Viral regulation of nutrient assimilation by algae and prokaryotes

Abstract

Viruses are the most abundant entities in the ocean and represent a large portion of lifes genetic diversity. As mortality agents, viruses catalyze transformations of particulate matter to dissolved forms. This viral catalytic activity may influence the microbial community structure and affect the flow of critical elements in the sea. However, the extent to which viruses mediate bacterial diversity and biogeochemical processes is poorly studied. The current thesis, using a single cell approach, provides rare and novel insights in to how viral infections of algae influence host carbon assimilation. Furthermore this thesis details how cell lysis by viruses regulates the temporal bacterial community structure and their subsequent uptake of algal viral lysates. Chapter 2 shows how viruses impair the release of the star-like structures of virally infected Phaeocystis globosa cells. The independent application of high resolution single cells techniques using atomic force microscopy (AFM) visualized the unique host morphological feature due to viral infection and nanoSIMS imaging quantified the impact of viral infection on the host carbon assimilation. Prior to cell lysis, substantial amounts of newly produced viruses (~ 68%) were attached to P. globosa cells. The hypothesis that impediment of star-like structures in infected P. globosa cells leads to enhanced grazing was proposed. The scenario of enhanced grazing is in sharp contrast to the current view that viral infections divert the organic carbon transfer from higher trophic levels (e.g., grazers). In chapter 3, during early hours of viral infection, the application of secondary-ion mass spectrometry (nanoSIMS) showed a high transfer of infected P. globosa biomass towards Alteromonas cells well before the latent period, which stimulated its initial doubling in abundance, attachment to algal cell surroundings. Following algal viral lysis, the succession of bacterial populations consisted of Alteromonas and Roseobacter cells and an efficient transfer of P. globosa viral lysates by these specific bacterial members (Day 2). The sharp increase of these two genera, which occurred in aggregate-association, declined in abundance due to plausible phage mediated lysis. The potential phage mediated lysis appeared to result in aggregate dissolution and was responsible for regeneration of dissolved inorganic carbon (55% of the particulate 13C-organic carbon) and generation of plentiful recalcitrant organic carbon. The findings such as algal leakage during infection substantiate a previously undocumented role of viruses, which appears to be responsible for alterations in the marine ecosystem process such as bacterial community structure and carbon availability. In chapter 4, it appears that viral infection of Micromonas pusilla cells led to the hindrance of pyrenoid synthesis (starch and proteins) and much of the newly assimilated material was diverted towards viral production. Viral lysis of M. pusilla led to dominance of Alteromonas cells and Bacteroidetes, where as Alteromonas cells dominated the bacterial communities in non-infected cultures through out the experiment. The ecological implication of viral mediated starch impediment in M. pusilla cells may lead to the release of labile proteins and increased levels of polysaccharides, which potentially directs the marine pelagic system to more regenerative processes.