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

This investigation was conducted in order to examine the interactions between an algal species, Tetraselmis indica (a new species described, manuscript under consideration elsewhere), collected in the wild 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 belonged to the genera Pseudomonas, Acinetobacter and Ruegeria. Morphologies of the bacterial strains were studied using epifluorescence microscopy and scanning electron microscopy. Re-association experiments were conducted with axenic cultures inoculated with the three bacterial strains in concentrations comparable to natural conditions, and the effect of each bacterial population on the growth of T. indica was determined. Tetraselmis indica exhibited differential growth success 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 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 T. indica (glucans, galactans, galactomannans and pectins), whereas no such utilisation was observed for other wall substrates such as cellulose, arabinoxylan, rhamnogalacturonan). The Pseudomonas sp. and the Acinetobacter sp. showed carbohydrase activity with Glucans, galactans, galactomannans and pectin, whereas the 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.