Photosynthesis and growth reactivation of kelps after the Polar Night in a warming Arctic

The ongoing temperature increase in the Arctic is happening four times faster than in the rest of the globe. Its impact is especially important during the dark winter, when kelp survival relies on carbon reserves, which may further impact the subsequent response to light. To elucidate the effects of increased temperature during the Polar Night and the onset of light, growth, photosynthetic performance, Rubisco quantity and activation state, respiration and biochemical composition were studied in two of the most abundant kelp species in Arctic rocky coastal ecosystems, Alaria esculenta and Saccharina latissima . Laboratory-grown sporophytes were transferred to complete darkness at 1 ºC and 5 ºC for 8 weeks, followed by 2 weeks of increasing light exposure simulating natural conditions. During darkness, photosynthetic pigments and Rubisco quantity only decreased in A. esculenta, with no temperature effect. However, Rubisco activation state remained unchanged in both species during this period and photosynthetic reactivation occurred within the first hour of light exposure. Warming reduced nitrate storage capacity in both species. Furthermore, temporal patterns in nitrate reserves suggest that this nutrient is not taken up during the Polar Night. Higher temperature accelerated biomass loss in darkness in S. latissima , and reduced growth rates during the 2 weeks of increasing light exposure in both species, mainly due to higher respiration rates. This study highlights that warming would affect carbon and nitrogen balances after the Polar Night, having a negative impact on growth recovery during early light exposure and potentially compromising the survival of marine kelp forests.