Melosira arctica : Algal superfood or nutritional dead-end for Arctic grazers?

Despite the ubiquitous distribution of the sub-ice diatom Melosira arctica across the Arctic Ocean, its biochemical composition and role as a food source for Arctic grazers remain poorly understood. We analyzed lipid-based trophic markers in Melosira assemblages from three research expeditions and contrasted them with unialgal M. arctica cultures. We also conducted a feeding experiment with two ice-associated (sympagic) amphipod species. To identify potential distinct biochemical traits, we compared trophic marker contents of Melosira samples with pelagic and ice-associated particulate organic matter, as well as with pelagic and sympagic diatom-dominated bloom communities. Despite pronounced variability among Melosira field assemblages, including fatty acid (FA) and stable isotope compositions, some consistent patterns emerged, such as overall high contributions of typical diatom-associated FAs. Polyunsaturated FAs, indicating nutritional quality, contributed up to 30% of total FAs in samples from Fram Strait and the Yermak Plateau/Nansen Basin, as well as in the cultures. Lower levels of polyunsaturated FAs (17%) in central Arctic Ocean samples likely reflected differences in cell developmental stages, nutrient supply, and ice properties. These Melosira assemblages had developed over a longer time than those collected from the other locations and, as expected, exhibited early indications of senescence. Across all samples, diatom-produced highly branched isoprenoids were absent, suggesting that Melosira does not produce these compounds. Overall, the chemical composition of Melosira was found to be typical of a centric diatom; only the sterol composition with a dominance of campesterol differed from that of pelagic (brassicasterol-dominated) and ice-associated (β-sitosterol-dominated) particulate organic matter. The experimental work indicated that amphipods interacted with Melosira, exhibiting varying degrees of algal consumption. Melosira can thus serve as both an under-ice habitat and a food source for Arctic grazers, but tracking its trophic interactions may be challenging given an absence of specific biochemical markers.