Nitrogen and phosphorus limitation effects on cell growth, biovolume, and toxin production in Ostreopsis cf. ovata

Ostreopsis cf. ovata is an epiphytic/benthic dinoflagellate that produces palytoxin-like compounds (putative palytoxin, ovatoxin-a, -b, -c, d and –e). Here we report on effects of nitrogen (N) and phosphorous (P) limited conditions on cell growth, cell size, biovolume, and toxin production of an O. cf. ovata strain isolated from the Adriatic Sea (Italy). Experiments were carried out in batch cultures using nitrate (NO3-) and phosphate (PO43-) as nutrient sources, and testing N:P ratios of 16, 5, 92 (control, N-limited and P-limited conditions, respectively). Residual N and P in the medium, cell yield, toxin concentrations, and toxin composition were analyzed throughout the growth. Two distinct cell size classes were identified and named class 1 (small cells) and class 2 (large cells), whose relative contribution under control condition was about 30 and 70%, respectively. N-limitation affected cell size, with significantly higher abundance (16%) of small cells being recorded under N stress than under control and P stress conditions. Conversely, P-limitation induced an increase of cell volume all over the growth cycle. Nutrient limitations affected growth rates and reduced final cell yields of 2.2-fold and 1.8-fold for N- and P-limited treatments versus control, respectively. Under all tested conditions O. cf. ovata showed the same qualitative profile, leading to a slight different contribution of each toxin to the total toxin content. On overall, toxins showed increasing concentrations from early to late stationary growth phase; particularly under control condition total toxin content increased from 13 to 24 pg cell-1. Nutrient limitations affected toxin production, which resulted significantly lower than control in late stationary phase, especially under N-deficiency: a 53% and 40% decrease in toxin cell content was observed under N- and P-limited conditions, respectively.