Algae are one species within the protist kingdom of living beings, which is mostly composed of single-celled, microscopic organisms that may share traits with animals, fungi and/or plants. The biological grouping also includes kelp and molds. There are two main groups of algae — seaweeds, or macroalgaes, and phytoplankton, or microalgaes — in addition to several different categories of algae based on color: green algae, red algae, brown algae and blue-green algae.

Algae are typically found in water, which is why you’ve probably seen a colony of algae accumulate at different bodies of water, like lakes and rivers. They contain chlorophyll — a compound also found in plants — which allows them to turn sunlight into food. A group of algae is called a bloom. But under certain circumstances, an algae bloom can rapidly grow and become an ecological and health hazard. Once a bloom is toxic, it’s known as a harmful algae bloom.

A variety of factors can lead to the creation of a harmful algae bloom. According to the U.S. National Oceanic and Atmospheric Administration (NOAA), nutrient pollution, high water temperatures, extreme weather events, slow water currents and low wind speeds can all lead to algae blooms to grow out of control. However, the agency notes, “how these factors come together to create a ‘bloom’ of algae is not well understood.”

These factors can lead to algae blooms, but not all blooms are harmful. According to the U.S. Centers for Disease Control and Prevention (CDC), there are three main algaes that can become harmful: cyanobacteria (called blue-green algae), dinoflagellates and diatoms (both of which are known as microalgae or red tide).

“The most frequent and severe blooms typically are caused by cyanobacteria, the only known freshwater algae with the potential for production of toxins potent enough to harm human health,” explains the U.S. Geological Survey on its website.

Read the full article about algae blooms by Bridget Reed Morawski at EcoWatch.