Nitric oxide production and antioxidant function during viral infection of the coccolithophore Emiliania huxleyi

Brittany M Schieler,Christopher M Brown,Benjamin A S Van Mooy,Marco J L Coolen,Helen Fredricks,Donald J Hirsh,Kay D Bidle,Megha V Soni

doi:10.1038/s41396-018-0325-4

Abstract

Emiliania huxleyi is a globally important marine phytoplankton that is routinely infected by viruses. Understanding the controls on the growth and demise of E. huxleyi blooms is essential for predicting the biogeochemical fate of their organic carbon and nutrients. In this study, we show that the production of nitric oxide (NO), a gaseous, membrane-permeable free radical, is a hallmark of early-stage lytic infection in E. huxleyi by Coccolithoviruses, both in culture and in natural populations in the North Atlantic. Enhanced NO production was detected both intra- and extra-cellularly in laboratory cultures, and treatment of cells with an NO scavenger significantly reduced viral production. Pre-treatment of exponentially growing E. huxleyi cultures with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) prior to challenge with hydrogen peroxide (H2O2) led to greater cell survival, suggesting that NO may have a cellular antioxidant function. Indeed, cell lysates generated from cultures treated with SNAP and undergoing infection displayed enhanced ability to detoxify H2O2. Lastly, we show that fluorescent indicators of cellular ROS, NO, and death, in combination with classic DNA- and lipid-based biomarkers of infection, can function as real-time diagnostic tools to identify and contextualize viral infection in natural E. huxleyi blooms.

Highlights

Emiliania huxleyi is a cosmopolitan species of coccolithophore, a group of unicellular, eukaryotic marine phytoplankton that produces intricate shells of calcium carbonate
Cell decline was concomitant with increases in both the proportion of dead or dying cells indicated by SYTOX Green (26% of culture at 48 hpi and 58% culture at 72 hpi; Fig. 1c) and intracellular reactive oxygen species (ROS) indicated by CM-H2DCFDA, increasing ~3-fold at 48 hpi and ~17-fold at 72 hpi (Fig. 1d)
Our findings demonstrate nitric oxide (NO) production by naturally occurring E. huxleyi populations undergoing various stages of viral infection in the eastern North Atlantic [12, 13, 15]

Summary

Introduction

Emiliania huxleyi is a cosmopolitan species of coccolithophore, a group of unicellular, eukaryotic marine phytoplankton that produces intricate shells (coccoliths) of calcium carbonate. E. huxleyi is known for forming large annual blooms in the North Atlantic, often spanning >105 km with cell densities exceeding 106 cells l−1 [1,2,3] These blooms are associated with dimethyl sulfide production in the surface ocean and flux into the atmosphere [4], increased albedo and heating of the surface ocean due to their coccoliths [5], and enhanced export flux of carbonate to the deep ocean [6]. This may be a result of increased cellular production of transparent exopolymeric particles (TEP) during infection, which acts to enhance particle aggregation and couple infection with microzooplankton grazing [19]

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Conclusion