Growth and grazing of microzooplankton in response to the harmful alga Heterosigma akashiwo in prey mixtures

Abstract

Microzooplankton grazers may play a role in regulating blooms of the ichthyotoxic alga Heterosigma akashiwo. This study tested the effects of H. akashiwo, when part of a mixed-prey assemblage, on the growth and feeding of microzooplankton. Laboratory cultures of 3 ciliates, Favella sp., Metacylis sp., and Strombidinopsis acuminatum, were exposed to reciprocal concentrations of H. akashiwo and a non-toxic prey, at saturating prey concentrations. Heterosigma akashiwo was toxic to Favella sp. and Metacylis sp. when H. akashiwo was the sole prey species; however, this toxicity was eliminated in the mixed-prey treatments, likely because of avoidance of H. akashiwo and selective feeding on non-toxic prey. In contrast, the growth rate of S. acuminatum was unaffected by H. akashiwo. Both Favella sp. and S. acuminatum ingested H. akashiwo but selected against the alga when other prey was available. In addition, natural planktonic communities, collected from East Sound, Orcas Island, northern Puget Sound, in September and October 2007, were exposed to bloomlevel concentrations of H. akashiwo. Abundances of the major microzooplankton types, primarily ciliates and Gyrodinium/Gymnodinium dinoflagellates, were unaffected by H. akashiwo, although slight changes in grazer size structure did occur. Heterosigma akashiwo was harmful to the smallest grazers, mainly aloricate ciliates and small Gyrodinium/Gymnodinium dinoflagellates, and beneficial to larger Gyrodinium/Gymnodinium dinoflagellates that were able to ingest and grow on the alga. The alga was not consumed by the majority of grazers. Preferential feeding on alternate prey reduces toxic effects of H. akashiwo on microzooplankton. Avoidance of H. akashiwo by a major group of grazers would promote bloom formation by reducing H. akashiwo mortality and focusing community grazing pressure on potential competitor species.

An inducible antipredatory defense in haploid cells of the marine microalga Emiliania huxleyi (Prymnesiophyceae)

“…This prey species promotes good ciliate growth and is easily discriminated from E. huxleyi under epifluorescence microscopy (see below) due to its much larger size. Between 24 and 36 h before experiments, the majority of remaining prey was removed from S. acuminatum cultures by gentle reverse sieving through 20 mm mesh (Graham and Strom 2010). After reverse sieving, cultures were diluted with ciliate medium to the desired S. acuminatum concentration.…”

Broad Salinity Tolerance as a Refuge from Predation in the Harmful Raphidophyte Alga Heterosigma akashiwo (Raphidophyceae)

“…, Clough and Strom , Bearon et al. , Graham and Strom , Fredrickson et al. , Harvey and Menden‐Deuer ).…”

Avoidance, movement, and mortality: The interactions between a protistan grazer and Heterosigma akashiwo, a harmful algal bloom species

“…It is known that Favella sp. will reject captured H. akashiwo cells (Taniguchi and Takeda 1988;Stoecker et al 1995), and will discriminate against H. akashiwo in prey mixtures (Graham and Strom 2010). However, when H. akashiwo is ingested by Favella sp., mortality of the ciliate rapidly follows (Verity and Stoecker 1982;Clough and Strom 2005;Graham and Strom 2010).…”

“…Culture of microorganisms-A strain of Heterosigma akashiwo (CCMP 2809), known to be toxic to some heterotrophic protists (Graham and Strom 2010) was used for all behavior experiments. Heterocapsa triquetra (unknown origin) and Isochrysis galbana (CCMP 1323) were used to rear ciliate cultures.…”

“…Laboratory research has shown that the heterotrophic dinoflagellates Oxyrrhis marina, Noctiluca scintillans, and Stoeckeria algicida feed readily on H. akashiwo (Jeong et al 2003;Clough and Strom 2005). However, other protists, such as large ciliates, die when they ingest H. akashiwo cells (Verity and Stoecker 1982;Clough and Strom 2005;Graham and Strom 2010). Thus, while some predators readily consume H. akashiwo others die or avoid feeding on this HAB species.…”