Viral-host interactions: from strain to natural planktonic communities

Being the most abundant and diverse entities in planet Earth, viruses are thought to play a relevant role in controlling the composition and diversity in phytoplanktonic microbial communities. Microbial communities sustain life in the oceans and even in terrestrial environments if we account for half of the oxygen in the atmosphere, which is produced by their photosynthetic members. Thus, understanding how viruses and their hosts interact at the vast oceanic scale, and the potential impact viruses might have on the development of marine microbial communities, remain of primary relevance. To what extent do viruses exert a significant pressure on the microbial communities they infect? To what extent does that interaction lead to the existence of a variety of “virus-driven” trade-offs between host traits, such as resistance and growth capacity? Despite the progress that has been in this area, especially with prokaryotes, we still lack assertive answers to these questions. This thesis aims to increase the current knowledge on marine viral role and their potential action in shaping marine microbial communities. To do so, cross-infectivity experiments were conducted and parameters such as growth rate (μ), resistance (R), and viral production (Vp), were investigated for two relevant eukaryotic phytoplankton systems: Micromonas / Micromonas Virus (MicV) (Paper I) and Emiliania huxleyi / Emiliania huxleyi Virus (EhV) (Paper II), respectively. Competition experiments between Micromonas strains with different resistance capacities and similar growth rate were also performed (Chapter 4.1). Viral impact was also measured at the broad level of complex natural marine microbial communities with six viral depletion microcosm experiments (Paper III). The significant trends observed on single virus-host interactions demonstrated strong co-interactions at different levels between the tested phytoplankton strains and their viruses; however, a potential viral role as major drivers behind a growthrate/ resistance trade-off was not consistently observed in any of the studied systems (Paper I, Paper II). In 4 out of 7 competition experiments was such trade-off possibly present, but even then not in an explicit manner. Surprisingly, higher viral production capacities were measured in generalist viral strains from both systems (Paper I, Paper II). For the viral depletion experiments (Paper III), the incubation period itself was sufficient to provoke significant changes in the composition of the microbial communities under study; however, viral impact was significant in half of the experiments, mostly in the prokaryotic community. Overall, this work challenges the conception of viruses as main drivers of marine microbial diversity, emphasizing the need for more knowledge about virus-host interactions in the oceans