AME 59:239-255 (2010)  -  DOI: https://doi.org/10.3354/ame01394

ABSTRACT: A bloom-forming dinoflagellate was isolated from coastal waters in western Korea during a red tide event in March 2008 and clonal cultures were established. The dinoflagellate was identified as Gymnodinium aureolum based on morphological and genetic analyses (GenBank accession no. FN392226). We report here for the first time that the red-tide dinoflagellate G. aureolum, which has previously been thought to be exclusively autotrophic, is a mixotrophic species. G. aureolum fed on algal prey using a peduncle. Among the algal prey provided, G. aureolum ingested heterotrophic bacteria, the cyanobacterium Synechococcus sp., and small algal species that had equivalent spherical diameters (ESDs) of ≤11.5 µm. However, it did not feed on larger algal species (ESD ≥12 µm) or the small diatom Skeletonema costatum. The specific growth rates for G. aureolum on the cryptophyte Teleaulax sp. increased continuously with increasing mean prey concentration before saturating at prey concentrations of ca. 190 ng C ml^–1 (11050 cells ml^–1). The maximum specific growth rate (mixotrophic growth) of G. aureolum on Teleaulax sp. was 0.169 d^–1, at 20°C under a 14:10 h light:dark cycle of 20 µE m^–2 s^–1, while its growth rate (phototrophic growth) under the same light conditions without added prey was 0.120 d^–1. The maximum ingestion and clearance rates of G. aureolum on Teleaulax sp. were 0.058 ng C grazer^–1 d^–1 (3.4 cells grazer^–1 d^–1) and 0.003 µl grazer^–1 h^–1, respectively. The calculated in situ grazing coefficient for G. aureolum on co-occurring cryptophytes ranged up to 0.498 d^–1. Bioassay results indicated that this strain of G. aureolum is not toxic. Results of the present study suggest that G. aureolum has a potentially considerable grazing impact on algal populations.

KEY WORDS: Growth · Harmful algal bloom · HAB · Ingestion · Mixotrophy · Peduncle · Protist