Biomolecular profiles of Arctic Sea-ice diatoms highlight the role of under-ice light in cellular energy allocation

Arctic sea-ice diatoms fuel polar marine food webs as they emerge from winter darkness into Spring. Through their photosynthetic activity they manufacture the nutrients and energy that underpin secondary production. Sea-ice diatom abundance and biomolecular composition vary in space and time. With climate change causing short-term extremes and long-term shifts in mean environmental conditions, understanding how and in what way diatoms adjust biomolecular stores with environmental perturbation is important to gain insight into future ecosystem energy production and nutrient transfer. Using synchrotron-based Fourier Transform Infra-Red microspectroscopy, we examined the biomolecular composition of five dominant sea-ice diatom taxa from landfast ice communities covering a range of under-ice light conditions during Spring, in Svalbard, Norway. In all five taxa we saw a doubling of lipid and fatty acid content when light transmitted to the ice-water interface was >5% but <15% (85–95% attenuation through snow and ice). We determined a threshold around 15% light transmittance after which biomolecular synthesis plateaued, likely due to photoinhibitory effects, except for Navicula spp, which continued to accumulate lipids. Increasing under-ice light availability led to increased energy allocation towards carbohydrates, but this was secondary to lipid synthesis, while protein content remained stable. It is predicted that under-ice light availability will change in the Arctic, increasing due to sea-ice thinning and potentially decreasing with higher snowfall. Our findings show that the nutritional content of sea-ice diatoms are taxon-specific and linked to these changes, highlighting potential implications for future energy and nutrient supply for the polar marine food web.