Interactions between the unicellular red alga Rhodella reticulata (Rhodophyta) and contaminated bacteria

AIMS: To define the role of the bacterial strains LR1 and LR3 in the Rhodella cell destruction caused by Cytophaga sp.LR2. METHODS AND RESULTS: The bacteria were obtained from algal culture with destruction. They were isolated in pure culture and tested for biochemical activities using Polymicrotest. The ability of bacteria to degrade and utilize the algal polysaccharide was investigated. The bacteria were grown in a media containing Rhodella polysaccharide as a sole carbon source. The level of the reducing sugars in the culture media was determined. Scanning electron microscopy (SEM) was used to define the location of bacteria in extensively and intensively cultivated Rhodella reticulata previously infected by Cytophaga sp. LR2. CONCLUSIONS: The lysis of Rhodella reticulata cells is due to the joint action of the three bacterial strains with the former pathogen Cytophaga sp. LR2 playing the main role. The accumulation of the polysaccharide and the excreted metabolites of the strains LR1 and LR3 stimulated the development of Cytophaga sp. LR2. The adaptation of the strain to particular conditions of alga cultivation and the utilization of polysaccharide as a sole carbon source supported its stable growth in alga suspension and destruction of Rhodella cells. SIGNIFICANCE AND IMPACT OF THE STUDY: The predominance of Cytophaga sp. LR2 over the two other contaminants and the lysis of Rhodella reticulata cells resulted from the ability of the bacterium to attach to the algal polysaccharide sheath. The formation of slime and extrusions facilitated the phenomenon of bacterial adhesion to the algal surface as well as the formation of colonial alga - bacterial spherules. The sedimentation of these aggregates decreased the ability of the algal strain to photosynthesize, led to the lysis of the cells and finally caused the death of Rhodella.