Structural and Functional Variations of Enzymes Indicate Latitudinal Adaptations of Subarctic and Arctic Sea Urchins

Rapid environmental changes in Arctic fjord systems due to global warming pose new challenges to benthic key species. Sea urchins of the genus Strongylocentrotus are the main grazers on habitat-forming kelp and have significant influence on their environment. Given their ecological relevance, it is crucial to address their potential impacts in future warming systems. We compared sea urchins from the Subarctic Porsangerfjord (Northern Norway) and the Arctic Kongsfjord (Svalbard) to investigate adaptation to local environmental conditions. The two fjords differ in temperature and ice cover. Due to progressive warming, the Arctic Kongsfjord is developing into a glacier- and ice-free fjord, as the Subarctic Porsangerfjord already is. Transcriptomes of two Strongylocentrotus species, S. droebachiensis and S. pallidus, were analyzed for convergent amino acid substitutions. We identified genetic variations in metabolic and energy-related pathways that may be linked to local thermal environments. Furthermore, we showed that colder-exposed Arctic sea urchins exhibited higher enzyme activities and lower Arrhenius activation energies, which might be associated with specific amino acid sequence changes. Our results suggest an overall higher biochemical and metabolic efficiency in Arctic sea urchins compared to their Subarctic counterparts, which indicates cold adaptation. The enzymatic properties of sea urchins do not only enhance food utilization under cold conditions but may also increase grazing activity and, thus, grazing pressure on kelp forests in the warming Arctic fjords. This, in turn, could have cascading effects on ecosystem structure and biodiversity in the future Arctic.