Ecological impact of algal-preying protists aplanochytrids (Labyrinthulea) as a source of docosahexaenoic acid for copepods in marine environments.

Docosahexaenoic acid (DHA) is an ω-3 polyunsaturated fatty acid with essential physiological functions in marine fish. Therefore, both fish and their prey must acquire DHA through the food web. Diatoms and especially dinoflagellates have traditionally been considered the primary producers of DHA. However, this assumption presents a paradox because, when considering the polyunsaturated fatty acid profiles produced by both groups, they contain substantially more eicosapentaenoic acid (EPA) than DHA, even though marine fishes exhibit nearly equivalent DHA:EPA ratios. In this study, based on biomass surveys conducted in the western North Pacific off northeastern Japan, we suggest that aplanochytrids (Labyrinthulea) are DHA suppliers comparable to dinoflagellates. Their accumulation in algae-rich regions, along with metabarcoding analysis and co-culture experiments, suggests their ecological role as predators in marine environments. Moreover, our estimates based on reported gut content analysis of calanoid copepods suggest that aplanochytrids efficiently transfer DHA to higher trophic levels, potentially surpassing diatoms and dinoflagellates in this role. This study is the first to indicate that aplanochytrids can contribute to DHA production and have the potential to prey on diverse algal groups within the grazing food chain. Therefore, for calanoid copepods, the primary prey of many marine fish species and lacking the ability to synthesize DHA de novo, relying instead on external sources, aplanochytrids represent a major and previously overlooked source of DHA that contributes substantially to its accumulation in fish. Labyrinthuleans are greater DHA contributors than recognized, thus requiring trophic flux studies including labyrinthuleans to clarify ecological significance.