Effects of viral infection on photosynthetic processes in the bloom-forming alga Heterosigma akashiwo

Pulse-amplitude-modulated (PAM) fluorescence is a technique that allows rapid assessment of photosynthetic activity of phytoplankton. This approach was used to evaluate the effect of viral infection on the photosynthesis of Heterosigma akashiwo, a toxic bloom-forming raphidophyte found in coastal waters of Japan and Canada. We found that viral infection caused by a single strand RNA virus (HaRNAV Strain 263) and 2 uncharacterised DNA viruses (Strains WBs1 and OIs1) gradually impaired photosynthetic activity during the lytic cycle and that photosynthetic electron transport was directly affected by viral infection. Photosystem (PS) II quantum yield of the active reaction centre was much less affected than the overall PSII-PSI electron transport. The decrease in the photosynthetic activity of the infected algae promoted the non-photochemical energy dissipation (heat). Furthermore, the lytic cycle of the viruses was of similar duration in darkness as in the light (ca. 100 h), and therefore it was not dependent on photophosphorylation. Our study demonstrates that the sensitivity of PAM fluorometry makes it a useful tool for studying viral infection in phytoplankton populations. Moreover, the results have implications for understanding the role of viral infection on the bloom dynamics of H. akashiwo by showing that viral replication is not light-dependent. Hence, viral production can occur below the photic zone.