Heterotrophic nanoflagellates and increased essential fatty acids during Microcystis decay

Abstract

To investigate the potential for heterotrophic organisms to upgrade the food quality of seston, we performed decay experiments using a non-toxic Microcystis aeruginosa (cyanobacteria) monoculture. The experiment was performed In darkness with aeration using a microbial inoculum collected from a hypereutrophic pond. Chlorophyll a concentrations decreased throughout the decay experiment. In contrast, eicosapentaenoic acid (EPA, 20:5ω3) and docosahexaenoic acid (DHA, 22:6ω3) concentrations increased and peaked on Day 5, while α-linolenic acid (α-LA, 18:3ω3) and stearidonic acid (18:4ω3) gradually decreased, suggesting that EPA and DHA might be converted from α-LA and stearidonic acid. Microscopic examination revealed that a heterotrophic nanoflagellate (HNF) and the ciliate Vorticella sp. dominated the biological community during this experiment. Further examination using scanning electron microscopy (SEM) identified the HNF as Paraphysomonas vestita ssp. vestita, whose biovolume was very strongly correlated with EPA concentrations. Size-fractionized fatty acid determinations carried out on Day 6 showed that approximately two-thirds of the seston's total EPA content was in the <5 μm size fraction, which corresponded to the P. vestita size fraction. This size fraction also had a fatty acid content (relative to carbon content) 3 times higher than the 5 to 10 or >10 μm seston size fractions.