Antagonism of Bacillus spp. towards Microcystis aeruginosa

Abstract

Freshwater resources are threatened by the presence and increase of harmful algal blooms (HABs) all over the world. The HABs are sometimes a direct result of anthropogenic pollution entering water bodies, such as partially treated nutrient-rich effluents and the leaching of fertilisers and animal wastes. Microcystis species are the dominant cyanobacteria (algae) that proliferate in these eutrophic waters. The impact of HABs on aquatic ecosystems and water resources, as well as their human health implications are well documented. Countermeasures have been proposed and implemented to manage HABs with varying levels of success. These control measures include the use of flocculants, mechanical removal of hyperscums and chemical algicides. The use of flocculants such as PhoslockTM is effective in reducing the phosphates in a water body thus depriving nutrients that are available to cyanobacteria. The mechanical option entails the manual removal of hyperscums thus reducing the numbers of cyanobacteria cells that may be the inoculum of the next bloom. The major disadvantage of these two measures is cost. Copper algicides have been used successfully to control HABs in raw water supplies intended for potable purposes. The major disadvantages are copper toxicity and release of microcystins from lysed cyanobacteria cells. Algicides accumulate in the sediments at concentration that are toxic to other aquatic organisms and may also cause long-term damage to the lake ecology. In some studies, microcystins have been implicated in the deaths of patients undergoing haemodialysis. Therefore there is an increasing need to reduce the use of chemicals for environmental and safety reasons. Thus, the development of environmentally friendly; safe non-chemical control measures such as biological control is of great importance to the management of HABs. Some papers, describe bacteria, which were isolated from eutrophic waters, such as Sphingomonas species with abilities to degrade microcystins and Saprospira albida with abilities to degrade Microcystis cells. Further research is required to evaluate whether these bacteria are antagonistic towards cyanobacteria. Ideally, a combination of strategies should be introduced; that is, combine predatory bacteria that directly lyse the cyanobacteria with microcystin degrading bacteria that then ‘mop up’ the released microcystins. The major objective of this study was to isolate organisms that have a similar antagonistic properties; determine their mechanism of action and then develop a model to account for the interaction between the predator and prey as the basis for the development of a biological control agent. During the screening for lytic organisms from eutrophic waters of Hartbeespoort dam, seven bacterial isolates were obtained. Based on electron microscope observation, two of the isolates were found aggregated around unhealthy Microcystis cells. These were identified as Pseudomonas stutzeri strain designated B2 and <i.Bacillus mycoides strain designated B16. Based on efficiency and efficacy experiments B. mycoides B16 was a more effective antagonist than P. stutzeri B2. Furthermore the <i.B. mycoides B16: Microcystis critical ratio was found to be 1:1 in 12 days. Thus altering the predator-prey ratio by increasing the predator bacteria numbers reduced the Microcystis lysis time to six days. The B. mycoides B16 managed to reduce the population of alive Microcystis cells by 85% under turbulent conditions and 97% under static conditions in six days. The increase in predator bacteria numbers coincided with a decrease in growth of Microcystis. The study on the interactions of Microcystis aeruginosa and Bacillus mycoides B16 indicated a series of morphological and ultrastructural changes within the cyanobacteria cell leading to its death. These are summarised in a conceptual model that was developed. The predatory bacteria, B. mycoides B16 attached onto the Microcystis cell through the use of fimbriae and or exopolymers. During this attachment the bacteria released extracellular substances that dissolved the Microcystis cell membrane and interfered with the photosynthesis process. The presence of numerous bacterial cells that aggregated around Microcystis cell provided a ‘shade’ that reduced the amount of light (hv) that reached the Microcystis cell. In response to these adverse conditions, the Microcystis cell did the following: It expanded its thylakoid system, the light harvesting system, to capture as much light as possible to enable it to carry out photosynthesis and it accumulated storage granules such as phosphate bodies, glycogen and cyanophycin and swollen cells. Other researchers have also reported the swelling phenomenon prior to cell lysis but did not account for what might be the cause. During the course of the lysis process the Microcystis cell underwent a transition stage that