Alexandrium population biology in the Gulf of Maine
D.M. Anderson and D.L. Erdner
Coastal waters of the northeastern U.S. are subject to recurrent outbreaks of paralytic shellfish poisoning (PSP) caused by the toxic dinoflagellate Alexandrium fundyense. Similarly, a new toxin syndrome caused by spirolide toxins originating with a related species called Alexandrium ostenfeldii now threatens public health in the region. Little is known about the extent of spirolide toxicity in the Gulf of Maine, but it is nevertheless clear that both spirolide and PSP toxicity are not uniform across the region. PSP, for which there is a large dataset from state shellfish monitoring programs, shows considerable spatial and temporal variability. This reflects in part A. fundyense growth and toxin accumulation in several separate habitats or zones defined by coastal circulation patterns and the discontinuous distribution of the dinoflagellate. The A. ostenfeldii geographic distribution has not been well characterized in the Gulf, but this species does co-occur with A. fundyense in many locations, and presumably has similar habitat restrictions. An additional factor underlying toxin variability in the Gulf is that toxicity varies dramatically among isolates of both species. Alexandrium fundyense isolates from northern waters of the Gulf, for example, produce significantly greater proportions of the more potent saxitoxins than do southern forms, resulting in toxin cell quotas that are often 10-20-fold higher. The suite of spirolide toxins produced by different isolates of A. ostenfeldii also varies significantly across fairly small geographic ranges.
Overall, this potential variability in PSP and spirolide toxicity represents a significant challenge from public health and fisheries management perspectives. We propose to investigate the extent of toxin variability in Alexandrium spp. in the Gulf of Maine and to demonstrate the manner in which genetic, environmental and hydrographic factors regulate that toxicity through changes in the Alexandrium population structure that directly affect toxin accumulation in shellfish. Based upon hydrographic data and our laboratory and field observations, we hypothesize that:
Alexandrium fundyense and A. ostenfeldii blooms in the Gulf of Maine are not single, homogeneous populations, but rather are comprised of distinct sub-populations, each with its own physiological characteristics, toxicity, and genetic history. The nature of these distinct populations, the extent of genetic mixing and recombination, and the selection pressures imposed by environmental forcings acting on these genotypes all influence Alexandrium population structure and contribute to the ultimate toxicity threat to humans.
The proposed project has the following specific aims:
- Identify a genetic marker capable of distinguishing different genotypes within A. fundyense and A. ostenfeldii populations;
- Determine the extent of natural genetic diversity of Alexandrium spp. in the Gulf of Maine;
- Characterize the relationships between toxicity, physiological variability and genotype in Alexandrium spp. from the Gulf of Maine;
- Track changes in the relative abundance of Alexandrium genotypes in Bay of Fundy source population through time;
- Track changes in the genotypic diversity of Alexandrium populations through time throughout the Gulf of Maine;
- Investigate the relationship between Alexandrium population structure and the quantity and composition of toxins in the plankton.