Modulation of ecdysal cyst and toxin dynamics of two Alexandrium (Dinophyceae) species under small-scale turbulence

Some dinoflagellate species have shown different physiological responses to certain turbulent conditions. Here we investigate how two levels of turbulent kinetic energy dis- sipation rates (ε = 0.4 and 27 cm2/s3) affect the PSP toxins and ecdysal cyst dynamics of two bloom forming species, Alexandrium minutum and A. catenella. The most striking responses were observed at the high ε generated by an orbital shaker. In the cultures of the two species shaken for more than 4 days, the cellular GTX(1+4) toxin contents were sig- nificantly lower than in the still control cultures. In A. minutum this trend was also observed in the C(1+2) toxin content. For the two species, inhibition of ecdysal cyst production occurred during the period of exposure of the cultures to stirring (4 or more days) at any time during their growth curve. Recovery of cyst abundances was always observed when turbulence stopped. When shaking persisted for more than 4 days, the net growth rate significantly decreased in A. minutum (from 0.25±0.01 day−1 to 0.19±0.02 day−1) and the final cell numbers were lower (ca. 55.4%) than in the still control cultures. In A. catenella, the net growth rate was not markedly modified by turbulence although under long exposure to shaking, the cultures entered earlier in the stationary phase and the final cell numbers were significantly lower (ca. 23%) than in the control flasks. The described responses were not observed in the experiments performed at the low turbulence intensities with an orbital grid system, where the population development was favoured. In those conditions, cells appeared to escape from the zone of the influence of the grids and concentrated in calmer thin layers either at the top or at the bottom of the containers. This ecophysiological study provides new evidences about the sensitivity to high levels of small-scale turbulence by two life cycle related processes, toxin production and en cystment, in dinoflagellates. This can contribute to the understanding of the dynamics of those organisms in nature.

This work has been a joint effort of the Spanish projects TURFI (REN2002-01591/MAR), TURDITOX (CTM2005-03547/MAR), TURECOTOX (CTM2006-13884- C02-00/MAR) and VARITEC (CTM2004-04442-C02) and by the EU project SEED (GOCE-CT-2005-003875). L. Bolli held a “Leonardo da Vinci” grant within the StudEX program of Switzerland and G. Llaveria an FPU fellowship from the Spanish Ministry of Science and Education (SMSE). Ò. Guadayol had a CSIC I3P doctoral contract. E. Berdalet and F. Peters were “Ramón y Cajal” scientists (SMSE), and K. van Lenning had a contract from the CRA (Centre de Referència en Aqüicultura, CIRIT-Generalitat de Catalunya, Spain).

Peer reviewed