The neurotoxin saxitoxin and related paralytic shellfish toxins are produced by multiple species of cyanobacteria and dinoflagellates. This study investigates the two saxitoxinproducing strains of Scytonema crispum, CAWBG524 and CAWBG72, isolated in New Zealand. Each strain was previously reported to have a distinct paralytic shellfish toxin profile, a rare observation between strains within the same species. Sequencing of the saxitoxin biosynthetic clusters (sxt) from S. crispum CAWBG524 and S. crispum CAWBG72 revealed the largest sxt gene clusters described to date. The distinct toxin profiles of each strain were correlated to genetic differences in sxt tailoring enzymes, specifically the open-reading frame disruption of the N-21 sulfotransferase sxtN, adenylylsulfate kinase sxtO, and the C-11 dioxygenase sxtDIOX within S. crispum CAWBG524 via genetic insertions. Heterologous overexpression of SxtN allowed for the proposal of saxitoxin and 3′-phosphoadenosine 5′phosphosulfate as substrate and cofactor, respectively, using florescence binding assays. Further, catalytic activity of SxtN was confirmed by the in vitro conversion of saxitoxin to the N-21 sulfonated analog gonyautoxin 5, making this the first known report to biochemically confirm the function of a sxt tailoring enzyme. Further, SxtN could not convert neosaxitoxin to its N-21 sulfonated analog gonyautoxin 6, indicating paralytic shellfish toxin biosynthesis most likely occurs along a predefined route. In this study, we identified key steps toward the biosynthetic conversation of saxitoxin to other paralytic shellfish toxins.

The diversity of the microbial communities associated with marine sponges has been extensively studied, but their functioning and interactions within the sponge holobiont are only recently being appreciated. Sponge-associated microorganisms are known for the production of a range of inhibitory metabolites with biotechnological application, but the ecological role that these compounds remains elusive. In this work, we explore the competitive interactions between cultivated sponge-associated bacteria to inspect whether bacteria that produce antimicrobial activities are able to inhibit potentially pathogenic bacteria. We isolated a Bacillus sp. bacterium with sponge-degrading activity, which likely has a negative impact on the host. This bacterium, along with other sponge isolates from the same genus, was found to be inhibited by a subpopulation of closely related sponge-derived Pseudovibrio spp. In some Pseudovibrio strains, these inhibitory activities were correlated with the genetic capacity to produce polyketides, such as erythronolide. Our observations suggest that antagonistic activities likely influence the composition of the sponge microbiome, including the abundance of bacteria that can be harmful to the host.

This review discusses cyanotoxin biosynthetic pathways and highlights the heterologous expression and biochemical studies used to characterise them.

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