Marine algal toxins: Biochemistry, genetics, and molecular biology

Toxic compounds are frequently, but not universally, associated with alg,al blooms. Several dinoflagellate and diatom species are capable of toxin synthesis. Some bacteria are also capable of synthesizing at least one family of “algal” toxins. Previous work on algal toxins can be broadly grouped in four categories: observations on the variability in toxin production by single species or among strains of a species, frequently as a function of environmental growth conditions; isotope feeding studies to reveal the identity of the substrates that are precursors to the toxin compounds; the genetics of toxin production; and the pharmacological aspects of toxins. Information gleaned from these studies provides a firm foundation for launching more contemporaneous research efforts to understand the biochemistry and molecular biology of toxins. The goals are to develop an understanding of the machinery (i.e. the enzymes and the genes that encode them) required to synthesize toxins, to understand how this machinery is regulated by environmental conditions, and gain insights as to how the tox n biosynthetic genes evolved and(or) have been spread through the marine community. The production of toxic compounds is a common, but not universal, characteristic of harmful algal blooms (HABs). Surprisingly, only a few HAB toxins have been characterized and these are synthesized by only a limited number of algal species (Steidinger 1993). Toxigenic algae may, however, exert an inordinately large adverse impact on other members of the community because toxins can flow through aquatic food chains in a manner analogous to the movement of carbon or energy (Smayda 1992). Toxigenic algae thus may have significant impacts on ecosystem processes as toxins affect viability, growth, fecundity, and recruitment of a wide range of organisms. This manuscript is concerned with the biochemistry and genetics of toxin production by HAB species. Most information is for marine toxins as these have been better documented than the freshwater toxins. Saxitoxins, the causative agent of paralytic shellfish poisoning, are used as a model throughout this review as they are the best characterized marine toxin in terms of biochemistry and genetics. The physiology and ecology of the organisms that synthesize saxitoxins are also better documented than for other HAB species; this information permits a more robust interpretation of biochemical and genetic data for the saxitoxins than for other HAB toxins. Inauspiciously, very little is known about the molecular biology of HAB toxins, making this topic difficult to review. The goal in writing the molecular section was to interpret the biochemical, genetic, and physiological data in a context that would encourage and arouse others to Acknowledgments I thank M. McKay, Z. Wci, and E. Smiley for their work in my lab. T. Martinson, W. Barber, D. M. Anderson, G. Taroncher-Oldenburg, and three anonymous reviewers provided useful comments on the manuscript. I thank D. M. Anderson and G. J. Doucette for providing unpublished data and M. Kodama, G. J. Doucette, and S. Franca for bacterial isolates described in the text. This work was sponsored by Sea Grant Alaska, NSF-Biological Oceanography, the Alaska Natural Resources Foundation, and the University of Alaska President’s Special Project Fund. The Instutite of Marine Science, University of Alaska, provided travel funds. pursue the development of molecular tools for the study of HAB toxin events. The rationale for molecular or biochemical studies of toxicity relates to many long-standing question about HAB toxins. For instance, results from several studies indicate that toxin synthesis is not a constitutive component of algal metabolism. Instead, both the extent to which algae accumulate toxins (i.e. the toxin content) as well as the number and quantity of individual toxins (i.e. the toxin composition) of algae are strongly influenced by environmental growth conditions. Thus, one obvious question pertains to how environmental and hydrological factors influence toxin synthesis. To answer this, we need to know how algae make toxins. There are man!7 related questions. Which metabolic pathways are involved in toxin synthesis? How did the genes encoding toxin biosynthetic enzymes evolve? How are these toxin biosyntheric genes passed on to offspring? Are these genes transmitted to other members of the population-community by sexual and (or) nonsexual mechanisms (e.g. transkingdom sex involving bacteria; viruses)? Do bacteria also have the gl=netic machinery required to make “algal” toxins? In essence, the development of molecular tools and a better understanding of biochemical pathways can provide a means whereby questions relating to toxins can be addressed in a straightforward fashion. Most of our previous approaches to these questions have, of necessity, been one of watching the hands of the clock rather than understanding what makes the hands move. Clearly, a better biochemical and molecular understanding of HAB toxin pl*oduction is needed. Moreover, the most expedient strategy for determining both “how” and “why” environment factors control toxin synthesis involves a multifaceted approal:h. First, more data are required on the effects of environmental conditions on the growth of and toxin production by HAB species. Second, information is needed on the genetics of toxin synthesis. Third, the pathways of toxin synthesis r,hould be elaborated. Fourth, using data derived from the three previous lines of research, it is important to develop a molecular understanding of the genes involved in toxin synthesis. This approach will allow us to