Variation in biochemical composition during encystment of the planktonic dinoflagellate Akashiwo sanguinea in N-limited cultures

Abstract

The planktonic dinoflagellate Akashiwo sanguinea is a commonly observed bloom-forming species in estuarine and coastal waters worldwide. Large scales of A. sanguinea bloom cause the mass mortality of fish, shellfish, and sea birds. The formation of resting cysts plays as a potential vector for its wide distribution and frequent recurrence of blooms; however, the biochemical characteristics of algal cells during their life cycle remain unclear. For the first time, the variations in cellular chlorophyll a (Chl a), protein, carbohydrate, and total lipid contents during encystment of A. sanguinea cultured in different nitrate concentrations, namely 0-, 25-, 50-, 100-, 200-, and 833-μM N-added treatment, were studied in the present study. The results indicated that resting cysts were formed in all the N treatments, with the highest encystment ratio of 3.34 ± 0.57% observed in the 100-μM N-added treatment, and the lowest rate of 0.80 ± 0.03% observed in the 883-μM N-added treatment. The levels of the four biochemical components varied significantly during the encystment process. The Chl a and protein levels were significantly lower in newly formed cysts than in vegetative cells; however, continuous accumulation of carbohydrates and total lipids occurred with the algal growth, particularly of carbohydrates in the resting cysts, which was more than tenfold greater than those in the vegetative cells. Low initial N concentrations were more favorable for carbohydrate accumulation in cysts than high N concentrations. The accumulated components may play vital roles for the substance of A. sanguinea during resting stage. The results provided fundamental information for an improved understanding of the physiological response of A. sanguinea during encystment.