The emergence of toxigenic cyanobacteria in Louisiana estuarine systems raises concerns regarding the impacts of fitness and public health risks of exposure to valuable shellfish species like the eastern oyster, Crassostrea virginica. The freshwater cyanobacterium, Microcystis aeruginosa, is of particular concern due to its salt tolerance, production of the cyanotoxin microcystin, ability to outcompete other phytoplankton species when pulses of nutrient-rich freshwater are delivered to receiving basins, and documented presence in oligo- and mesohaline Louisiana estuaries. This spatiotemporal overlap of eastern oysters and M. aeruginosa makes monitoring efforts and research resolving feeding of the eastern oyster on this, and similar, species of pressing importance. However, individual cells of M. aeruginosa are small (2-4 µm) but can also form large colonies (100 µm) comprised of hundreds to thousands of cells, both of which are difficult to enumerate accurately using traditional microscopy. Furthermore, morphological identification of M. aeruginosa, or other cyanobacteria species that produce toxins non-constitutively, does not tell us if a given cell is producing harmful cyanotoxins.
Through the application of molecular techniques (real-time quantitative PCR; RT-qPCR) we are detecting the presence of low concentrations (250 cells ml-1) of M. aeruginosa in local oyster habitats and determining which portion of those cells possess the genes to produce microcystin (approx. 65%). These data are providing important data that help explain microcystin concentrations in oyster tissues ranging from 0.194 to 0.941 ng MC g-1 oyster tissue wet weight (MCY-DM ELISA). We also used RT-qPCR to quantify possible rejection of a non-toxic strain of M. aeruginosa in lab-based oyster feeding experiments. These results showed that rejection of M. aeruginosa was consistent even with alternative prey available (Fig. 1), although total pseudofeces production rates increased with a diet rich in the cyanobacterium. Molecular methods are proving critical tools in enhancing our understanding of toxigenic cyanobacteria as components of the natural estuarine phytoplankton communities, relating species abundance to toxin accumulation in oyster tissue, and understanding oyster feeding behavior as these cells become more abundant in prey communities.