Assimilation of Cyanobacteria by the Freshwater Bivalve Nodularia douglasiae: Insights from Long-Term Laboratory and Field Feeding Experiments

In this study, we conducted long-term laboratory and field feeding experiments to investigate the ability of a freshwater bivalve, Nodularia douglasiae (recently renamed to N. nipponensis) to utilize cyanobacterial cells as food. In the laboratory experiment, N. douglasiae which was fed with 15Nlabeled Microcystis aeruginosa cells over 80 days showed an assimilation efficiency of 47%. The fatty acid compositions in bivalves reflected a slight but apparent increase in the relative ratio of two cyanobacterial fatty acids, linoleic acid (LA) and α-linolenic acid (ALA), to eicosapentaenoic acid (EPA). In the field-feeding experiment, N. douglasiae was fed natural seston in the littoral area of the eutrophic Lake Hachiro (Akita, Japan) during the summers of 2017 and 2018 when cyanobacterial blooms occurred. The ratios of LA and ALA to EPA in bivalves increased from 0.45 to 0.70 and from 0.40 to 0.77 during the summers; the final values were consistent with those obtained in the laboratory feeding experiment. The results indicated that N. douglasiae can utilize cyanobacterial cells as food and maintain assimilation activity at least over several months, and the species may serve as a useful biomanipulation tool for controlling cyanobacterial blooms in freshwater lakes.