Coimmobilization of the freshwater microalga Chlorella vulgaris and the plant-growth-promoting bacterium Azospirillum brasilense in small alginate beads resulted in a significantly increased growth of the microalga. 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. Light microscopy revealed that both microorganisms colonized the same cavities inside the beads, though the microalgae tended to concentrate in the more aerated periphery while the bacteria colonized the entire bead. The effect of indole-3-acetic acid addition to microalgal culture prior to immobilization of microorganisms in alginate beads partially imitated the effect of A. brasilense. We propose that coimmobilization of microalgae and plant-growthpromoting bacteria is an effective means of increasing microalgal populations within confined environments.Microalgae have many uses. They can serve as water bioremediation agents (40), as feed for aquaculture (17), as food for humans and animals (10), in pigment production (25), in bioremoval of heavy metals (49), and in agriculture (33). It is usually desirable to establish large populations of microalgae, especially in aquatic environments where they are often employed. One means of increasing microalgal populations may be to inoculate them with other microorganisms, a strategy that is being tested for its potential to increase yields of agriculturally important plants (5,23,32).One candidate microorganism for coinoculation with microalgae is Azospirillum brasilense, a member of the group of plant rhizosphere bacteria known as plant-growth-promoting bacteria (PGPB) (6,20,29). This relatively well-studied diazotrophic bacterium (5) promotes the growth of many terrestrial plants upon seed or root inoculation and increases the yields of numerous crop plants (8,37). All the known Azospirillum species produce plant hormones, mainly auxins, as do many other PGPB. It is thought that interference with the hormonal metabolism of the host plant is one of the major ways in which PGPB affect plant growth (15,41).The aim of this study was to increase the growth of the freshwater microalga Chlorella vulgaris, an important organism in tertiary wastewater treatment (21, 30, 44) and for several industrial research studies (27,48), by inoculating it with A. brasilense when grown within a confined environment. This study is the first report of the deliberate inoculation of Chlorella sp. with a terrestrial PGPB, perhaps because of the different origins of the two microorganisms. C. vulgaris is not known to harbor any associative beneficial bacteria, and Azospirillum sp. is rarely used for inoculation in aquatic environments (42).To ensure the close proximity of the two microorganisms in the liquid medium essential for C. vulgaris, they were coimmobilized in alginate beads and were cocultivated under controlled conditions suitable for both, in batch cultures and in con...
