Microcystins do not provide anti-herbivore defence against mixotrophic flagellates.pdf

AQUATIC MICROBIAL ECOLOGY Aquat Microb Ecol Vol. 59: 207–216, 2010 doi: 10.3354/ame01395 Published online April 14 INTRODUCTION Cyanobacterial blooms have increasingly become a nuisance in many freshwater and brackish ecosystems (Chorus Bartram 1999, Carmichael 2001, Huisman et al. 2005). They are facilitated by high water tempera- tures and nutrient load and therefore benefit from anthropogenic influences like eutrophication and cli- mate warming (Codd 2000, J?hnk et al. 2008, Paerl Huisman 2008). Microcystin-producing cyanobacteria of the Microcystis genus are widely distributed bloom- forming species. The hepatotoxic microcystins inhibit protein phosphatases in eukaryotes (MacKintosh et al. 1990) and are therefore toxic to many potential grazers of Microcystis (Fulton Paerl 1987b, DeMott et al. 1991). Other factors contributing to the low edibility of Microcystis are the formation of colonies leading to interference with the feeding behaviour of many zoo- plankton species (Fulton Paerl 1987a, Yang et al. 2006) and the low content of long-chain polyunsatu- rated fatty acids and sterols, which are essential for many herbivores (Wacker von Elert 2001, von Elert et al. 2003). While most experiments investigating effects of toxic cyanobacteria on pelagic grazers have been done with metazoans, several protozoa are able to maintain high growth rates when feeding on cyanobacteria including Microcystis (Cole Wynne 1974, Nishibe et al. 2002, Kim et al. 2006). Especially mixotrophic chrysophytes, such as Ochromonas spp. and Poterioochromonas spp., can graze efficiently on Microcystis and even degrade the cyanotoxins (Ou et al. 2005, Zhang et al. 2008, Van Donk et al. 2009). The genus Ochromonas is abundant in freshwaters and has been reported to frequently co- occur with Microcystis (Van Donk et al. 2009). Ochro- monas has therefore been proposed as a potential bio- logical control agent against Microcystis blooms (Cole Wynne 1974, Zhang et al. 2008), although little