Abstract
We investigated whether trophic repackaging of autotrophic picoplankton by phagotrophic protists is associated with an improvement in food quality for the metazooplankton Daphnia magna (i.e., whether trophic upgrading occurs in this system). The nutritional value of the autotrophic species Microcystis aeruginosa PCC7806, Synechococcus sp. strain BO8809, Synechococcus elongatus SAG 89.79, and Choricystis minor KR1988/ 8, and of the heterotrophic nanoflagellate Paraphysomonas sp. grown on these different picoplanktonic species was evaluated in standardized growth experiments with D. magna . In order to investigate the functional role of the flagellate in the simplified autotrophic picoplankton‐ Daphnia food chain, Paraphysomonas sp. was grown on the different picoplanktonic organisms and subsequently separated from the food items before being fed to D. magna. The presence of Paraphysomonas sp. as an intermediary trophic step enhanced somatic growth and reproduction of D. magna. Supplementation of Synechococcus sp. with lipids from Paraphysomonas sp. (grown on Synechococcus sp.) revealed that trophic upgrading of autotrophic picoplankton is due to the additional lipids present in the flagellate. Paraphysomonas sp. synthesized polyunsaturated fatty acids and sterols de novo, which most likely explains the trophic upgrading. Paraphysomonas sp. also improved the food quality of M. aeruginosa PCC7806, which is toxic for D. magna . The heterotrophic flagellate Paraphysomonas sp. is capable of trophically upgrading a poor quality food source not only by producing essential lipids, but also by detoxifying the cyanobacterial food organism.