Protistal Grazers Increase Grazing on Unicellular Cyanobacteria Diazotroph at Night

Lixia Deng,Hongbin Liu,Shunyan Cheung

doi:10.3389/fmars.2020.00135

Abstract

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.

Highlights

Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary productivity in the ocean (Moore et al, 2013)
The range of the diel variation of C:N ratio was larger in C. watsonii than in C. autotrophica
The C:N ratio of C. watsonii varied from a maximum of 10.0 at the end of daytime to 6.9 during nighttime while that of C. autotrophica ranged from 8.5 to 7.8

Summary

Introduction

Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary productivity in the ocean (Moore et al, 2013). N2 fixation by diazotroph is an important source for biologically available N in the euphotic zone, in which its significance equals or exceeds the flux of dissolved inorganic N from deep waters (Capone et al, 2005). The fixed N is especially a significant N source, contributing to approximately half of primary production (Karl et al, 1997). The most studied diazotroph Trichodesmium could release its fixed N directly (Ohlendieck et al, 2000), through extracellular release (Capone et al, 1994), viral cell lysis (Ian et al, 2004), programmed cell death (Berman-Frank et al, 2004) and grazing (O’Neil et al, 1996). Unlike Trichodesmium, how the unicellular cyanobacterial diazotrophs (UCDs) transfer their fixed N to the environment and other organisms is unclear. There was a growing concern about trophic interaction between UCDs and higher trophic levels (Wilson et al, 2017; Horii et al, 2018), with the majority of Protistal Grazing on Unicellular Cyanobacteria Diazotroph related studies focusing on mesozooplankton (0.2–20 mm) grazing on UCDs (Scavotto et al, 2015; Conroy et al, 2017)

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Conclusion