Marine biotoxins and climate change

I worry about food safety and so it seems do 60% of respondents in a global survey involving 150,000 people in 142 countries. However, while they are mostly concerned about the safety of the current food supply, I worry about the impact of climate change in worsening the food safety situation. We have already covered the impact of climate change on the accumulation of heavy metals and growth of moulds producing mycotoxins.

But of course there is more.

In this last blog in the series covering the food safety impact of climate change we will look at increases in the presence of marine biotoxins produced by blooms of harmful algae.

Toxins produced by some algal species

During recent decades, there has been an apparent increase in the occurrence of harmful algal blooms in many marine and coastal regions. Changes in climate may be creating a marine environment particularly suited to the growth of harmful species of algae. Two major functional groups of marine algae, or phytoplankton, are involved in causing toxic blooms – diatoms and dinoflagellates. There are also toxic cyanobacteria, sometimes called blue-green algae, that are not strictly speaking algae but very similar in action.

Certain toxins produced by these organisms are particularly dangerous to humans. A number of illnesses are caused by ingesting seafood contaminated by the toxins.

The most important harmful algae and their poisoning syndromes include diatoms from the genus Pseudo-nitzschia (amnesic shellfish poisoning), and species of dinoflagellates from the genera Alexandrium, Pyrodinium, and Gymnodinium (paralytic shellfish poisoning), Karenia (neurotoxic shellfish poisoning), Dinophysis and Prorocentrum (diarrhetic shellfish poisoning), and Gambierdiscus (ciguatera fish poisoning). There are also cyanobacteria that produce a range of toxins that can affect humans drinking or swimming in contaminated water causing a similar range of symptoms. Their toxins include microcystin, nodularin, cylindrospermopsin, anatoxin-a, anatoin-a(s), lyngbyatoxin and saxitoxins.

As the names of the syndromes imply the toxins can cause memory loss, digestive problems, seizures, lesions and skin irritations, and finally paralysis that may include the respiratory system. Indeed an impressive list.

Some of these toxins can be acutely lethal and are among the most powerful natural substances known. They affect fish, birds and mammals including humans. Because these toxins are tasteless, odourless, and heat and acid stable, normal screening and food preparation procedures will not prevent intoxication.

Increase in the growth of harmful algae

Dinoflagellate abundances have increased to the detriment of diatom populations in some marine ecosystems linked to increases in sea surface temperatures. This can have serious consequences.

As an example a calculation was performed of the impact of climate change on the length of the period of toxic blooms in Puget Sound, an important area of shellfish farming. Results suggested that by year 2100 the period of optimal growth of the toxic dinoflagellate Alexandrium catenella may potentially expand from 68 days to up to 259 days due to warmer water temperatures. This would have severe implications for regional food safety as A. catenella produces paralytic shellfish poisoning. It would totally close the area for shellfish harvesting for most of the year devastating the local economy.

Another example of a dinoflagellate known to generally favour warmer conditions is Gambierdiscus toxicus, one of the species producing ciguatoxin. Increases in ciguatera fish poisoning has been observed with elevated sea surface temperatures. Clinical signs in humans eating fish containing the toxin include gastrointestinal, neurologic, and cardiovascular signs. Gastrointestinal signs include vomiting, diarrhea, abdominal pain and cramps. Neurologic signs include itching, pain, visual blurring, weakness, depression and headache. Cardiovascular signs include arrhythmia, bradycardia, hypotension, and cardiac block.

Cyanobacteria can reproduce quickly in favourable conditions, where there is abundant sunlight, still or slow-flowing water and sufficient levels of nutrients, especially nitrogen and phosphorus. In still conditions, surface water may form a separate warm top layer in which cyanobacteria is able to access sunlight and nutrients. If these combined factors are present for several days, cyanobacteria multiply and form large blooms. The problem seems to be getting worse. Polluted farm runoff continues largely unabated, and the climate crisis is producing warmer weather and water temperatures, along with more rainfall – all conditions that feed the blooms. News reports of blooms in the USA have increased every year since 2010, when there were a total of 71 stories about outbreaks. In 2018 there were 452 reports about harmful outbreaks.

Incomplete understanding

As already mentioned above harmful algal blooms usually increase during the warm summer months. As daily temperatures continue to rise, the number of days ideal for harmful algal growth increases. As the planet’s oceans warm, coastal regions are seeing more and more algal blooms, often worsened by fertilizer and manure that runs off from farms. With toxic algal blooms becoming more potent and lasting longer, scientists are taking a closer look at their links to a changing climate. What was once considered a summertime matter is now being considered a year-round issue.

However, the extent to which regional climate change will influence harmful algal bloom dynamics is uncertain as separating the effects of climate change from natural variability remains a key scientific challenge. Climate change pressures will influence marine planktonic systems globally, and it is conceivable that harmful algal blooms may increase in frequency and severity. Nonetheless there is only basic information to speculate upon in which regions or habitats harmful algae may be the most resilient or susceptible. 

We can continue to test for the presence of toxins in seafood as is currently the practice in many countries. But the potential escalation of outbreaks could easily overwhelm the system. Should we risk it? I for one worry about the future given the current trajectory of global warming.

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