Carbohydrate-degrading bacteria closely associated with Tetraselmis indica: influence on algal growth

In the present study, we examined the interactions between the algal species Tetraselmis indica and strains of bacteria with which it is closely associated. Three bacterial strains were isolated and sequence analysis of the 16S rDNA indicated that the organisms belong to the genera Pseudomonas, Acinetobacter and Ruegeria. Morphologies of the bacterial strains were studied using epifluorescence microscopy and scanning electron microscopy. Reassociation experiments were conducted with axenic cultures inoculated with the 3 bacterial strains in concentrations comparable to natural conditions, and the effect of each bacterial population on the growth of T. indica was determined. T. indica exhibited differential growth with the various bacterial cultures, and in particular Acinetobacter sp. was observed to promote growth of the algae. These experiments revealed that microbes associated with the alga differentially influence algal growth dynamics. Bacterial presence on the cast-off cell wall products of the alga suggested the likely utilisation of algal cell wall by bacteria. The bacterial strains were tested for carbohydrate metabolism using various sugars and screened for carbohydrase activity. Bacterial strains were found to produce carbohydrases for degradation of polysaccharides generally present in the cell wall of Tetraselmis (glucans, galactans, galactomannans and pectins), whereas no such utilisation was observed for other wall substrates (such as cellulose, arabinoxylan, rhamnogalacturonan). Pseudomonas sp. and Acinetobacter sp. showed carbohydrase activity with glucans, galactans, galactomannans and pectin, whereas Ruegeria sp. showed much less carbohydrase activity and only with pectin. The carbohydrate utilisation studies using artificial substrates suggested the potential utilisation of cast-off algal cell wall products.