Protozoa-contaminated microalgae treated with bleach can be safely used as feed for growing oyster larvae

Protozoa contamination of microalgal cultures is a recurring issue in laboratories and bivalve hatcheries, often leading to significant schedule alterations and economic losses. This study evaluated the effectiveness of bleach (sodium hypochlorite) treatment in eliminating Uronema-like ciliates from cultures of Tisochrysis lutea. It assessed its impact on microalgal survival and integrity, as well as its potential use as feed for oyster (Crassostrea gigas) larvae. Growth, pediveliger yield, and metamorphic success of oyster larvae were assessed in 500 L culture tanks using both live and bleach-treated microalgae as feed. Bleach concentration significantly decreased within 4 min of addition to seawater, with the dissipation rate being highest in the presence of T. lutea. Aeration had no significant effect on microalgal cell density, regardless of whether the cells were treated with bleach, in both small (1 L) and large (500 L) culture volumes. Notably, 60 or 120 min of exposure to 375 and 750 μL-1 bleach L-1 effectively diminished the protozoa population without compromising the integrity of the microalgal cells. Oyster larvae fed on bleach-treated microalgae (375 μL L-1) exhibited a slower shell growth rate (7.2 μm d-1) and longer metamorphic period (8 days) than control larvae fed live microalgae (8.5 μm d-1 and 5 days, respectively). In contrast, the yield of metamorphosed larvae was higher in bleach-treated larvae. These results indicate that bleach, when followed by neutralization, can be safely used to eliminate protozoa in microalgal cultures and further use them as feed for rearing oyster larvae, offering a practical, effective, and inexpensive solution for hatcheries facing potential microalgal crashes due to protozoa contamination.