A pilot study investigating the potential of antimicrobial photodynamic therapy (aPDT) to control Vibrio spp. development in microalgae and seawater

Antimicrobial photodynamic therapy (aPDT) uses light-specific wavelengths and nontoxic photosensitisers to eradicate target microbial cells. Bivalve hatchery systems are challenged with bacterial contamination of microalgae feed, which can be detrimental to all production life stages. The objectives of this study were to investigate the antimicrobial potential of blue LED and red laser aPDTs in a marine environment. The bacteria Vibrio spp. were targeted, as they have been associated with significant Pacific oyster Crassostrea gigas mortalities worldwide. The compartments tested were (a) microalgae feed (Tetraselmis sp., Isochrysis sp. and Chaetoceros muelleri) and (b) tank seawater. C. gigas seed-fed light-treated and untreated microalgae for 8 weeks were also screened for Vibrio spp. Molecular diagnostics (PCR, qPCR and Sanger sequencing) were used. The aPDTs were deemed non-destructive to the microalgae. V. splendidus was detected exclusively in C. muelleri growth cultures. Tank seawater samples were positive for V. splendidus pre-treatment. A complete eradication of V. splendidus RNA (indicative of pathogen viability) was observed in seawater samples treated by aPDTs, while no V. splendidus DNA (pathogen may be viable or not) was observed in seawater samples treated by red laser and a 33% reduction occurred when treated with LED. Of significance, a reduction of V. splendidus DNA and RNA was observed in oysters fed the aPDT-treated microalgae versus those oysters fed the untreated diet. This pilot study indicates that there is the potential to use aPDT to treat against pathogenic bacteria, in particular Vibrio spp., in hatchery microalgal feed and tank water.