By Leanne Pearson
Photosynthetic microorganisms known as cyanobacteria are found in a variety of aquatic and terrestrial habitats worldwide, including oceans, lakes, polar melt-water ponds, hot springs and desert soil crusts. Sometimes erroneously termed ‘blue-green algae’, they are in fact a morphologically and physiologically diverse phylum of bacteria.
Cyanobacteria were among the earliest lifeforms to colonise Earth, and it was their expansion in the shallow seas of the Paleoproterozoic era 2.4 billion years ago that led to the Great Oxidation Event and the subsequent rise of multicellular organisms. Rare examples of fossilised reefs (stromatolites) created by cyanobacteria can still be viewed today off the coast of the Bahamas and the UNESCO World Heritage-listed Shark Bay, Western Australia (pictured below).
Like plants, cyanobacteria use energy from sunlight to convert carbon dioxide (CO2) and water into sugars and oxygen (O2). In fact, plastids (chloroplasts) found in higher plants and algae are thought to have evolved from ancient cyanobacteria engulfed by eukaryotic cells. Cyanobacteria can be considered the true ‘lungs of the Earth’, rivalling the rainforests in terms of oxygen production. Their photosynthetic activity also regulates CO2 levels in the atmosphere, counteracting anthropogenic inputs and slowing climate change. Some filamentous genera, such as Nostoc and Anabaena, can also convert (or fix) atmospheric nitrogen (N2) into ammonia within specialised cells (heterocysts), making this essential element available to other organisms.
As primary producers of bioavailable carbon and nitrogen, cyanobacteria are a critical component of food webs in both marine and freshwater ecosystems, particularly nutrient-poor (oligotrophic) zones. However, under nutrient-rich (eutrophic) conditions, cyanobacteria can proliferate to such an extent that they can throw ecosystems out of balance, harming or killing other organisms (see below figure).
When dense cyanobacterial blooms die off and are consumed by other microbes, the water surrounding them becomes devoid of oxygen (anoxic) and can no longer support aerobic life. Most alarmingly, some strains of cyanobacteria are capable of producing potent toxins (cyanotoxins), which can poison aquatic wildlife, livestock and humans via exposure to contaminated aerosols, water or seafood. Among the arsenal of toxic compounds produced by cyanobacteria are the hepatotoxic microcystins and the neurotoxic saxitoxins (pictured below), the latter of which are included in Schedule 1 of the Chemical Weapons Convention alongside ricin.
Toxic cyanobacteria are particularly problematic in rivers, lakes and reservoirs with elevated levels of phosphates and nitrates. Agricultural run-off (e.g. fertilizers and manure), domestic wastewater, and effluent are major sources of nutrient pollution contributing to harmful bloom events. In addition to posing a significant threat to human health and natural ecosystems, toxic blooms are a significant burden on the economy due to the cost of monitoring regimes and the closure or disruption of recreational, agricultural and domestic waters.
While scientists are beginning to understand how cyanobacteria produce toxins and the environmental conditions regulating blooms, the native function of these compounds largely remains a mystery.
Further Reading
Environmental conditions that influence toxin biosynthesis in cyanobacteria. Neilan BA, Pearson LA, Muenchhoff J, Moffitt MC, Dittmann E.Environ Microbiol. 2013 May;15(5):1239-53.
Cyanobacterial toxins: biosynthetic routes and evolutionary roots.
Dittmann E, Fewer DP, Neilan BA.FEMS Microbiol Rev. 2013 Jan;37(1):23-43.
The genetics, biosynthesis and regulation of toxic specialized metabolites of cyanobacteria.
Pearson LA, Dittmann E, Mazmouz R, Ongley SE, D’Agostino PM, Neilan BA.Harmful Algae. 2016 Apr;54:98-111.