Microalgae, why are they toxic ?

Microscopic algae or microalgae are phyto-planktonic microorganisms that can proliferate under certain circumstances giving rise to spectacular coloration events (e.g. red tides). We speak about ‘Harmful Algal Blooms’ (HABs) when microalgae induce oxygen depletion (eutrophication) and/or release toxic compounds (phycotoxins).

Essential node of the trophic network, microalgae have a key-role in the aquatic ecosystem functioning. Phylogenetically, they represent a large diversity of organisms that produce an unparalleled panel of bioactive secondary metabolites. 

Although only 2% of these compounds are known to be toxic to animals and humans, microalgae blooms are a serious threat for the consumption of sea products, especially bivalve molluscs that filter water and accumulate toxins, as well as fishes that bioaccumulate them through the trophic chain.

About sixty microalgae species have been identified as toxigenic, belonging primarily to Dinoflagellates and Diatoms. The toxins they can produce are usually classified based on their physiological effects on humans (see table, non-exhaustive list):

The past decades have witnessed recurrent occurences of microalgae blooms. The impact of human activities on this phenomenon which is likely to aggravate in the future is no longer questioned by the scientific community: a higher frequency of heatwave events, ecosystem imbalances due to the supply of nutrients from the land (fertilizer runoffs or septic tank overflows), and loss of biodiversity all favor growth of microalgae at the expense of the rest of the biotope.

Some phycotoxins can be produced by different species while some other are apparently associated to one single species. Different strains from a same genus or species can also have a toxigenic activity more or less important depending on their geographical localization or seasonal environmental parameters, even on a daily scale under the influence of water stratification, tides or wind effect.

It is also important to notice that certain toxins are excreted, either released out of the cells or remaining cell-bounded, but most of toxins produced by microalgae are indeed endotoxins that are liberated only when cells die and explode, thus when blooms collapse and decline.

As regard to this variability, the occurrence of HABs is of major concern in a global context of environmental risk assessment, first because it is an indicator of a source of pollution, and second because of the toxicity issue. In many countries, water management and public health authorities have been taking that seriously and developed ambitious programs to improve detection methods, to provide strict regulations and/or guidelines based on tolerance thresholds securing sea product consumption and access to bathing areas.

These thresholds have been usually established based on estimation of toxin-producer cell densities, or by measuring toxin concentration in water. In the majority of aquaculture facilities or beach monitored bathing areas, these methodologies are employed in routine and applied accordingly, leading to extended closures of sites, sometimes on the whole season. Reacting to these situations in a more accurate and proactive manner, providing transparent information towards the public, would be obviously beneficial. For all the shellfish and fish growers especially, improving the risk management, reducing the duration of sales bans and giving clear information to the customers represent a major economical concern. 

Performant and robust scientific tools supporting decision-making are crucial to reach these objectives.