Distribution of epiphytic bacteria on cells of Pseudo-nitzschia multiseries in culture

Toxicity of the domoic-acid-producing pennate diatom Pseudo-nitzschia multiseries is enhanced by bacterial association. Yet, apart from initial molecular studies nearly a decade ago, the diversity of bacteria in cultures of P. multiseries has not been explored. The goal of this study was to examine the frequency and distribution pattern of epiphytic bacteria growing on cultured cells of P. multiseries. Diatoms were grown in flasks of f/2 medium for 31 days. Every 5-12 days, 5 one-mL subsamples were transferred to a filtration apparatus and grown undisturbed for 2 days in order to maintain natural associations between the diatom host and its bacterial epiphytes. The subsamples were then gently prepared for SEM examination. From 15-40% of the diatom cells had attached bacteria. Most diatom cells carried only 1-5 bacteria, although some had up to 74 bacteria. Even in late stationary phase, many diatom cells were bacteria-free. A morphologically diverse bacterial flora was observed attached to the host diatoms, including: 1) stalked bacteria, 2) rod-shaped bacteria attached by their narrow side, 3) rods attached along their long side, and 4) cocci. Stalked bacteria were most common on the girdle band junctions of the cell. They were also attached to the raphe and striae. Rods resting on their long side were most common on the striae of the valve face. These preliminary results indicate that cultured diatoms co-exist with a diverse bacterial epiphytic flora, having a specific distribution pattern and frequency. Their effect on domoic acid production has yet to be determined.