Expansion of the Chlorovirus Genus by Studies on Virus Natural History and Chlorella Host Metabolism

Most animal-microbe symbiotic interactions must be advantageous to the hostand provide nutritional benefits to the endosymbiont. When the host providesnutrients, it can gain the capacity to control the interaction, promote self-growthand increase its fitness. Chlorella-like green algae engage in symbioticrelationships with certain protozoans, a partnership which significantly impactsthe physiology of both organisms. Consequently, it is challenging to grow axenicchlorella cultures after isolation from the host, as they are nutrient fastidious andsusceptible to virus infection. We hypothesize that the establishment of asymbiotic relationship spurred natural selection on nutritional and metabolic traitsthat differentiate symbiotic algae from their free-living counterparts. Here, wecompare metabolic capabilities of five symbiotic and four free-living Chlorellaalgae by determining growth levels on combinations of nitrogen and carbonsources. Data analysis by hierarchical clustering reveals clear separation of thesymbiotic and free-living Chlorella into two distinct clades. Symbiotic algaecannot metabolize NO3 but can utilize three symbiont-specific amino acids (Asn,Pro and Ser). These amino acids were exclusively affected by thepresence/absence of Ca in the medium, and differences were magnified ifgalactose but not sucrose or glucose was provided. Additionally, Chlorella