Unveiling the responses of Alexandrium pacificum to phosphorus utilization by physiological and transcriptomic analysis

Phosphorus (P) is an essential macronutrient impacting bloom formation of marine dinoflagellates. The dinoflagellate Alexandrium pacificum is a cosmopolitan species known to frequently cause dense blooms in estuarine and coastal waters worldwide, while the physiological and molecular responses of A. pacificum to P utilization are still not well understood. Herein, the growth, P utilization, toxin production and transcriptomes of A. pacificum grown under P-deficient, inorganic P-replete, and organic P-replete conditions were compared. The results indicated that P-deficient adversely affected the growth of A. pacificum and significantly down-regulated the expression of genes related to P transport and material metabolism, but enhanced the production of toxin. On the other hand, no significant differences were observed in growth and toxin production between the organic and inorganic P-replete treatments. However, genes involved in P transport, utilization and TCA cycle were significantly changed in the organic P-replete compared with the inorganic P-replete group, and the mechanisms underlying the use of various organic P compounds were different. These findings suggested that A. pacificum evolved diverse organic P utilization strategies to adapt to low P conditions, which might be a crucial factor driving bloom formation in a low inorganic P environment.