In Praise of 'Scum'?

Cyanobacteria or blue-green algae get a very bad 'press'. The world would, however, be very different without them. Cyanobacteria cause concern, because 5 of their 2000 identified species produce some of the deadliest toxins known to Science. This may contaminate drinking water for humans. Their role in eutrophication (rapid algal growth in water bodies, when nutrients accumulate) is also well-known. The consequent depletion of oxygen can lead to the deaths of fish and other aquatic organisms (https://www.theguardian.com/environment/2024/may/01/blue-green-algae-cyanobacteria-organisms). Cyanobacteria were, however, the first Earth organisms to use photosynthesis. They exploited the power of sunlight, to make carbohydrates from carbon dioxide and water, releasing oxygen as a 'waste' product. This generated the oxygen-rich (21%) atmosphere, on which most animals now depend. Some folk even suggest that cyanobacteria have become the chloroplasts in plants. This is termed 'endosymbiosis' and is seen when one organism 'chooses' to live in the body of another, for mutual benefits. The cyanobacteria/chloroplasts get the protection of the plant cells with regular provision of carbon dioxide and water. The plants get access to the products of photosyntheis. This hypothesis is supported, by chloroplasts (like mitochondria) having their own annular DNA. This reflects the way in which DNA is found in the bodies of all bacteria.