Heterotrophically cultivated cells use organic substrate(s) for carbon and energy while mixotrophically grown cells can use light and organic carbon for energy and use either organic or inorganic carbon for biomass synthesis [180, 181, 284]. Many algae can utilize various organic carbon substrates: sugars (glucose, mannose, fructose, lactose, etc.), VFA (acetate), glycerol, molasses, and organic carbon from wastewater. Under mixotrophic conditions, algae exhibit a five — to tenfold higher growth rate compared to photoautotrophic growth [285]. Species that grow mixotrophically include:

• Green algae: C. reinhardtii [286-288]; Chlorella sp. [289-293]; Scenedesmus [294-296]; Tetraselmis suecica [297]; Platymonas subcordiformis [298]; Botryococcus braunii [299]; Micractiniumpusillum [300]; Haematococcusplu — vialis [301-303]; Haematococcus lacustris [304, 305].

• Red algae: Porphyridium cruentum [306, 307]; Galdieria sulphuraria [308].

• Diatoms: Phaeodactylum tricornutum [256,285,309-312]; Nannochloropsis sp. [313-315]; Navicula saprophila [316]; Nitzschia [316, 317].

• Cyanobacteria: Synechococcus [318, 319]; Arthrospira [320-323]; Nostoc flagelliforme [324]; Anabaena variabilis [325].

The main disadvantage of mixotrophic growth is the high cost of organic carbon sources. Main advantages of mixotrophic growth are the elimination of light pene­tration limitation that allows high concentration of algae (opportunity to reduce harvesting cost), better process control, increased growth rate and production of lipids, and potential to use waste streams as an organic carbon source. One attractive solution is the coupling of wastewater treatment and algal production [326]. Another possible source of organic carbon is acetate from a modified AD system where the major products of anaerobic fermentation are acetate and hydrogen. The drawback of systems grown on waste streams is bacterial and viral contamination.

Light Conditions

Light characteristics have a significant impact on pigment content, photosynthetic activity, and lipid content [327, 328]. Generally, high irradiation inhibits the forma­tion of polar lipids, but stimulates synthesis of storage carbohydrates and neutral lipids (usually triglycerids). Light limitation favors the production of total proteins and structural polar lipids associated with chloroplasts [258, 329-332]. B. braunii cultivated under continuous illumination gave the highest yield of exopolysaccha­rides. A light-dark cycle of 16-8 h resulted in the highest hydrocarbon yield [333]. Blue light was found to promote protein synthesis, while red light stimulated carbo­hydrate production [334].