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Algae and bacteria have coexisted ever since the early stages of evolution. This coevolution has revolutionized life on earth in many aspects. Algae and bacteria together influence ecosystems as varied as deep seas to lichens and represent all conceivable modes of interactions - from mutualism to parasitism. Several studies have shown that algae and bacteria synergistically affect each other's physiology and metabolism, a classic case being algae-roseobacter interaction. These interactions are ubiquitous and define the primary productivity in most ecosystems. In recent years, algae have received much attention for industrial exploitation but their interaction with bacteria is often considered a contamination during commercialization. A few recent studies have shown that bacteria not only enhance algal growth but also help in flocculation, both essential processes in algal biotechnology. Hence, there is a need to understand these interactions from an evolutionary and ecological standpoint, and integrate this understanding for industrial use. Here we reflect on the diversity of such relationships and their associated mechanisms, as well as the habitats that they mutually influence. This review also outlines the role of these interactions in key evolutionary events such as endosymbiosis, besides their ecological role in biogeochemical cycles. Finally, we focus on extending such studies on algal-bacterial interactions to various environmental and bio-technological applications.

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The production of microcystins (MC) from Microcystis aeruginosa UTEX 2388 was investigated in a P-limited continuous culture. MC (MC-LR, MC-RR, and MC-YR) from lyophilized M. aeruginosa were extracted with 5% acetic acid, purified by a Sep-Pak C 18 cartridge, and then analyzed by high-performance liquid chromatography with a UV detector and Nucleosil C 18 reverse-phase column. The specific growth rate () of M. aeruginosa was within the range of 0.1 to 0.8/day and was a function of the cellular P content under a P limitation. The N/P atomic ratio of steady-state cells in a P-limited medium varied from 24 to 15 with an increasing . The MC-LR and MC-RR contents on a dry weight basis were highest at of 0.1/day at 339 and 774 g g ؊1 , respectively, while MC-YR was not detected. The MC content of M. aeruginosa was higher at a lower , whereas the MC-producing rate was linearly proportional to . The C fixation rate at an ambient irradiance (160 microeinsteins m ؊2 s ؊1 ) increased with . The ratios of the MC-producing rate to the C fixation rate were higher at a lower . Accordingly, the growth of M. aeruginosa was reduced under a P limitation due to a low C fixation rate, whereas the MC content was higher. Consequently, increases in the MC content per dry weight along with the production of the more toxic form, MC-LR, were observed under more P-limited conditions.The bloom of cyanobacterium Microcystis aeruginosa is a ubiquitous phenomenon in eutrophic lakes and reservoirs in many countries of the world. Many strains of Microcystis are known to produce cyanobacterial hepatotoxins called microcystins. The toxin, a soluble peptide, is lethal to many kinds of aquatic organisms and damages zooplankton, fish (14), and the liver of higher animals (2, 22).Many studies on the effects of environmental factors on microcystin production by cyanobacteria have been conducted with Microcystis (7,13,19,20,(22)(23)(24), Anabaena (15, 16), Oscillatoria (17), and Synechocystis (11) species. The toxin of M. aeruginosa is at a maximum at light intensities between 40 and 50 microeinsteins m Ϫ2 s Ϫ1 (19, 23). The microcystin contents of Anabaena and M. aeruginosa are highest at 25°C (16) and between 20 and 24°C (21), respectively. Lower and higher temperatures decrease their amounts. In contrast, the effects of N and P on the toxin production by cyanobacteria are highly variable (13). Batch-cultured M. aeruginosa decrease in toxicity when N and inorganic C are removed from the medium (2). The concentration of microcystin in Anabaena increases with P (16). Toxin production by Oscillatoria agardhii depends on a low-level concentration of P (0.1 to 0.4 mg of P per liter), and higher concentrations have no additional effect (17). It was recently reported that the net microcystin production rate decreases as the specific cell division rate decreases in N-limited M. aeruginosa cultures (13). However, information about microcystin production related to the nutrient status in cells or in ambient circumstances is still insufficient.In this study, M. ...