Tracking an Algal Predator: Monitoring the Dynamics of Vampirovibrio Chlorellavorus in Outdoor Culture

Abstract

The environmental conditions created in the Southwestern deserts of the United States are conducive to the production of green microalgae biomass, for use as a feedstock intended for conversion to carbon neutral liquid biodiesel. One promising heat-tolerant, rapidly-growing, high lipid content species is the chlorophyte, Chlorella sorokiniana (Shihira and Krauss, 1965) (isolate DOE 1412), which has been selected for pilot-scale production as part of a larger algal biofuels project to assess its potential for long-term productivity in open, outdoor monoculture production systems. Molecular analysis exposed the presence of the pathogenic bacterium, Vampirovibrio chlorellavorus (Gromov & Mamkaeva, 1972) causing infection and death of DOE 1412, which occurred most rapidly at air temperatures exceeding 34 °C. This Gram-negative bacterium has been reported to attach to and utilize the cellular contents of several Chlorella species, leading to yellowing and flocculation of algal cells, and death of the host. A quantitative PCR assay was developed to monitor pathogen accumulation using the 16S ribosomal RNA gene, in addition to the algal 18S ribosomal RNA gene for normalization. The assay is highly sensitive, with limits of quantification for the 16S and 18S gene targets calculated to be 19 and 131 copies, respectively. The qPCR assay was used to monitor several outdoor reactors inoculated with the DOE1412, throughout the warm season growth-to-harvest cycle to understand the disease cycle and inform disease management decisions. Further, the bacterium was monitored in paddlewheel DOE 1412 cultures treated with benzalkonium chloride (BAC), a biocide tested for the ability V. chlorellavorus attack of DOE 1412. The treatment resulted in a reduced growth rate for DOE 1412, but prolonged the duration of the production cycle resulting in increased total harvestable yield, compared to untreated control cultures.