Differential gene expression is tied to photo chemical efficiency reduction in virally infected Emiliania huxleyi

Abstract

Photophysiological changes in phytoplankton have important implications for the structure and function of planktonic communities and ocean biogeochemistry. Viruses have been shown to decrease photochemical efficiency (Fv/Fm) of Emiliania huxleyi late in infection, although the precise timing and mechanism of this reduction remain largely unexplored. We coupled high-resolution temporal measurements of photosystem II (PSII) efficiency during coccolithovirus EhV-86 infection with expression of host psbA (D1), PSII assembly and repair genes (ctpA, ftsH, and HCF136), and antioxidant genes (ascorbate peroxidase [apx] and superoxide dismutase [sod2]) using digital PCR. Fv/Fm declined significantly in infected cultures from 1.25 h postinoculation (p.i.), while non-photochemical quenching (NPQ), measured as the normalized Stern-Volmer parameter, significantly increased from 4.0 h p.i. These changes were preceded by significant decreases in expression of ctpA and ftsH (25 min p.i.) and of HCF136 (1 h p.i.). Significant down-regulation of sod2 homologs and apx was observed later, starting at 1 to 3 h p.i. Our data suggest that virus-induced photophysiological changes may begin with a reduction in PSII repair and assembly, followed by a reduction in antioxidants that may result in an accumulation of damaged PSII complexes. Elevated NPQ during infection may help dissipate excess energy resulting from damage to PSII. This study illustrated that viral infection leads to a cascade of significant changes in the expression of genes assessed during this study, which may lead to the observed reduction in photosynthetic performance.