Viral lysis of <i>Micromonas pusilla</i>: impacts on dissolved organic matter production and composition

The viral mediated transformation of phytoplankton organic carbon to dissolved forms (“viral shunt”) has been suggested as a major source of dissolved organic carbon (DOC) in marine systems. Despite the potential implications of viral activity on the global carbon fluxes, studies investigating changes in the DOC composition from viral lysis is still lacking. Micromonas pusilla is an ecologically relevant picoeukaryotic phytoplankter, widely distributed in both coastal and oceanic marine waters. Viruses have been found to play a key role in regulating the population dynamics of this species. In this study we used axenic cultures of exponentially growing M. pusilla to determine the impact of viral lysis on the DOC concentration and composition, as estimated from lysate-derived production of transparent exopolymer particles (TEP) and two fractions of fluorescent dissolved organic matter (DOM): aromatic amino acids (excitation/emission; 280/320 nm; F(280/320)) and marine humic-like fluorescent DOM (320/410 nm; F(320/410)). DOC concentration increased 4.5 times faster and reached 2.6 times higher end concentration in the viral infected compared with the non-infected cultures. The production of F(280/320) and F(320/410) were 4.1 and 2.8 times higher in the infected cultures, and the elevated ratio between F(280/320) and F(320/410) in lysates suggested a higher contribution of labile (protein) components in viral produced DOM than in algal exudates. The TEP production was 1.8 times faster and reached a 1.5 times higher level in the viral infected M. pusilla culture compared with the non- infected cultures. The measured increase in both DOC and TEP concentrations suggests that viral lysis has multiple and opposite implications for the production and export processes in the pelagic ocean: (1) by releasing host biomass as DOC it decreases the organic matter sedimentation and promotes respiration and nutrient retention in the photic zone, whereas (2) the observed enhanced TEP production could stimulate particle aggregation and thus carbon export out of the photic zone