Pigment composition and photoprotection of Arctic sea ice algae during spring

From the beginning of spring to the melt period, ice algae in the bottom of Arctic sea ice experience a large irradiance range, varying from <0.1% up to 25-30% of the incoming visible radiation. The increase in spring is usually rapid, with a varying photoacclimative response by bottom ice algae to protect themselves against excess light, such as changes in cellular pigment composition. This study focused on the temporal variation in pigment composition of bottom ice algae under 2 contrasting snow depths (thin and thick) during spring. Controlled experiments were also carried out to investigate the photoprotective capacity of ice algae to relatively high irradiances during a short-term period (<6 h). Ice algae were able to photoacclimate rapidly and effectively to irradiance ranging from 10 to 100 mu mol photons m(-2) s(-1). However, we observed contrasting responses in photoacclimation depending on the ice algal community composition and their light history. Our experimental results suggest that the xanthophyll cycle (diadinoxanthin to diatoxanthin conversion) and D1-protein recycling play an important role in stabilizing photoprotection in ice algae. In addition, bottom ice algae likely employed a 'cellular light-exposure memory' strategy in order to improve their photoacclimative response to changing light exposure. According to our data, this process could be maintained over at least 2 wk. Hence, ice algae may be more resilient to varying light conditions than previously thought and may be well-adapted for the expected future light regime changes associated with variability in snow and sea ice cover.