Feeding by phototrophic red-tide dinoflagellates: five species newly revealed and six species previously known to be mixotrophic

Abstract

We report here for the first time that 5 red-tide dinoflagellates (Gymnodinium catenatum, G. impudicum, Lingulodinium polyedrum, Prorocentrum donghaiense, and P. triestinum) which had been previously thought to be exclusively autotrophic dinoflagellates are mixotrophic species. We in- vestigated the feeding behaviors, the kinds of prey species that 11 mixotrophic red-tide dinoflagellates (Akashiwo sanguinea, Alexandrium tamarense, G. catenatum, G. impudicum, Heterocapsa triquetra, L. polyedrum, P. donghaiense, P. micans, P. minimum, P. triestinum, and Scrippsiella trochoidea) fed on, and the effects of the prey concentration on the growth and ingestion rates of P. donghaiense, H. triquetra, P. micans, and L. polyedrum when feeding on algal prey. We have also calculated grazing coefficients by combining field data on abundances of P. donghaiense, H. triquetra, P. micans, and L. polyedrum and co-occurring prey species. All algal predators tested in the present study ingested small phytoplankton species that had equivalent spherical diameters (ESDs) < 12 μm. A. sanguinea and L. polyedrum were able to ingest large phytoplankton species such as H. triquetra, S. trochoidea, and A. tamarense. Prorocentrum spp. fed on prey by engulfing the prey cell through body sutures, while S. trochoidea engulfed prey through the apical horn as well as through the sulcus. Specific growth rates of P. donghaiense, H. triquetra, and P. micans on a cryptophyte and L. polyedrum on P. minimum and S. trochoidea increased with increasing mean prey concentration, with saturation occur- ring at mean prey concentrations of 110 to 480 ng C ml -1 . The maximum specific growth rates (mixotrophic growth) of P. donghaiense, H. triquetra, and P. micans on the cryptophyte were 0.510, 0.283, and 0.197 d -1 , respectively, under a 14:10 h light:dark cycle of 20 μE m -2 s -1 , while their growth rates (phototrophic growth) under the same light conditions without added prey were 0.375, 0.184, and 0.106 d -1 , respectively. The maximum specific growth rates of L. polyedrum on P. minimum and S. trochoidea were 0.254 and 0.303 d -1 , respectively, under a 14:10 h light:dark cycle of 50 μE m -2 s -1 , while their growth rates without added prey were 0.157 and 0.182 d -1 , respectively. Maximum ingestion rates of P. donghaiense, H. triquetra, and P. micans on the cryptophyte were much lower than those of L. polyedrum on S. trochoidea and P. minimum. The calculated grazing coefficients of P. donghaiense, H. triquetra, and P. micans on the cryptophyte were up to 2.67, 0.091, and 0.041 h -1 , respectively, while those of L. polyedrum on small Prorocentrum spp. and S. trochoidea were up to 0.026 and 0.011 h -1 , respectively. The results of the present study suggest that the algal predators sometimes have a potentially considerable grazing impact on populations of the algal prey.