Effects of two strains of the parasitic dinoflagellate Amoebophrya on growth, photosynthesis, light absorption, and quantum yield of bloom-forming dinoflagellates

Abstract

Eukaryotic parasites are believed to play important roles in bloom dynamics of red-tide dinoflagellates; however, little is known about their impact on host physiology and behavior. To address those issues, we examined the influence of parasitic dinoflagelates, Amoebophrya spp., on growth, photosynthesis, light absorption, and quantum yield of the bloom-forming dinoflagellates Akashiwo sanguinea and Gymnodinium instriatum. Parasites of the 2 host species differed in their site of infection, developing in the nucleus of A. sanguinea but in the cytoplasm of G. instriatum, and had divergent effects on host photophysiology. Neither host species appeared competent to repro- duce once infected, as growth of fully infected populations was negligible and cell division of infected hosts was never observed. Uninfected populations of both host species exhibited strong diel period- icity in photosynthesis, with parasitized cultures showing distinctly different patterns. Infected A. sanguinea displayed little or no photosynthetic periodicity, whereas diel periodicity continued in par- asitized G. instriatum but was less pronounced than that of uninfected host. Chlorophyll a (chl a) con- tent of A. sanguinea declined steadily over the infection cycle, while per cell and per chl a photo- synthetic rates decreased sharply until 16 h and then stabilized at ~50 pgC cell -1 h -1 and ~1 mgC (mgchla) -1 h -1 , respectively. By comparison, chl a content of infected G. instriatum was comparable with that of uninfected cells, with photosynthetic performance remaining high (~80% of uninfected hosts) until very late in the infection cycle. Light absorption by hosts increased in the blue region and decreased in the red region of the spectrum during the infection cycle, consequently enhancing chl a- specific absorption coefficients relative to uninfected cells by as much as 22 to 56% for A. sanguinea and 59% for G. instriatum. Furthermore, parasitism lowered maximum quantum yields in photosyn- thesis of both hosts by a factor of ~2, particularly in late infection stages. The contrasting effects of intranuclear and intracytoplasmic strains of Amoebophrya on photosynthetic performances and photo- physiological properties of host cells suggest that these parasites may exert somewhat different influ- ences on primary production and microbial activities during epidemic outbreaks in natural systems.