Broad photoacclimation strategies in Arctic sea-ice diatoms revealed by species-specific growth responses to different light conditions

The Arctic region is experiencing rapid rates of warming due to anthropogenic climate change, leading to a dramatic decrease in sea-ice cover. Impacts of these changes on sea-ice biological communities, biogeochemical cycles, and Arctic food webs remain understudied, partially due to methodological challenges studying sea-ice biology. The main novelty of this study lies in the estimation of species-specific growth rates of sea-ice diatoms—the dominant primary producers in Arctic sea ice—from natural communities. Sea-ice algal communities from two Svalbard fjords, Tempelfjorden and Van Mijenfjorden, were incubated at different irradiance levels, including darkness, using PDMPO, a fluorescent probe that labels newly deposited biogenic silica, to track diatom cell division under near in situ conditions. Building on this approach, we were able to provide the first estimates of production for individual diatom species, based on species-specific growth rates and abundance data, and to link these production estimates to environmental conditions in the field. Our results demonstrate strong interspecific variability in light response and show that, in contrast to photosynthesis, growth can occur in darkness. No clear relationship between the species abundance and their growth rates was found, illustrating that the most abundant taxa are not necessarily the fastest growing and that growth history needs to be taken into account. Furthermore, our estimates of growth production based on the staining approach presented in this study differed substantially from carbon-based primary production estimates, highlighting the need to consider physiological processes beyond photosynthetic carbon fixation. Together, these findings highlight the need for a species approach in ecological models aiming to predict sea-ice ecosystem responses to ongoing Arctic change.