The rapid proliferation of algae and cyanobacteria in aquatic environments, known as an algal bloom, represents an ecological challenge. When these growths produce toxins or cause severe environmental damage, they are termed Harmful Algal Blooms (HABs). These blooms can lead to hypoxia when the massive algal biomass eventually dies and decomposes, creating “dead zones” that kill fish and other aquatic life. Beyond deoxygenation, many species produce potent toxins, such as the liver toxin microcystin or various neurotoxins. This global problem, often fueled by excess nutrients like nitrogen and phosphorus from human activity, prompts the question of whether natural mechanisms, specifically bacteria, can effectively counteract this runaway growth.
Algicidal Bacteria: Nature’s Algae Controllers
Aquatic ecosystems contain specialized microorganisms known as algicidal bacteria that actively target and destroy phytoplankton. These bacteria are ubiquitous in both marine and freshwater environments, where they constantly interact with the primary producers.
The relationship between these bacteria and algae is one of antagonism, where the bacteria act as a form of microscopic predator or parasite. They are recognized as drivers in the natural “crash” phase of a bloom cycle, where the algal population suddenly declines. Numerous bacterial groups possess this ability, including members of the phyla Bacteroidetes and Gammaproteobacteria. Specific genera often isolated and studied for their lytic capabilities include Pseudomonas, Alteromonas, and Bacillus.
These specialized bacterial strains control the proliferation of microalgae, playing an important role in shaping the species composition within pelagic environments. The ability of these bacteria to lyse, or rupture, algal cells demonstrates a natural control mechanism that prevents a single algal species from completely dominating the ecosystem.
How Bacteria Destroy Algal Cells
Algicidal bacteria employ different strategies to induce the death and lysis of algal cells, which can be broadly divided into contact-dependent and contact-independent mechanisms. The method of attack determines whether the bacteria must physically engage with their target or can kill remotely using secreted chemical agents.
The direct-contact mode requires the bacterial cell to physically attach to the surface of the algal cell before inducing lysis. This physical association often occurs within the phycosphere, the immediate, nutrient-rich vicinity surrounding the algal cell. Once attached, the bacteria may release lytic enzymes or compounds directly onto the cell wall or membrane of the algae, causing the cell to rupture. For instance, some strains, like certain Streptomyces species, have been observed to physically twine around cyanobacteria cells.
The indirect, or remote, killing mechanism involves the secretion of algicidal compounds into the surrounding water. These chemicals exert their toxic effect without the need for physical contact between the bacteria and the algae. Researchers confirm this mode by demonstrating that a cell-free filtrate, with all bacteria removed, retains the ability to kill the target algae.
The compounds released are chemically diverse, including enzymes, antibiotics, and toxins. Certain bacteria, such as Rheinheimera species, use L-amino acid oxidases, which generate hydrogen peroxide as a byproduct to kill algae. These algicides disrupt processes in the algal cell, such as damaging the cell structure, inhibiting photosynthesis, or triggering programmed cell death.
A third, less immediate mechanism is nutrient competition, where bacteria outcompete algae for scarce resources like iron or nitrogen. Bacteria produce specialized molecules, such as siderophores, to scavenge essential iron from the water, effectively starving the algae. This mechanism suppresses algal growth, influencing the overall bacterial impact on algal bloom dynamics.
Leveraging Bacteria for Algal Bloom Management
The natural ability of algicidal bacteria to control algae has prompted extensive research into using them as a targeted, environmentally conscious method for managing harmful algal blooms. This approach, known as microbial biocontrol, seeks to harness specific bacterial strains or their compounds for direct application in affected water bodies. The advantage over chemical treatments is the potential for specific targeting and avoiding secondary pollution.
Current research focuses on isolating and cultivating strains that show high algicidal efficiency against specific bloom-forming species, such as the dinoflagellate Karenia mikimotoi. The goal is to develop a product, such as a dried powder or a concentrated liquid, that can be introduced into the environment to eliminate the bloom. This biological approach is an alternative to methods that are often expensive or can indiscriminately harm non-target aquatic life.
However, the transition from successful laboratory trials to large-scale field application presents challenges. One major difficulty is maintaining the viability and required dosage of the bacteria once they are introduced into a complex, dynamic aquatic ecosystem. While some strains are highly specific, others may kill multiple species, raising concerns about unintended ecological consequences. Furthermore, introducing algicidal bacteria can inadvertently boost the release of dissolved phosphorus from the dying algae, which could then fuel the growth of other algal species. The development of bacterial biocontrol agents remains a rapidly evolving and promising field for sustainable water management.