key organisms respond to them largely remain elusive. Here, we have applied an integrative approach based on a combination of photosynthetic analyses, elemental stoichiometry, transcriptomics and epigenetics to identify physiological mechanisms underpinning the response of the model diatom Phaeodactylum tricornutum to sequential marine heatwaves and subsequent warming. Our results show that P. tricornutum coped well with MHWs, albeit with minor negative effects on the growth rate, photosynthetic pigments and cellular metabolism. However, this short-term exposure to heat stress significantly reduced its resilience to subsequent warming, potentially due to DNA damage and an impaired photosynthetic machinery, as indicated by gene expression changes. We also found that expression patterns of transposable elements (TEs) in sequential MHWs and subsequent high temperature and high CO2 (HTHC) were influenced by their insertion time, cytosine (5mC) and adenine (6mA) methylation levels, and GC content. Furthermore, 5mC methylation in CHG and CHH contexts gradually increased during these conditions, suggesting their role in acclimation. Thus, our result suggests that MHWs as isolated events do not appear to dramatically impact the growth of marine diatoms, but they reduce their resilience to cope with subsequent MHWs and general ocean warming, which has implications for our understanding of how ocean warming might influence key primary producers.