Assessment of microzooplankton grazing on Heterosigma akashiwo using a species-specific approach combining quantitative real-time PCR (QPCR) and dilution methods.

Delaware's Inland Bays (DIB) are subject to numerous mixed blooms of harmful raphidophytes each year, and Heterosigma akashiwo is one of the consistently occurring species. Often, Chattonella subsalsa, C. cf. verruculosa, and Fibrocapsa japonica co-occur with H. akashiwo, indicating a dynamic consortium of raphidophyte species. In this study, microzooplankton grazing pressure was assessed as a top-down control mechanism on H. akashiwo populations in mixed communities. Quantitative real-time polymerase chain reaction (QPCR) with species-specific primers and probes were used in conjunction with the dilution method to assess grazing pressure on H. akashiwo and other raphidophytes. As a comparison, we measured changes in chlorophyll a (chl a) to determine whole community growth and mortality caused by grazing. We detected grazing on H akashiwo using QPCR in samples where chl a analyses indicated little or no grazing on the total phytoplankton community. Overall, specific microzooplankton grazing pressure on H akashiwo ranged from 0.88 to 1.88 day(-1) at various sites. Experiments conducted on larger sympatric raphidophytes (C. subsalsa, C. cf. verruculosa and F japonica) demonstrated no significant microzooplankton grazing on these species. Grazing pressure on H akashiwo may provide a competitive advantage to other raphidophytes such as Chattonella spp. that are too large to be consumed at high rates by microzooplankton and help to shape the dynamics of this harmful algal bloom consortium. Our results show that QPCR can be used in conjunction with the dilution method for evaluation of microzooplankton grazing pressure on specific phytoplankton species within a mixed community.