Marine guanidinium neurotoxins: Biogenic origins and interactions, biosynthesis and pharmacology

Naturally occurring neurotoxins belonging to two structurally distinct groups of guanidinium alkaloids known collectively as saxitoxins (STXs) and tetrodotoxins (TTXs) share a high affinity and ion flux blockage capacity for voltage-gated sodium ion channels (NaV). Both toxin groups are produced by marine microorganisms and widely distributed among vector species in the oceans, but are also found in terrestrial species. The STXs are often referred to as paralytic shellfish toxins (PSTs) based on their accumulation in shellfish and the symptoms in humans after consumption of toxic seafood. Biosynthesis of STXs is confirmed in four genera of marine dinoflagellates and among about a dozen species of primarily freshwater and brackish water strains of filamentous cyanobacteria. The origin of the STX biosynthetic genes in dinoflagellates remains controversial and may represent multiple horizontal gene transfer (HGT) events from progenitor bacteria and/or cyanobacteria. The recent identification of the biosynthetic genes for STX analogs in both cyanobacteria and dinoflagellates has yielded insights into mechanisms of toxin heterogeneity among species and the evolutionary origins of the respective elements of the toxin gene cluster. The biogenic origins of TTXs and tetrodotoxicity remain even more enigmatic. The TTXs occur primarily in marine pufferfish species, and hence tetrodotoxicity is frequently described as pufferfish poisoning (PFP) after the toxin syndrome in human consumers of such toxic fish. In marine environments, TTXs also appear in invertebrate species, particularly of benthic feeders on suspended particulates and carnivorous vector species. Symbiotic colonizing bacteria or free-living bacteria sequestered via feeding from the water column or sediments are most often invoked as proximal sources of TTXs in marine macrofauna, but endogenous biosynthesis independent of bacteria cannot be excluded. The TTX biosynthetic pathway has not been completely elucidated, and the biosynthetic genes are unknown

Associated Bacteria and Their Effects on Growth and Toxigenicity of the Dinoflagellate Prorocentrum lima Species Complex From Epibenthic Substrates Along Mexican Coasts

The dinoflagellate genus Prorocentrum is globally represented by a wide variety of species found upon benthic and/or epiphytic substrates. Many epibenthic Prorocentrum species produce lipophilic polyether toxins, some of which act as potent protein phosphatase inhibitors and tumor-promoters associated with Diarrheic Shellfish Poisoning (DSP). Most members of the Prorocentrum lima species complex (PLSC) commonly found in the tropics and sub-tropics are toxigenic. Epiphytic and planktonic bacteria co-occur with toxigenic Prorocentrum but reciprocal allelochemical interactions are under-investigated. The aim of the present study was to identify the culturable bacteria collected together with isolates of the PLSC from seagrass (Thalassia testudinum) and macroalgae along tropical Atlantic coasts of Mexico, and to explore potential species interactions with selected isolates. Twenty-one bacterial genera belonging to Proteobacteria, Actinobacteria, and Bacteroidetes were identified by amplification of the 16S rRNA gene marker from nine clonal Prorocentrum cultures, with gamma-proteobacteria comprising the dominant class. A positive correlation was found between the bacterial genera associated with two Prorocentrum clones and the esterified toxin analog DTX1a-D8, but there was no apparent correlation between the other PLSC clones and their associated bacteria with the other five DSP toxins detected. No bacteriostatic or allelochemical response was found for cell-and culture medium extracts of five Prorocentrum isolates assayed for bioactivity against Staphylococcus sp. DMBS2 and Vibrio sp. HEL66. Bulk cell-washing of Prorocentrum PA1, followed by growth with antibiotics, was only effective in reducing bacterial load in the initial growth stages, but did not yield axenic cultures or lower bacterial cell densities throughout the culture cycle. Antibiotic treatment did not impair growth or survival of the dinoflagellate, or apparently affect DSP toxin production. There was no significant correlation between Prorocentrum cell volume, growth rate, bacterial cell counts, or cellular toxin concentration over the entire time-series culture cycle. Benthic Prorocentrum and associated bacterial communities comprise highly diverse and characteristic microbiomes upon substrates, and among compartments in culture, but this study provides little evidence that allelochemical interactions among Prorocentrum cells and associated bacteria originating from epibenthic substrates play a definable role in growth and toxigenicity