The loss of mixotrophy in Alexandrium pseudogonyaulax: Implications for trade-offs between toxicity, mucus trap production, and phagotrophy

Abstract

Two important competitive traits in protists, particularly in harmful algal bloom forming species, are lytic toxicity and mixotrophy. This study focuses on Alexandrium pseudogonyaulax, a dinoflagellate species that is both toxic and mixotrophic, and in addition uses a mucus trap to immobilize and capture potential prey. A single strain of A. pseudogonyaulax was subcultured and grown under two different conditions; one subculture was grown autotrophically, while the other was periodically offered algal prey. After 3 yr, the substrain that was fed remained an avid mixotroph, while the autotrophically grown substrain almost completely lost its ability to feed. The emergence of these two “substrains,” with different nutritional strategies, allowed us to investigate possible trade-offs between lytic toxicity, mixotrophy, and mucus trap production. Both substrains were still capable of producing mucus traps, and successfully captured other cells. Although the nonfeeding substrain lost its ability to feed, it was more lytic than the feeding substrain, which may suggest a trade-off between lytic toxicity and feeding ability. However, this increased toxicity was not enough to outcompete other faster growing competitors such as Teleaulax acuta and Heterocapsa rotundata. In contrast, the feeding substrain was able to inhibit the growth of these same competing species. The results indicate that for A. pseudogonyaulax, the benefits of phagotrophy far outweigh those of lytic toxicity or the mucus trap alone. The nonfeeding strain used here provides evidence of the loss of mixotrophy under culture conditions, highlighting one of the obstacles in the study of mixotrophic protists.