Cyanobacteria and Cyanotoxins
By Mark Boniface
Cyanobacteria are a group of bacteria that harness their energy through the process of photosynthesis, absorbing the sun’s energy and converting it into usable energy. These bacteria get their name from their bluish-green color. These bacteria are significantly important in many ecosystems. Cyanobacteria are some of the only organisms that produce gaseous oxygen as a product of their photosynthetic process. Cyanobacteria, and their descendants, are credited with the production of all the oxygen gas on Earth. These bacteria are the cause of a massive blooming of biodiversity of aerobic organisms, and the cause of the near-extinction of oxygen-intolerant organisms.
Along with the production of essential and highly demanded oxygen gas and the fixing of carbon dioxide and nitrogen, cyanobacteria also can produce a variety of toxins. These harmful agents come primarily as neurotoxins, cytotoxins, endotoxins, or hepatotoxins. The cyanobacteria produce these toxins to kill off other local species to provide more room for growth. These toxins arise when the environment provides optimal for cyanobacteria to reproduce explosively and exponentially, resulting in algal blooms. These blooms are not only harmful for many aquatic species, but also for humans who ingest contaminated fish and shellfish or swim in or are exposed to contaminated water. It is thought that cyanobacteria are an environmental cause of degenerative neurological diseases such as Amyotrophic Lateral Sclerosis (ALS), Parkinson’s disease, and Alzheimer’s disease.
Cyanobacteria have also been identified as a potential source of renewable energy in their ability to produce sugars and ethanol as well as produce agents with potentially antibiotic properties.
Cyanobacteria have a typical cell structure. These bacteria are gram-positive cells with a thick peptidoglycan layer that lies between the cell membrane and outer membrane. This layer is what allows many microbes to be stained purple by staining dyes used to make bacteria visible. Just inside the cell wall, cyanobacteria have layers of thylakoids, membrane bound structures where the “light” photosynthetic reactions occur. Each thylakoid is covered with phycobilisomes which act like antennae that harvest light. These thylakoids are often present as stacks of disc like structures, called grana, which allow maximum capturing of light energy. These discs are interconnected by smaller thylakoids which allow each stack of discs to function as a single unit. Buried within the layers of thylakoids are structures called cyanophycin, which possibly function as nitrogen and carbon storage. Further inside the spherical cells, the thylakoids stop and ribosomes become present, these are the protein synthesizers that are present in almost all cells. Finally at the center of the cell lays the DNA of the cell as well as the cyanobacteria’s carboxysomes. These are bacterial microcompartments that hold the Rubisco enzymes which are the carbon dioxide fixing enzymes. Rubisco is tethered to the inside of these carboxysomes. This arrangement is beneficial to the cell since the enzyme carbonic anhydrase is also held inside these compartments. These enzymes together use substrate channeling, the passing of a product of one enzyme directly to the next enzyme, to increase Rubisco’s carbon fixation efficiency .
Cyanobacteria lack any flagella, making it difficult for them to move under their own power, though some species can move by gliding along surfaces. This movement requires contact with a solid surface such as a rock face occurs in the direction that is parallel to the longest axis of the cell or filament. Special small protein fibers and other organelle like structures allow this gliding movement to occur, thought to be due to the secretion of a mucilage during movement but the exact mechanism of gliding is still unclear . Some species of cyanobacteria such as Oscillatoria form multicellular films that are capable of movement through use of oscillating movement. Other species move through flotation by forming gas vesicles bounded by protein sheaths.
Traditionally, cyanobacteria are connected to the plant kingdom as the predecessor to the chloroplast, but recently they have been separated due to several unique features. First of which is the ability for cyanobacteria to fix carbon. Cyanobacteria use the energy of captured sunlight to drive photosynthesis, the process of splitting water molecules into oxygen, protons, and electrons. Since cyanobacteria are typically in an all marine environment, they tend to employ strategies which are known as carbon concentrating mechanisms to help acquire carbon in the form of carbon dioxide or bicarbonate. To accomplish this, cyanobacteria use the carboxysomes that were previously mentioned. These structures hold the carbon fixing enzymes, Rubisco. The cyanobacteria use metabolic channeling to increase local carbon concentrations and make Rubisco more efficient. Unlike eukaryotic cells, cyanobacteria lack compartmentalization of the thylakoid membrane which runs parallel to the plasma membrane. This prevents the sharing of energy carrier pools such as cytochrome c and ferredoxins and preventing interactions between photosynthetic and respiratory metabolism. Despite this setback, cyanobacteria have huge diversity between respiratory systems between species, this diversity is termed the “branched electron transport chain.”
Cyanobacteria do not have any internal membrane or nuclei but do have many folds on their external membrane that function similarly to the way an internal membrane would in the mechanism of photosynthesis. The bluish-green pigment of cyanobacteria functions to capture light energy from the sun and then use the surrounding water as an electron donor and produces oxygen gas as a byproduct. The captured carbon dioxide or bicarbonate is reduced by taking electrons from the water and giving it to the carbon. This forms carbohydrates through the Calvin cycle, the same cycle used by eukaryotic cells.
Cyanobacteria are one phylum of microbes that have the ability to explosively reproduce given the proper environmental conditions. The end results of these periods of high growth are algal blooms. These blooms can potentially cover hundreds of square miles and are often seen by satellite imagery, and while the individual cyanobacteria live only a few days, the blooms tend to last a few weeks. These large fields of organisms look like a blanket of foam or scum, often bright green in color due to the chlorophylls of the bacteria but can also appear blue or even a brownish-red depending on the species present. For both saltwater and freshwater systems, algal blooms most often come around as an ecosystem’s response to a large addition of natural or artificial substances such as phosphates and nitrogen in the form of detergents, fertilizers, even sewage . This process is termed eutrophication, and while the majority of eutrophication events are artificially driven, they can happen naturally but over a much larger time period and to a much less extreme effect. The presence of high levels of phosphate and nitrogen allow rapid plant growth and decay, mainly of simple algae which can quickly take up the available resources and prevent more complex plant life take root in the new phosphate rich environment. However, the period of high growth must come to an eventual end, the phosphates are consumed and growth slows, eventually most, if not all of the cyanobacteria die and begin to decay. This process consumes oxygen which in turn makes the localized area a hypoxic dead zone, killing any marine life present. Despite this, the majority of the time these blooms are directly harmless, but this is not always so. Sometimes harmful algal blooms arise, often called “red tides” or simply, harmful algal blooms, called HABs for short. These tides contain toxins and pathogens which kill marine life and are potentially fatal for humans . Altogether, over 84 cyanotoxins are known to exist, but only a very small number have been studied . The most common forms that these toxins take are as alkaloids, amino acids, cyclic peptides, and polyketides.
 Hodgkin, J. and Partridge, F.A. "Caenorhabditis elegans meets microsporidia: the nematode killers from Paris." 2008. PLoS Biology 6:2634-2637.
Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2015, Kenyon College.