Cyanobacteria are ancient, single-celled organisms that resemble algae but are technically photosynthetic bacteria, often referred to as blue-green algae. These organisms are fundamental to aquatic ecosystems as primary producers, using sunlight to create energy and serving as a food source for a variety of other life forms. Cyanobacteria can multiply rapidly under certain conditions, leading to dense surface scums known as blooms that can impact water quality. Despite their sometimes overwhelming abundance and reputation for toxicity, cyanobacteria are routinely consumed and controlled by a diverse array of predators across different trophic levels.
Primary Consumers: Aquatic Grazers and Filter Feeders
Multicellular aquatic organisms, particularly zooplankton, represent the most visible consumers of cyanobacteria in the water column. These grazers employ mechanical feeding strategies, either filtering water to capture cells or actively scraping aggregated mats. Among the zooplankton, large-bodied cladocerans, such as Daphnia, are recognized as effective grazers capable of ingesting filamentous and colonial cyanobacteria.
These water fleas use their thoracic appendages to create a current, filtering particles from the water and transferring them to the mouth. Larger cladocerans, those exceeding 3,000 micrometers, can better handle the physical challenge posed by larger cyanobacterial colonies and filaments. Rotifers, microscopic invertebrates, are also common filter feeders that consume cyanobacteria, though their smaller size often restricts them to individual cells or small, fragmented colonies.
The grazing pressure exerted by these primary consumers is a significant natural mechanism for controlling the density of cyanobacterial populations. While certain species of zooplankton, such as some copepods, may avoid large cyanobacteria, the collective filtration by cladocerans and rotifers can help mitigate the growth of large-scale blooms. The consumption of cyanobacteria by these organisms transfers carbon and energy up the aquatic food web, linking the tiny bacteria to larger organisms like fish.
Specialized Microbial Eaters
Below the level of zooplankton, a microscopic world of unicellular organisms specializes in preying on cyanobacteria. This group primarily consists of various protozoa, which are single-celled eukaryotes that engulf their prey whole. Heterotrophic protozoans, including ciliates, flagellates, and amoebae, are major consumers of individual cyanobacterial cells.
Ciliates and amoebae use phagocytosis, a process where they surround and internalize the cyanobacterial cell into a food vacuole for digestion. Some amoebae are efficient predators, capable of engulfing prey many times their own size by breaking down filamentous structures. Predatory bacteria also play a role, using distinct mechanisms to consume their prokaryotic relatives.
Certain predatory bacteria, such as Bdellovibrio bacteriovorus, are endobiotic; they invade the cyanobacterial cell and consume it from the inside before replicating and lysing the host. Other species, like Candidatus Venantispira tubingensis, are spiral-shaped bacteria that move along the prey filament, causing the host cells to lyse upon contact. These microbial consumers exert a high level of mortality on cyanobacterial populations, often targeting the individual cells that make up a larger bloom.
The Unique Role of Cyanophages
Cyanophages are viruses specifically engineered to infect and destroy cyanobacteria, representing a unique and specialized form of predation. These viruses are pervasive in both freshwater and marine environments, acting as a rapid, targeted control mechanism for cyanobacterial populations. Cyanophages typically operate on a lytic life cycle, which results in the swift destruction of the host cell.
The process begins when the cyanophage attaches to the surface of a cyanobacterial cell and injects its genetic material. This foreign DNA hijacks the cell’s internal machinery, forcing it to stop normal functions and produce new viral components. Once new viral particles have been assembled, the host cell’s membrane is ruptured through lysis, releasing the new viruses to infect surrounding cyanobacteria.
This mechanism is significant in terminating dense blooms, often following the “kill the winner” hypothesis, where the most dominant cyanobacterial species is targeted by a specialized phage. The sudden lysis of cells releases dissolved organic material and nutrients back into the water, rapidly collapsing the bloom structure. While effective, phages are often strain-specific, meaning a single phage may only target one type of cyanobacterium, necessitating a diverse viral community to control a mixed-species bloom.
Why Predation Is Challenging: Defense Mechanisms
Despite the array of predators, cyanobacterial blooms still occur because these organisms have evolved sophisticated defense mechanisms. One primary physical defense involves morphological changes, specifically the formation of large colonies or long, filamentous chains. This increased size makes the cyanobacteria physically too large for many filter-feeding zooplankton, such as smaller rotifers and cladocerans, to ingest or handle.
Many cyanobacteria secrete a thick, slimy layer of mucilage or sheath material that surrounds the cells or colonies, further deterring grazers by making them unpalatable or difficult to capture. Additionally, some species possess gas vesicles, which allow them to regulate their buoyancy and move vertically in the water column, often rising to the surface to form dense scums and escape deeper-dwelling predators.
The most well-known defense is chemical, involving the production of potent compounds called cyanotoxins, such as microcystins and saxitoxins. These toxins act as feeding deterrents, reducing the ingestion rates of zooplankton and even poisoning some grazers. Cyanobacteria are also often a nutritionally poor food source for zooplankton, lacking essential fatty acids and sterols, meaning they do not provide the necessary building blocks for the predator’s growth and reproduction.