CO2 is a nontoxic, nonflammable, abundant, and renewable feedstock and its bio­transformation into industrially important chemicals can not only have a positive impact on the global carbon balance but also provide novel routes for the green biotechnology. As one of the oldest life forms on earth, microalgae have very high CO2 biofixation capacity, grow fast, and accumulate large quantities of lipids and carbohydrates and hence became the most promising feedstock for production of next generation biofuels like biodiesel and bioethanol [78]. Considering the fact that CO2 is a very cheap carbon source, microalgal systems should also be considered as potential resources for EPS production. However, in the literature, there are very few reports on microalgal polysaccharide production. In general, for production of value added products, the biggest advantage in using open microalgae culture is the direct use of solar energy which in turn is highly energy efficient and cheap [79]. Actually, these systems applied to phototropic and mixotrophic cultures are considered to be the most technically and economically feasible methods at commercial scale [78]. On the other hand, this advantage does not hold for EPS production where use of monocultures, closed, and controlled cultivation systems are required to reach high levels of productivity [80]. Although photobioreactors and fermenters are advanta­geous in providing optimum conditions for biomass growth and EPS production, these systems are expensive and energy intensive when compared with open systems [81].

There are various types of bioreactors that can be used for EPS production such as airlift flat plate photobioreactors (well reviewed by Zhang et al. [82]). Gener­ally, culture conditions for lipid-rich biomass production and EPS production are remarkably different. A systematic study conducted with the green colonial fresh water microalgae Botryococcus braunii strains on the effect of culture conditions on their growth, hydrocarbon and EPS production also revealed two distinct culture conditions so that cultivation in 16:8 h light dark cycle yielded higher hydrocarbons whereas continuous illumination with agitation yielded higher amounts of EPSs with

1.6 g/L—maximum yield obtained from B. braunii LB 572 strain [83]. In a study on the effect of salinity with the same strain, EPS yields of 2-3 g/L were also reported [84]. The difference in cultivation conditions could also be used for the high-level EPS production by use of a two-stage culture as reported for spirulan production by Spirulina platensis [85]. In this method, whereas the first stage focuses on rapidly increasing microalgal biomass, culture conditions in the second stage are modified to maximize the polysaccharide yield. Rhodella violacea [86] and Porphyridium cruen — tum [87] are well known as producers for viscous bioactive EPS [88] and the highest yield of 543.1 mg/L EPS production was reported for P cruentum after optimization of initial pH, light intensity, inoculation ratio, and liquid volume of shaking batch cultures [89]. By culturing P cruentum semi-continuously in flat plate photobioreac­tors, a production rate of 68.64 mg/L per day could be reached by Sun et al. [90]. Very low EPS concentrations (less than 30 mg/L) were reported for planktonic diatoms like Amphora holsatica, Navicula directa, and Melosira nummuloides [5, 91]. How­ever, these yields can be improved by further studies on optimizing the bioreactor conditions in favor of EPS production.

Another important issue for microalgal cultivation is the need for using high con­centrations of chemical fertilizers as a source for nitrogen and phosphorus. Whereas high nitrogen concentrations in the cultivation medium favors polysaccharide syn­thesis pathways and biomass formation, lipid accumulation is favored under nitrogen limited conditions where polysaccharide pathways are blocked and the photosynthet­ically fixed carbon is directed towards fatty acid synthesis [92]. Microalgae could become a favorable source for EPS production if the high expenses associated with fertilizers could be reduced by replacing them with their low cost alternatives. Be­sides the use of wastewater as an inexpensive source, the literature is very limited in such studies.