Identification and localization of polyketide synthase genes from cultures of diarrheic shellfish poisoning toxin producing dinoflagellates of the genus Prorocentrum

Aquatic toxins are responsible for a number of acute and chronic diseases in humans. Okadaic acid (OA) and other dinoflagellate derived polyketide toxins pose serious health risks on a global scale. Ingestion of OA contaminated shellfish causes diarrheic shellfish poisoning (DSP). Some evidence also suggests tumor promotion in the liver by OA. Microcystin-LR (MC-LR) is produced by cyanobacteria and is believed to be the most common freshwater toxin in the US. Humans may be exposed to this acute hepatotoxin through drinking or recreational use of contaminated waters. ^ OA producing dinoflagellates have not been cultured axenically. The presence of associated bacteria raises questions about the ultimate source of OA. Identification of the toxin-producing organism(s) is the first step in identifying the biosynthetic pathways involved in toxin production. Polyketide synthase (PKS) genes of toxic and non-toxic species were surveyed by construction of clonal libraries from PCR amplicons of various toxic and non-toxic species of Prorocentrum in an effort to identify genes, which may be part of the biosynthetic pathway of OA. Analysis of the PKS sequences revealed that toxic species shared identical PKS genes not present in non-toxic species. Interestingly, the same PKS genes were identified in a library constructed from associated bacteria. ^ Subsequent bacterial small subunit RNA (16S) clonal libraries identified several common bacterial species. The most frequent 16S sequences found were identified as species of the genus Roseobacter which has previously been implicated in the production of OA. Attempts to culture commonly occurring bacteria resulted in the isolation of Oceanicaulis alexandrii , a novel marine bacterium previously isolated from the dinoflagellate Alexandrium tamarense, from both P. lima, and P. hoffmanianum. ^ Metabolic studies of microcystin-LR, were conducted to probe the activity of the major human liver cytochromes (CYP) towards the toxin. CYPs may provide alternate routes of detoxification of toxins when the usual routes have been inhibited. For example, some research indicates that cyanobacterial xenobiotics, in particular, lipopolysaccharides may inhibit glutathione S-transferases allowing the toxin to persist long enough to be acted upon by other enzymes. These studies found that at least one human liver CYP was capable of metabolizing the toxin. ^

Potential role of sponge communities in controlling phytoplankton blooms in Florida Bay

An unprecedented series of ecological disturbances have been recurring within Florida Bay since the summer of 1987. Persistent and widespread phytoplankton and cyanobacteria blooms have coincided with the large scale decimation of sponge communities. One hypothesis is that the large scale loss of suspension-feeding sponges has rendered the Florida Bay ecosystem susceptible to these recurring blooms. The primary objective of this study was to experimentally evaluate the potential for suspension-feeding sponges to control nuisance phytoplankton blooms within Florida Bay prior to a large sponge die-off event. To achieve this objective, we determined the extent and biomass of the surviving sponge community in the different basins of Florida Bay. Many areas within Florida Bay possessed sponge densities and biomasses of 1 to 3 ind. m–2 or 100 to 300 g m–2 respectively. The dominant species includedSpheciospongia vesparia, Chondrilla nucula, Cinachyra alloclada, Tedania ignis and Ircinia sp., which accounted for 68% of individual sponges observed and 88% of sponge biomass. Laboratory grazing rates of these dominant sponges were experimentally determined on 4 different algal food treatments: a monoculture of cyanobacteria Synechococcus elongatus, a monoculture of the diatom Cyclotella choctawhatcheeana, a monoculture of the dinoflagellate Prorocentrum hoffmanianum, and an equal volume of the 3 monocultures combined. To estimate the impact of a mass sponge mortality event on the system-wide filtration rate of Florida Bay, we combined estimates of the current sponge biomass and laboratory sponge filtration rates with estimates of mean volumes of the sub-basins of Florida Bay. This study implies that the current blooms occurring within the central region of Florida Bay can be explained by the loss of the dominant suspension feeder in this system, and there is no need to invoke a new addition of nutrients within this region for the blooms to occur.