Feeding by raphidophytes on the cyanobacterium Synechococcus sp.

We investigated feeding by the raphidophytes Chattonella ovata, C. subsalsa, Fibrocapsa japonica, and Heterosigma akashiwo on the cyanobacterium. Synechococcus sp. To explore whether each species is able to feed on Synechococcus sp., we carefully observed inside target grazer cells using an epifluorescence microscope and tranbsmission electron microscope (TEM). We also explored the feeding behaviors of C. ovata and H. akashiwo on Synechococcus using high-resolution video microscopy. In addition, we measured ingestion rates of C. ovata, C. subsalsa and H. akashiwo on Synechococcus sp. as a function of prey concentration. We calculated grazing coefficients by combining the field data on abundances of H. akashiwo and co-occurring Synechococcus spp. with laboratory data on ingestion rates. Both C. ovata and H. akashiwo were able to ingest single Synechococcus cells. However, neither TEM nor video microscopy showed any Synechococcus cells inside or ingested by F japonica, One to two ingested Synechococcus cells inside the protoplasm of F japonica cells were very rarely observed. C. ovata and H. akashiwo engulfed a single Synechococcus cell captured by the mucus excreted from mucocysts. The ingestion rates of C. ovata, C. subsalsa, or H. akashiwo on Synechococcus increased continuously with increasing prey concentration at prey concentrations ! 4 x 10(6) to 5.5 x 10(6) cells ml(-1). At a given prey concentration, the highest ingestion rates of the raphidophytes on Synechococcus were 18.6 cells raphidophyte(-1) h(-1) for C. ovata, 20.5 cells raphidophyte(-1) h(-1) for C. subsalsa, and 3.9 cells raphidophyte(-1) h(-1) for H. akashiwo. The calculated grazing coefficients attributable to H. akashiwo on co-occurring Synechococcus spp. were up to 1.24 d(-1). The results of the present study suggest that raphidophytes sometimes have a considerable grazing impact on populations of Synechococcus.