Microalgae Growth-promoting Bacteria: a Novel Approach in Water Science; a Micro-review

Plant growth-promoting bacteria (PGPB) of the genus Azospirillum are known to enhance the growth of numerous agricultural crops. We are proposing the use of these bacteria as “microalgae-growth promoting bacteria” (MGPB) to enhance the cleaning capacity of polluted water by the freshwater microalgae Chlorella vulgaris and C. sorokiniana. Deliberate inoculation of Chlorella sp. with a terrestrial PGPB was not reported prior to these studies, perhaps because of the different origins of the two microorganisms. Chlorella sp. is not known to harbor any plant growth-promoting bacteria, and Azospirillum sp. is rarely used for inoculation in aquatic environments. Co-immobilization of C. vulgaris and A. brasilense Cd in small alginate beads resulted in significant increases in numerous growth parameters of the microalgae. Dry and fresh weight, total number of cells, size of the microalgal clusters (colonies) within the bead, number of microalgal cells per cluster, and the levels of microalgal pigments significantly increased. Additionally, lipids and the variety of fatty acids significantly increased, and the combination of microalgae and the MGPB had superior capacity of removing ammonium and phosphorus from polluted synthetic and municipal wastewaters. Other PGPB (like Flavobacterium sp., Azospirillum sp., and Azotobacter sp.) are currently studied in aquaculture; carp cultivation by enhancing phytoplankton growth, and for stabilization of mass culture of marine microalgae used as a feed for marine organisms, all with promising results. This aspect of the effect of PGPB on water microorganisms is currently in its infancy. We propose that co-immobilization of microalgae and plant growth-promoting bacteria are an effective means of increasing microalgal populations and also their capacity for cleaning polluted water. INTRODUCTION In natural or artificial aquatic environments, freshwater or marine, microalgae are always associated with bacteria (Mouget et al., 1995). It is unknown whether these bacteria are associative, growth promoters, symbionts, or simply co-exist with the microalgae (like saprophytes in terrestrial environments). Apart from a few, these bacteria were seldom isolated or characterized (Suminto and Hirayama, 1997; Gonzalez-Bashan et al., 2000), and their effect on the microalgae, some used for wastewater treatment (Tam and Wong, 2000; Valderrama et al., 2002), is unknown. The working hypothesis of the studies described in this short review is that some of the aquatic bacteria are plant growth promoters. As microalgae can be considered a plant, albeit of single cells, these bacteria may promote the growth and the activity of the microalgae. As a first step, and since almost no PGPB was ever demonstrated to have an effect on microalgal growth, the use of Azospirillum PGPB of