Toxicity variability in Alexandrium pacificum isolated from East China Sea: impact of temperature and light intensity

Suitable temperature and light intensity play important roles in the formation of harmful algae blooms (HABs), which can pose serious threats to aquatic ecosystems and human health. In this study, we measured the growth, physiological function, and paralytic shellfish toxins (PSTs) production of Alexandrium pacificum (CCMA-272), a strain isolated from East China Sea, at different temperatures (15, 20, and 25 °C) and light intensities (30, 60, and 90 µmol photons/(m²·s)). Results indicate that temperature and light intensity significantly affected the growth, physiology, and toxigenic potentials of A. pacificum. The optimal conditions for the growth of A. pacificum were observed at 20 °C under 60 µmol photons/(m²·s). Regarding the production of PSTs, this strain of A. pacificum produced 12 PSTs, including carbamate toxins: saxitoxin (STX), neosaxitoxin (NEO), and gonyautoxin 1–4 (GTX1, GTX2, GTX3, GTX4); dicarbamoyl toxins: dicarbamoylsaxitoxin (dcSTX), dicarbamoylgonyautoxin 2, 3 (dcGTX2, dcGTX3); and N-sulfocarbamoyl toxins: N-sulfocarbamoylgonyautoxin 1, 2 (C1, C2), and gonyautoxin 5 (GTX5). Among all the PSTs, C2 was the most abundant. Low temperature (15 °C) and high light intensity (90 µmol photons/(m²·s)) were beneficial for the production of PSTs in A. pacificum. When cultured at 20 and 25 °C, A. pacificum generated comparable total quantities of PSTs, yet the toxicity levels were lower at 25 °C. Intracellular PSTs contents were greater than extracellular PSTs contents, except those under the condition of 25 °C with 30 µmol photons/(m²·s). However, as the increase of temperature, A. pacificum released more amounts of analogues with higher toxicity levels (e. g., STX and dcGTX₂) into the environment than intracellularly. These findings emphasize the significant sensitivity of A. pacificum to temperature and light intensity, highlighting the importance of evaluating both intracellular and extracellular PSTs for assessing its toxicity and aiding in the prediction and management of HABs.