When the Dose Makes the Poison: Harmful Algal Blooms

Algae: you wouldn’t be here reading this article, and I sure wouldn’t be here writing it, if it weren’t for them. It might seem like a bit of a stretch to make that connection, but algae are at the base of the food web in aquatic ecosystems. As primary producers, they make their own food without having to rely on other organisms. They do however rely on nutrients, such as nitrogen and phosphorus, the sun and carbon dioxide. The ocean absorbs nearly 30% of the carbon dioxide released in the atmosphere, and algae use it as a building block for photosynthesis, turning energy into (their own) biomass, releasing oxygen as byproduct.  

When we think about algae, we picture the commonly known seaweed, which shapes habitats and creates a home for many species of fish. However, the algae that end up indirectly feeding the whole marine ecosystem aren’t visible to human eyes: they’re microscopic, and take the name of phytoplankton, which includes diatoms, dinoflagellates and cyanobacteria. As the word “plankton” suggests, this type of algae doesn’t stay anchored to the seafloor, but drifts along with the sea current and the wind. The other known type of plankton is zooplankton, a non-photosynthetic organism that feeds on phytoplankton but can also feed on species of zooplankton itself. For some animals, like whales, corals and small fish, zooplankton is the main source of nutrients. Whales are called filter feeders exactly because of this reason: their survival depends on the availability of zooplankton, and zooplankton depends on the availability of phytoplankton. Thus, phytoplankton, albeit minuscule, carries the weight of the world in its metaphoric shoulders, ensuring that the delicate balance of the Earth system is maintained.  

The poison 

Lately, on social media, the phenomenon of Harmful Algal Blooms (HABs) – algae releasing harmful toxins in the water – has become a viral topic after Nature published a paper titled “Alzheimer’s disease signatures in the brain transcriptome of estuarine dolphins”. In short, the paper connects the effects of HABs to signs of Alzheimer’s disease in dolphins. Though on X (former Twitter), a post went viral linking Alzheimer’s disease in dolphins to lead and mercury pollution in the water. Someone had taken the title of the paper and wrote their own narrative, without bothering to read the actual study – not even the abstract. Now, 122k people have interacted with a text that spreads total misinformation about HABs and pollutants. Many people have good intentions by sharing news on climate change, pollution and global warming, but like in any case, intention isn’t nearly enough. Education is at the base of a just, correct approach to climate change. If someone is so extremely blinded by their feelings, as righteous as they may be, that they skip education, then it becomes harmful. The saying goes: “The dose makes the poison”, and funnily enough, it is true both for social media activism and Harmful Algal Blooms. 

The science behind the bloom 

Many algal blooms happen naturally, and not all of them are harmful. The phenomenon occurs when there is an overgrowth of algae in fresh water or marine systems. It becomes harmful if it negatively impacts other organisms and disrupts the local ecosystems.  

Some algae, when they overgrow, produce phycotoxins that can kill fish, mammals and birds. For example, the harmful bloom of cyanobacteria, that happens near lakes, rivers or estuaries, produces a toxin that damages the liver of mammals. In coastal areas, dinoflagellates and diatoms produce powerful neurotoxins that accumulate in the bodies of shellfish, sardines and anchovies. Whoever ends up eating the intoxicated animals, whether they’re a human, a seabird or a cetacean, is at serious risk of seizures, injury and death. Other types of algae don’t produce toxins, but if the increase in the population is extreme, they can consume large quantities of oxygen when they die, leading to hypoxic – low oxygen – conditions in bodies of water. 

From land, these blooms can be observed with the naked eye due to their intense colors, but not all the colorful blooms are harmful. That is why scientists are studying ways to detect HABs with cheaper, faster methods, so they can easily analyze the phenomenon and warn the public whenever a harmful bloom is occurring. 

Although harmful algal blooms commonly happen in nature, anthropic activity is exacerbating the frequency of their occurrence, along with their effects. Just like climate change, HABs are bound to happen, but even a tiny disruption in the delicate balance of the ecosystems can have devastating consequences.  

The cause of HABs is multifactorial, and while the factors are known, the actual process is yet to be understood. One of the most common causes is related to the availability of nitrates and phosphates in the water. An excess of these nutrients, namely nutrient pollution, stimulates the overgrowth of algae. Nutrient pollution is mainly influenced by human activities, such as the use of synthetic fertilizer, sewage waste, stormwater runoff and wastewater discharge. Thermal pollution from power plants and climate change are among the other triggering factors. 

The consequences  

Climate change is altering the water temperature globally, threatening the survival of marine life. A recent study published in the journal PLOS ONE established that increasing water temperature is decimating the krill populations – a species of zooplankton – making life difficult for blue whales, that are struggling to find nutrients, and “singing” less. An unprecedented HAB that occurred in 2015, following anomalous heatwave events, is another factor connected to the decrease in blue whale communication. Global warming and ocean acidification, triggered by a rise in CO2 emissions, favor the growth of harmful algae, further decreasing and scattering the krill population. Weak and stressed, the whales are going quiet. The once loud symphony of blue whales is disappearing beneath the sea in an inevitable decrescendo.  

In the end, the massive damage isn’t just ecological: our health and economy are severely threatened by HABs, along with many critical climate factors. Without benign phytoplankton, humanity dies, animals die, plants die. Afterall, algal blooms have greatly contributed to the deadliest mass extinction in the history of the Earth, the Permian-Triassic event, also called the “Great Dying”. If you destroy the foundation of a house, it collapses. 

Among the billions of stars and planets of our galaxy, we might just be the most complex expression of nature. From blue whales and dolphins, from the sound of trees and falling water, to the symphonies humans compose, the Earth hosts the most unique piece of music. It is our duty not to let it go silent. 

Elisabetta Fenu