involved changes from alive (with an intact membrane) to membrane compromised (selective permeability), to death (no membrane) and eventual cell debris. Due to the depleted Microcystis cells, the B. mycoides B16 (non-motile, non-spore former) formed chains, i.e., exhibited rhizoidal growth in search of new Microcystis cells to attack. In conclusion, the present evidence in this study suggests that B. mycoides B16 is an ectoparasite (close contact is essential) in its lysis of Microcystis aeruginosa under laboratory conditions. These findings that B. mycoides B16 is a predatory bacterium towards Microcystis aeruginosa need to be further evaluated under field conditions in mesocosm experiments (secluded areas in a lake) to determine the possibility of using this organism as a biological control agent. The study on the interactions of Microcystis aeruginosa and Bacillus mycoides B16 indicated a series of morphological and ultrastructural changes within the cyanobacteria cell leading to its death. These are summarised in a conceptual model that was developed. The predatory bacteria, B. mycoides B16 attached onto the Microcystis cell through the use of fimbriae and or exopolymers. During this attachment the bacteria released extracellular substances that dissolved the Microcystis cell membrane and interfered with the photosynthesis process. The presence of numerous bacterial cells that aggregated around Microcystis cell provided a ‘shade’ that reduced the amount of light (hv) that reached the Microcystis cell. In response to these adverse conditions, the Microcystis cell did the following: It expanded its thylakoid system, the light harvesting system, to capture as much light as possible to enable it to carry out photosynthesis and it accumulated storage granules such as phosphate bodies, glycogen and cyanophycin and swollen cells. Other researchers have also reported the swelling phenomenon prior to cell lysis but did not account for what might be the cause. During the course of the lysis process the Microcystis cell underwent a transition stage that involved changes from alive (with an intact membrane) to membrane compromised (selective permeability), to death (no membrane) and eventual cell debris. Due to the depleted Microcystis cells, the B. mycoides B16 (non-motile, non-spore former) formed chains, i.e., exhibited rhizoidal growth in search of new Microcystis cells to attack. In conclusion, the present evidence in this study suggests that B. mycoides B16 is an ectoparasite (close contact is essential) in its lysis of Microcystis aeruginosa under laboratory conditions. These findings that B. mycoides B16 is a predatory bacterium towards Microcystis aeruginosa need to be further evaluated under field conditions in mesocosm experiments (secluded areas in a lake) to determine the possibility of using this organism as a biological control agent. The study on the interactions of Microcystis aeruginosa and Bacillus mycoides B16 indicated a series of morphological and ultrastructural changes within the cyanobacteria cell leading to its death. These are summarised in a conceptual model that was developed. The predatory bacteria, B. mycoides B16 attached onto the Microcystis cell through the use of fimbriae and or exopolymers. During this attachment the bacteria released extracellular substances that dissolved the Microcystis cell membrane and interfered with the photosynthesis process. The presence of numerous bacterial cells that aggregated around Microcystis cell provided a ‘shade’ that reduced the amount of light (hv) that reached the Microcystis cell. In response to these adverse conditions, the Microcystis cell did the following: It expanded its thylakoid system, the light harvesting system, to capture as much light as possible to enable it to carry out photosynthesis and it accumulated storage granules such as phosphate bodies, glycogen and cyanophycin and swollen cells. Other researchers have also reported the swelling phenomenon prior to cell lysis but did not account for what might be the cause. During the course of the lysis process the Microcystis cell underwent a transition stage that involved changes from alive (with an intact membrane) to membrane compromised (selective permeability), to death (no membrane) and eventual cell debris. Due to the depleted Microcystis cells, the B. mycoides B16 (non-motile, non-spore former) formed chains, i.e., exhibited rhizoidal growth in search of new Microcystis cells to attack. In conclusion, the present evidence in this study suggests that B. mycoides B16 is an ectoparasite (close contact is essential) in its lysis of Microcystis aeruginosa under laboratory conditions. These findings that B. mycoides B16 is a predatory bacterium towards Microcystis aeruginosa need to be further evaluated under field conditions in mesocosm experiments (secluded areas in a lake) to determine the possibility of using this organism as a biological control agent.