Effect of Temperature and Salt on Laboratory Growth of Vampirovibrio chlorellavorus and Killing of a Cultivated Chlorella Host

Abstract

Vampirovibrio chlorellavorus (Gromov et Mamkaeva, 1980) is a member of the phylum cyanobacteria that has been described as an obligate pathogen of several of the green microalga, Chlorella. It utilizes as yet unknown functions to access the contents of individual Chlorella sp. host cells, which results in cell death. Its presence in a cultivated Chlorella sorokiniana culture was first discovered using polymerase chain reaction (PCR) to amplify the 16S ribosomal DNA gene, followed by DNA sequencing. Its continued routine detection throughout much of the cultivation season suggested it was an endemic member of the phycosphere community in this open cultivation system, located in Tucson, Arizona. Ultimately, its presence resulted in rapid death of C. sorokiniana in open pond systems and reduced biomass harvest. PCR analysis of total DNA isolated from sand and soil layers removed from a nearby riverbed indicated that V. chlorellavorus resides naturally in the riverbed. The ability to manage this bacterial pathogen in cultivated Chlorella host species is hindered by the limited information available in the literature regarding the biological and genomic characteristics of V. chlorellavorus. The objective of this study was to identify environmental factors that trigger the apparent increased growth rate of V. chlorellavorus and rapid algal death during the cultivation cycle. In laboratory experiments, V. chlorellavorus was shown to cause death of C. sorokiniana when the temperature exceeded 28°C, whereas, algal death was not observed when the temperature was 24°C or lower, among the temperatures tested. Also, the bacterium was more pathogenic to C. sorokiniana, grown in open cultivation systems during the summer months, compared to the cooler season months. Futhermore, when C. sorokiniana and V. chlorellavorus were co-cultivated in the presence of sodium chloride ranging from 0-10 g per liter, the growth of the bacterium was not impeded to any extent that might suggest C. sorokiniana was rendered less susceptibility to pathogen attack. Future work involves examining more triggers and ways to inhibit V. chlorellavorus growth.