Abstract:

Trophic interaction between marine diazotrophs and higher trophic levels is an increasingly concerned but still poorly understood topic in recent years. The mechanisms and controlling factors of microzooplankton grazing on diazotrophs were especially unclear. In this study, we have for first time analysed diel microzooplankton grazing selectivity on unicellular cyanobacterial diazotroph (i.e. Crocosphaera watsonii WH8501) and non-diazotrophic unicellular microalgae (i.e. Chlorella autotrophica). A mixed diet consisting of C. watsonii and C. autotrophica was supplied to 4 species of protistal microzooplankton at daytime and night-time, respectively. Given that C. watsonii fixes N2 at night-time, it showed a stronger diel pattern of C:N ratio (10 at daytime and 6.8 at night-time) than C. autotrophica (8.2 at daytime and 7.8 at night-time). Concurrently, all four grazers ingested more C. watsonii in night-time than in daytime, suggesting the influence of nutritional quality (inferred from C:N ratio) of prey on the ingestion behaviour of grazers. In particular, Oxyrrhis marina changed from preferring C. autotrophica in daytime to preferring C. watsonii in night-time. Nevertheless, except O. marina, the rest grazers showed species specific grazing preferences. Euplotes sp. preferred C. watsonii to C. autotrophica, while Scuticociliate sp. and Lepidodium sp. preferred C. autotrophica to C. watsonii. Considering that C. watsonii produced more extracellular polymeric substances (EPS) than C. autotrophica, such grazing preferences were explained by grazer’s capability of ingesting the EPS surrounding cells, which was influenced by the feeding mode, cingulum size and cell size of grazers.

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