Exploration of Phoslock® clay in mitigating Prymnesium parvum fish-killing algal blooms in aquaculture ponds

When in 2009 a tropical Australian barramundi farm suffered from fish-killing <i>Prymnesium parvum</i> blooms, the farm manager decided to manipulate N:P nutrients by adding Phoslock^® bentonite clay and adjusting pH through the addition of molasses. Phosphate levels were maintained at <0.03 mg/L, phytoplankton biomass and dissolved oxygen became more stable and <i>Prymnesium</i> disappeared from the system altogether. We conducted <i>Prymnesium</i> culture experiments under different N:P ratios to interpret field observations, while measuring toxins using the RTgill-Wl cell line assay. Methanol extracts of nitrogen deficient cells were the most toxic during stationary phase, followed by phosphorus deficient cells and with nutrient sufficient cultures least toxic (7, 20, 65% gill cell viability, respectively). Sonicated whole-cell cultures were more toxic after 4h (0-13% gill cell viability) but lost significant toxicity after 24h in the dark, suggesting toxin degradation. Phoslock^® effectively removed 60-100% of water soluble toxicity of live <i>Prymnesium</i> and offers great potential as on-farm emergency response. We are currently refining clay type and dosage, including exploration of application of clays to other ichthyotoxic algae.