Alexandrium tamarense is a unicellular protist that causes Harmful algal blooms (HABs) and paralytic shellfish poisoning. The impacts of HABs on marine ecosystems and the seafood industry are substantial. A recent study estimated the average economic impact in the United States from HABs, from 1987 to 1992, to be $49 million a year including $18 million in damage to commercial fisheries. This figure does not include a loss of potential income generated by the vast shellfish resources in Alaska and Georges Bank that are closed due to paralytic shellfish poisoning caused by Alexandrium tamarense.
In addition, the saxitoxins produced by Alexandrium tamarense are suspected as a cause of mortality in sea birds and humpback whales and human intoxication and death. Very little is known about the factors that influence the formation of HABs or their recent spread to new areas. A critical requirement for controlling Alexandrium tamarense is the knowledge about its basic biology and toxin production, areas of research that will be significantly aided by the availability of a genomic resource for this species. Characterization of Alexandrium tamarense will also shed light into genome evolution in dinoflagellates, taxa that are renowned for their ability to incorporate multiple endosymbionts and to accumulate foreign genes through horizontal gene transfer (HGT).
Paralytic Shellfish Poisoning (PSP) is a potentially fatal syndrome associated with the consumption of shellfish that have accumulated toxins produced by microscopic algae. This phenomenon is considered to be the most widespread of the poisoning syndromes caused by blooms of toxic algae "red tides" affects freshwater drinking supplies as well as coastal fisheries worldwide and is a significant and growing threat to human health, fisheries resources, and the health of aquatic ecosystem. Saxitoxin, the most widely known PSP toxin, is produced by a limited number of marine dinoflagellates (e.g., Alexandrium tamarense) and freshwater filamentous cyanobacteria (e.g., Anabaena circinalis). The biosynthetic pathway for saxitoxin synthesis is poorly understood and none of the genes involved in saxitoxin synthesis have been conclusively identified. The cyanobacteria represent an attractive model for identifying putative saxitoxin genes due to their small genomes and the wealth of genomic information available in databases. In contrast, the most significant impacts from Saxitoxin toxicity derive from dinoflagellate blooms in marine systems.
The incidence and severity of HAB events is increasing globally and both national and international plans for coping with the impacts of HABs call for strategies to identify the genes responsible for toxin production. Specifically, identification of the saxitoxin genes falls under, Harmful Algal Research and Response National Environmental Science Strategy (HARRNESS), which defines the US strategy to understand "red tides" and to ameliorate their negative impacts on ecosystems and human health. Our work will lay the foundation for understanding the evolution and dispersal of saxitoxin genes in both cyanobacteria and dinoflagellates and will provide a set of molecular probes that can be used in future projects to support monitoring and management of HAB events.
Saxitoxins (STX) are produced by a limited number of marine dinoflagellates and freshwater filamentous cyanobacteria. Radio−tracer studies indicate an unexpected pathway is involved in saxitoxin synthesis; rather than being a purine/xanthine derivative, saxitoxin is synthesized from the Claisen condensation of arginine (ARG) with acetyl group carbons, plus an S−methyladenosine−CH3 group and two additional guanine groups derived from ARG. A single enzyme has thus far been characterized as a presumptively pathway−ending sulfotransferase with activity toward R2 and R3. None of the genes involved in Saxitoxin synthesis in dinoflagellates have been identified, despite attempts to clone Saxitoxin genes from knowledge of this sulfotransferase.
Cysts of Alexandrium tamarense lay dormat on the ocean floor, buried in sediment. Left undisturbed, they can stay in this safe for years. When O2 is present and consitions are right, germination may begin.
Warmer tempratures and increase light at certain times of year can stimulate cysts to germinate. The cysts break open and a swimming cell emerges. Within a few days of "hatching," the cell reproduces by simple division.
With abundant nutrients and optimal conditions, cells will reproduce exponentially. A single cell can divide into several hundred cells within weeks. If enough cell bloom, shellfish cen become contaminated, poisoning animals and humans who eat them.
Dinoflagellates, such as Alexandrium tamarense, are models for evolutionary research due to their unparalleled ability to capture plastids through serial endosymbioses. Alexandrium tamarense has accumulated genes from different endosymbionts and putatively from other algae through lateral transfer.
Finally, Alexandrium tamarense, like most dinoflagellates, has a nuclear structure that is unique among eukaryotes. Because it lacks histones, it is likely that novel methods of compacting DNA and controlling gene expression that have evolved in these species.