Protistal Grazers Increase Grazing on Unicellular Cyanobacteria Diazotroph at Night

In this study, we have for the first time analysed diel microzooplankton grazing selectivity on unicellular cyanobacterial diazotroph (i.e. Crocosphaera watsonii WH8501) and non-diazotrophic unicellular microalga (i.e. Chlorella autotrophica). A mixed diet consisting of these two algae was supplied to 4 species of protistan microzooplankton at daytime and night-time, respectively. C. watsonii fixes nitrogen at night-time and showed a stronger diel pattern of cellular C:N ratio than C. autotrophica. All four grazers ingested more C. watsonii at night-time than in daytime, suggesting the diazotroph became more nutritious (inferred by C:N ratio) and thus a preferred prey for grazers when it fixes nitrogen. In particular, Oxyrrhis marina changed from preferring C. autotrophica in daytime to preferring C. watsonii at night-time. The rest grazers showed species-specific grazing preferences, which could be explained by extracellular polysaccharide production of C. watsonii , feeding mode, cingulum size and cell size of grazers.