Comparative analysis of fatty acid profile in three eutardigrade species

Tardigrades colonize a wide range of habitats in which they can be predators, prey or primary consumers in food webs. Most species are herbivorous, feeding on cell fluid of algae and mosses, while others feed on bacteria, or prey on micrometazoans. Despite the wide range of food sources, details on food preference and on consequent lipid composition of tardigrade species are in practice unknown. Aiming to fill the gap of knowledge, we investigated the fatty acid composition of three eutardigrade species, since fatty acids are the main component of lipids and they play an important role in the function of cell membranes and in the physiological responses of organisms. The species, differing in colonized habitat and probably in diet, were: Acutuncus antarcticus (Hypsibiidae), a freshwater Antarctic species cultured using Chlorococcum sp. as food source, and the moss-dwelling species Macrobiotus macrocalix and Richtersius coronifer (Macrobiotidae). For each species, lipids were extracted from ten replicates of 150-250 animals with chloroform/methanol and the total extracts were used to obtain the fatty acid metylesters that were injected into a gas chromatograph. In all species, the same 21 fatty acids belonging to saturated, monounsaturated (MUFA) and polyunsaturated (PUFA) groups were identified. In A. antarcticus the most represented fatty acids were: palmitic (C16:0), stearic (C18:0), oleic (C18:1n-9), and myristic (C14:0) acids; saturated fatty acids (56.6%) were the most abundant with respect to MUFA (22.3%) and PUFA (21.1%). In M. macrocalix the most represented were: oleic (C18:1n-9), palmitic (C16:0), stearic (C18:0), and linoleic (C18:2n-6) acids; the saturated fatty acids (38.4%), MUFA (28.8%) and PUFA (32.8%) were uniformly distributed. In R. coronifer, alpha-linolenic (C18:3n-3), palmitic (C16:0), stearic (C18:0), and arachidonic (C20:4n-6) acids are the most represented; the percentage of PUFA (52.8%) was higher than that of MUFA (8.2%) and saturated fatty acids (38.9%). These data indicate clear differences in the fatty acid composition and amount among species. The fatty acid profiles reflect the food source and can be used as indicator to assess the feeding diet of tardigrades. Interestingly, species inhabiting the same substrate and eating the same food (moss cell content) use/transform the fatty acids in different way indicating different biochemical needs.