Vibrio parahaemolyticus is a gastrointestinal pathogen that is abundant in coastal marine environments. Elevated numbers of V. parahaemolyticus cells have been correlated with marine microalgae blooms, particularly blooms of diatoms and dinoflagellates, but the nature of the relationship between V. parahaemolyticus and microalgae is unknown. We performed in vitro assays using 27 environmental V. parahaemolyticus strains and various phototrophs; a diatom, a dinoflagellate, unarmored and armored forms of a coccolithophore, and two species of cyanobacteria. The V. parahaemolyticus strains we employed contained different combinations of virulence-correlated genes, the hemolysin genes tdh and trh, the Type III Secretion System 2 (T3SS2) marker gene vscC2, and the Type VI Secretion System (T6SS) marker gene vipA1. We determined that all V. parahaemolyticus strains, even strains in which no virulence factor genes were detected, were able to cause decreases in diatom, dinoflagellate, and unarmored coccolithophore biomass in vitro. No correlation between content of any virulence gene and damage to microalgae was apparent. We hypothesize that marine microalgae represent a reservoir of nutrients that the copiotroph V. parahaemolyticus can utilize in salt marsh environments, which are often poor in labile carbon and energy sources. This helps to explain the recent correlations between V. parahaemolyticus and microalgae blooms in such environments.
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