blue. Cyanobacteria are found almost everywhere; in oceans, lakes and rivers as well as on land. They flourish in Arctic and Antarctic lakes, hotsprings and wastewater treatment plants. They even inhabit the fur of polar bears, to which they impart a greenish tinge. Cyanobacteria produce potent toxins, but they also produce helpful bioactive compounds, including substances with antitumour, antiviral, anticancer, antibiotic and antifungal activity, UV protectants and specific inhibitors of enzymes. Harmful algal blooms Cyanotoxins are often implicated in what are commonly called red tides or harmful algal blooms. Lakes and oceans contain many single-celled organisms called phytoplankton. Under certain conditions, particularly when nutrient concentrations are high, these organisms reproduce exponentially. The resulting dense swarm of phytoplankton is called an algal bloom; these can cover hundreds of square kilometres and can be easily seen in satellite images. Individual phytoplankton rarely live more than a few days, but blooms can last weeks. Generally, these blooms are harmless, but if not they are called harmful algal blooms, or HABs. HABs can contain toxins or pathogens which result in fish kill and can also be fatal to humans. In marine environments, HABs are mostly caused by dinoflagellates, though species of other algae taxa can also cause HABs (diatoms, flagellates, haptophytes and raphidophytes). Marine dinoflagellate species are often toxic, but freshwater species are not known to be toxic. Neither are diatoms known to be toxic, at least to humans. In freshwater ecosystems, algal blooms are most commonly caused by high levels of nutrients (eutrophication). The blooms can look like foam, scum or mats or like paint floating on the surface of the water, but they are not always visible. Nor are the blooms always green; they can be blue, and some cyanobacteria species are coloured brownish-red. The water can smell bad when the cyanobacteria in the bloom die. Strong cyanobacterial blooms reduce visibility to one or two centimetres. Species which are not reliant on sight (such as cyanobacteria themselves) survive, but species which need to see to find food and partners are compromised. During the day blooming cyanobacteria saturate the water with oxygen. At night respiring aquatic organisms can deplete the oxygen to the point where sensitive species, such as certain fish, die. This is more likely to happen near the sea floor or a thermocline. Water acidity also cycles daily during a bloom, with the pH reaching 9 or more during the day