The competitive dynamics of toxic and non toxic ribotypes of the harmful dinoflagellate Alexandrium tamarense

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A number of species of marine phytoplankton produce natural biotoxins. Filter feeding shellfish ingest these phytoplankton without any apparent negative effects. However, bio-accumulation of the toxins in the shellfish flesh allows concentrations to reach sufficiency elevated levels that serious health consequences may result for humans that ingest the shellfish. Owing to the severity and possible fatality of the symptoms, paralytic shellfish poisoning (PSP) is perhaps the globally most important shellfish poisoning syndrome. PSP is caused by potent neurotoxins (saxitoxins) produced by the dinoflagellate genus Alexandrium. PSP occurs worldwide, with toxicity of shellfish in UK waters being characterised by frequent low level toxicity (monitoring programs indicating 100's of occurrences annually) and sporadic major toxicity events. Blooms of the causative species vary both spatially and temporally, and as yet we have very little understanding of the environmental factors that govern them. Human health is safeguarded by government biotoxin and phytoplankton monitoring programmes operated by Food Safety Authorities at significant cost to the economy. Of the various species of Alexandrium, the species complex Alexandrium tamarense is of particular concern in UK waters. Non toxic, group III, A.tamarense have historically been found in France, Spain and Portugal with their northernmost extent being the south of the UK. In contrast, toxic cells of group I A.tamarense are characteristic of northerly latitudes. However, recent studies have indicated changing distributions, with the non toxic group III cells now being found as far north as Shetland. Waters around the UK are known to be warming, suggesting that this, or other associated environmental changes (e.g. localised salinity, pH or light conditions), have allowed group III cells to 'invade' northern waters, where they now compete with group I. This is consistent with monitoring observations of reduced PSP events in recent years, but with no significant change in the total abundance of A.tamarense. Study of the factors driving the changing distributions of A.tamarense will provide better understanding and predictive ability of PSP events. This will allow more targeted biotoxin monitoring and better safeguard human health. Furthermore, such study will allow us to investigate how climate change is influencing the distribution of marine phytoplankton in UK waters. Recently we have isolated into laboratory culture strains of both group I and group III A.tamarense providing, for the first time, cultures of both toxic and non toxic strains of the organism from a single region. We shall study the influence of environmental conditions on strain growth and toxicity both individually and in competition. In addition, as A.tamarense forms an overwintering resting stage (a cyst) which settles to the sea floor and only germinates when conditions are favourable, we shall study the effect of environmental conditions on cyst germination. As both A.tamarense strains are of a single species, it is not possible to discriminate between them based on morphology. We will therefore utilise recently developed oligonucleotide probes for A.tamarense. After treatment with these molecular probes, and viewed under ultra violet light on a flourescence microscope, the non-toxic group III cells glow green and the toxic groups I cells glow gold, allowing for easy discrimination and enumeration even when cells are grown in combination to study their competition. Finally, we shall use our experimental results to derive and parameterise mathematical models for A.tamarense. These models will be used to simulate in situ data on the abundance of the different strains of A.tamarense we are currently collecting at a number of sites. Once developed, these models will increase our ability to predict the likelihood of a PSP event based on knowledge of the environmental conditions.