The importance of morphological versus chemical defences for the bloom-forming cyanobacterium Microcystis against amoebae grazing

Abstract

Amoebae grazing can be an important loss factor for blooms of the common cyanobacterium Microcystis. Some Microcystis strains seem to be protected against amoebae grazing, but it is unclear whether this is achieved by their colony morphology or biochemically. These factors were investigated in grazing experiments using two Microcystis-grazing amoebae (Korotnevella sp. and Vannella sp.) and two Microcystis strains with differing colony morphology (aeruginosa and viridis morphotype) and different sensitivity to amoebae grazing. Amoebae did not increase in density and failed to reduce the growth rate of cultures of the amoebae insensitive viridis strain, irrespective of whether the Microcystis strain was colonial or unicellular. This suggests that the extended mucilage matrix surrounding viridis colonies is not the main defence mechanism against amoebae grazing. At the same time, the growth rate of both unicellular and colonial cultures of the amoebae-sensitive aeruginosa strain was heavily reduced by the growing amoebae. The addition of filtered viridis-conditioned medium to aeruginosa cultures significantly decreased both amoebae growth and its effect on aeruginosa growth rates, which indicates that extracellular compounds constitutively produced by viridis are at least partially responsible for their insensitivity to amoebae grazing. These results demonstrate the potential importance of chemical interactions between lower trophic levels (protists) for Microcystis bloom dynamics

Microbial players involved in the decline of filamentous and colonial cyanobacterial blooms with a focus on fungal parasitism

“…Regarding that a major difference between both morphotypes was the thick of their mucilage matrix ( M. viridis presenting the densest, extended mucilage), the authors suggested that the thick mucilage layer of Microcystis colonies as well as its biochemical composition may be real physical and chemical barriers for the amoeba grazing. Moreover, some compounds excreted by M. viridis seem to play an important role in the grazing rate of amoebae (Van Wichelen et al ., ). Recently, Urrutia‐Cordero and colleagues () showed that cyanotoxins can alter the cytoskeleton structure of the grazer Acanthamoeba castellanii , which leads to its death.…”

Sedimentation Patterns of Toxin-Producing Microcystis Morphospecies in Freshwater Reservoirs

“…We hypothesize that the reduced settling velocities of M. flos-aquae at Valmayor may be related to a progressive disintegration of colonies into single cells, generally more susceptible to grazing losses than integer colonies. However, this phenomenon may vary among grazers as amoebas feed more easily on colonies than on single Microcystis cells [33]. …”

“Surface Browsing” May Allow “Filter-Feeding” Protozoa to Exert Top-Down Control on Colony-Forming Toxic Cyanobacterial Blooms

“…Additionally, protozoa may ingest the mucus matrix on the biofilm as a supplementary source of nutrients during grazing on the bacterial/algal biofilm. 46,51 Thus, through surface browsing, protozoa may have further impacts on Microcystis colonies.…”

“…Likewise, the mucus on the Microcystis colony surface can serve as a physical barrier against zooplankton grazing, but some protozoa, such as amoebae and ciliates, seem able to overcome this barrier, , which is supported by our observations (Supporting Information Video S3). Additionally, protozoa may ingest the mucus matrix on the biofilm as a supplementary source of nutrients during grazing on the bacterial/algal biofilm. , Thus, through surface browsing, protozoa may have further impacts on Microcystis colonies.…”