FunNature & AnimalFormation and risks of red tides

Formation and risks of red tides


According to biblical tradition, the god of the Christians launched ten great plagues on Egypt that mercilessly devastated the region. One of them, the first, was the conversion of the waters of the Nile River into blood .

When I hit the Nile River with the stick, the water
will turn to blood. The fish in the river will die, and the water
it will smell so bad that the Egyptians will be disgusted to drink it.

Exodus, 7: 17-18

There is no physically or biologically possible method to transform a body of water into a biological fluid like the one described in that plague, the only way to achieve something similar would be the effusion of large amounts of animal blood into the river waters. However, there is an ecological process that, although it does not transform water into blood, generates a similar-looking effect , and that the ancient inhabitants of North Africa, without any knowledge of microbiology, could interpret as blood. This is the red tide .

The so-called red tide is a disproportionate proliferation , sometimes uncontrolled, of microscopic organisms with a reddish hue , which generally stain the water that color. However, the term “red tide” is not entirely correct. First of all, because it is not a movement of water masses caused by gravitational forces, like a tide. And, on the other hand, there are other forms of massive proliferation of microorganisms, from other groups, that do not necessarily alter the color of the waters or that can dye them other colors, yellowish, greenish or different shades of brown.

The red tides themselves are formed, mainly, by a group of microorganisms called dinoflagellates .

What are dinoflagellates?

Dinoflagellates are a group of microorganisms, mostly unicellular, although there are species of colonial organization. They have traditionally been classified as algae, although they have no phylogenetic relationship with them. A good number of species of dinoflagellates are photosynthetic, with chlorophyll A, C2 and carotenoids —the pigments that give them their red color—. Those that are not are heterotrophs and feed on other unicellular organisms. Many species are mixotrophs —that is, they can photosynthesize and feed on other organisms—and some are parasitic.

Some dinoflagellates, such as zooxanthellae, form symbiosis with corals, anemones and other invertebrates , providing them with oxygen and nutrients in exchange for protection and waste substances that the microorganism can use as a resource. This ability, in fact, is vital for coral reef-building corals. When these microorganisms die, the coral discolors, bleaches, and dies too. However, most species are free-living.

Dinoflagellates have a highly variable structure, but they always share certain specific features: on the one hand, they always have two flagella , one usually coils around the cell body —transverse— and the other is directed towards the back —longitudinal, with function motorboat. In addition, they usually have a cellular covering, called amphiesma , which is made up of flattened vesicles called cortical alveoli . In addition, some species —called “ thecae ”— present more or less flattened cellulose plates, of variable size and shape depending on the species, which form a rigid and hard cover called theca . Other species that lack this structure are called ” atecates “.

How does a red tide occur?

The massive proliferation of dinoflagellates, like many other microorganisms, may have a completely natural origin. With some frequency, some natural events generate an abundant release of nitrates and phosphates in the aquatic environment . These nutrients, in the sea, are usually limiting, and a large-scale contribution can trigger a growth in the population.

But, in general, red tides are caused by human activities . Coastal water pollution or discharges into rivers, lakes and reservoirs, from industrial, wastewater or agricultural water, can provide excess nutrients that lead to a massive proliferation of microorganisms.

The effects derived from global change, such as the increase in greenhouse gases or the increase in global temperature, may become engines that increase red tides, but much remains to be studied.

The episodic and irregular nature of these events requires monitoring systems that provide data continuously, with a constant frequency —ideally daily— and in the very long term, decades. These limitations create serious difficulties when studying and, therefore, predicting the behavior of the massive proliferation of dinoflagellates in a future scenario of global change.

What dangers does the red tide have?

There are many different species of dinoflagellates that can produce a red tide, and each one has its particularities, causing the proliferation to have different effects. Many dinoflagellates contain toxins such as okadaic acid, azaspiracid, saxitoxins, brevetoxin, ciguatoxin, scaritoxin or, one of the most toxic substances on the planet, maitotoxin . These substances, in the high concentrations found during a red tide, can have disastrous effects on ecosystems, the animals that live in them, and even cause public health problems.

Being microorganisms that remain suspended in the water, forming part of the plankton, filter animals , such as mussels, clams, oysters and other molluscs, obtain large amounts of dinoflagellates with the mere process of feeding.

Fish can also suffer serious consequences from red tide poisoning. Barracuda, grouper, snapper, red mullet… either because they consume contaminated animals, or because they acquire these toxins through their gills, they can be seriously affected. In very extreme cases, large fish kills have been observed as a result of a massive proliferation of dinoflagellates.

Instantly the water turned to blood, the fish in the river
died, and the water stank so bad that the Egyptians couldn’t
they could drink it. The whole country was filled with blood!

Exodus, 7: 20-21

These toxins bioaccumulate in the tissues of the affected organisms and any animal that later consumes them – including humans – suffers its consequences. Poisonings in humans from consuming contaminated animals include ciguatera, diarrheal, neurotoxic, or paralytic shellfish poisoning.

Even mere contact with water during a red tide can pose a risk. Swimming or carrying out activities submerged for long periods of time, consuming it —if it is fresh water— or even breathing water droplets left in the air when waves break can be sources of exposure to some toxins .

Blooms of organisms other than dinoflagellates — such as certain cyanobacteria or diatoms of the genus Pseudo-nitzschia — can also produce harmful toxins, turning the water green or brown. On the other hand, certain red tides and other proliferations of various colors are harmless. Faced with a massive proliferation of microorganisms, there is no way of knowing, at a glance, which organism is causing it, so, in order to prevent possible poisoning, in all cases, the best option is to avoid contact with these waters and with the animals that live in them.


CDC. 2021. Illness and Symptoms: Marine (Saltwater) Algal Blooms. Center for Disease Control and Prevention.

Gómez, F. 2012. A checklist and classification of living dinoflagellates (Dinoflagellata, Alveolata). CICIMAR Oceánides, 27(1), 65-140. DOI: 10.37543/oceanides.v27i1.111

Moore, S. K. et al. 2008. Impacts of climate variability and future climate change on harmful algal blooms and human health. Environmental Health, 7(2), S4. DOI: 10.1186/1476-069X-7-S2-S4

Noga, EJ et al. 2006. Phylum Dinoflagellata. In PTK Woo et al., Fish Diseases and Disorders (pp. 16-45). CABI.

Stoecker, D. K. 1999. Mixotrophy among Dinoflagellates. Journal of Eukaryotic Microbiology, 46(4), 397-401. DOI: 10.1111/j.1550-7408.1999.tb04619.x

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