Effects of different levels of N- and P-deficiency on cell yield,okadaic acid,DTX-1, protein and carbohydrate dynamics in the benthic dinoflagellate Prorocentrum lima

Prorocentrum lima (Ehrenberg) Dodge is a cosmopolitan epiphytic dinoflagellate that produces biotoxins which are causative of diarrhetic shellfish poisoning (DPS). Here we report on effects of several nitrogen (N) and phosphorous (P) limited conditions on cell yield, okadaic acid (OA) and dinophysistoxin-1 (DTX-1) contents synoptically with cell carbohydrate, exopolysaccharide (EPS) and cell protein concentrations in a P. lima strain isolated from the Sacca di Goro lagoon (Northern Adriatic Sea). Batch culture experiments were set to assess changes induced by four nitrogen-limited levels (1/3-N, 1/10-N, 1/20-N, and 1/50-N) and four phosphorus-limited levels (1/3-P, 1/10-P, 1/20-P, and 1/50-P) with respect to control nutrient conditions (f/2 medium; NO₃⁻ and PO₄³⁻ concentrations: 883 and 36.3 μM, respectively; N/P ratio: 24). Low nutrients availability determined lower cell yields starting from 1/10-N and 1/3-P levels and the pattern observed was dependent on nutrient dynamics, as shown by N and P analyses performed in culture media during growth. Final cell yield decreased significantly up to 4.7- and 5.6-fold under 1/50-N and 1/50-P-limited levels with respect to control values, while cell volume increased with respect to control (up to 30% and 35% for N- and P-experiment, respectively). On overall, OA concentration ranged from 6.69 to 15.80 pg cell⁻¹, while DTX-1 ranged from 0.12 and 0.39 pg cell⁻¹ resulting in unusual high OA/DTX-1 ratios. The study indicates that protein, carbohydrate, EPS, and toxin concentrations displayed remarkable different patterns under the two kinds of nutrient deficiencies. The main differences can be summarised as: (i) significant decrease of cell protein concentration (up to 2-fold) under N-limitation, conversely no significant changes in protein concentration under P-limitation; (ii) significant increase of cell carbohydrate (up to 2.8-fold and 3.4-fold for N- and P-limitation, respectively) and cell OA amount (up to 1.9-fold and 2.3-fold, N- and P-limitation, respectively) under both N- and P-limitations, however different level-deficiency dependent patterns were displayed under the two nutrient conditions; (iii) significant increase of EPS concentration (up to 6.50-fold) under P-limitation, conversely no significant changes in EPS concentration under N-limitation. Data presented here indicate that P. lima adopts different eco-physiological strategies to face N-limitation or P-limitation. This study provides the first evidence for an increase in EPS production by benthic dinoflagellates under P-limited conditions; the ecological significance of this increase is discussed.