Abstract
Outbreaks of cyanobacterial blooms in freshwaters have been increasingly reported worldwide during the last few decades. One of the major problems with cyanobacterial blooms is that some species can be toxic. Their toxins (cyanotoxins) can have mild to serious health effects on humans and animals due to direct contact or ingestion with drinking water and food. Due to the unavailability of efficient testing methods for cyanotoxins especially in developing countries, people may consume toxin contaminated water and foods. Non-toxic cyanobacterial blooms may cause other ecological and socioeconomic impacts. Therefore, physical and chemical control methods are currently being employed as direct control or removal of cyanobacterial blooms. However, due to some intrinsic adverse consequences of physical and chemical control of blooms on the ecosystem and high operational cost, biological control has gained attention as an alternative sustainable bloom management strategy. A diverse array of control agents such as viruses, bacteria, fungi, actinomycetes, protozoa, flagellates, and macrophytes with the potential to terminate or suppress the growth of cyanobacteria have been isolated and identified from freshwater bodies in different parts of the world. Among the controlling agents, heterotrophic bacteria in the aquatic environment are of high interest owing to their high specificity on the targeted cyanobacteria and high survival ability during non-bloom conditions. Heterotrophic bacteria antagonize cyanobacteria either through direct contact or indirectly by the secretion of allelopathic compounds. The mechanisms of antagonism are known to be physiological and metabolic dysfunctions and transcriptional regulation of genes. In addition, there are some evidence to show regulation of antagonism through cell density-dependent quorum-sensing (QS) mechanisms. In response, cyanobacteria induce defensive mechanisms such as alteration of colony morphology and activation of chemical defences against microbial antagonists. This review article aimed to encompass the current status of knowledge on the biological control of freshwater cyanobacterial blooms and to highlight the existing knowledge gap in the available literature. We highlight that although a large number of microbial antagonists have been isolated, identified, and demonstrated their cyanobacteria lytic activity in the laboratory, their field application is still challenging. Understanding the spatial and temporal variations in antagonists and cyanobacteria in a particular aquatic ecosystem is essential for planning an effective bloom management strategy. We also highlight the necessity in expanding research focus towards novel strategies to enhance application potential and overcome existing challenges.