Patterns of Evolutionary Conservation and Divergence in the Short-Term Hyposalinity Stress Response of a Euryhaline Diatom, Skeletonema marinoi
Survival under fluctuating environmental conditions, such as those increasing in frequency and magnitude under environmental change, requires a successful response to stress. Interspecific differences in stress responses may result in differential survival of species, even within a lineage. Diatoms may constitute one such lineage, as salinity tolerance among extant species is diverse, and the observation of frequent historic habitat transitions between marine and freshwater environments indicates that diatoms successfully mitigated (low) salinity stress in the past, followed by adaptation and diversification over evolutionary time scales. To understand to what extent the diatom hypoosmotic stress response consists of conserved and variable elements, we used RNA sequencing during an 8 hour time series to characterize the short-term stress response to hyposalinity of the ancestrally marine, euryhaline, diatom Skeletonema marinoi, and compared it to its distant relative C. cryptica which shows a broader salinity tolerance. Our data show that upon exposure to low salinity, S. marinoi mounts a rapid response to manage osmotic and oxidative stress (15–30 min), after which the diatom transitions into an energy-intensive recovery phase (2–4 h). By 8 hours, S. marinoi approaches acclimation to low salinity. Comparison with C. cryptica showed that chloroplastic K+ efflux and a broad response to oxidative stress constitute conserved mechanisms related to an euryhaline lifestyle across the diatom lineage. However, our data also suggested that S. marinoi is less adept in mitigating low salinity stress than C. cryptica, which unlike S. marinoi tolerates freshwaters and shows distinct mechanisms for initial survival and long-term growth in low salinity. Altogether, our study highlights that the cellular mechanisms behind low salinity tolerance in diatoms include elements that are both conserved and variable across the diatom lineage. Given the crucial ecological roles of diatoms in aquatic food webs, understanding the various strategies of marine diatoms to overcome freshening of their environments is central to predicting the impacts of climate change on coastal primary production.
diatom, RNA-seq, salinity, stress, transcriptomics, Behavior and Ethology, Integrative Biology
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