Chemical manipulations of algae are reflected in their biochemical constituents. Nitrogen starvation increased lipid production from 117 to 204 mg L-1d-1 (Rodolfi et al., 2008). By manipulating the nutrients in Scenedesmus obliquus, up to 58.3% lipid was attained, which was five — to tenfold higher than controls (Mandal and Mallick, 2009). It is of interest to note that carotenoids increased only in Dunaliella salina as the salinity increased (Gomez et al., 2003; Coesel et al., 2008). The carotenoid levels (mg L-1) corresponded to 6.9, 10.8, and 12.9 mg L-1 in Provosoli medium of 1 M, 2 M, and 3 M sodium chloride (NaCl), respectively; in an arti­ficial medium, they were more pronounced and were 8, 12.9, and 29.5 mg L-1 in 1 M, 2 M, and 3 M NaCl, respectively. Takagi et al. (2006) showed that the salt content of the medium could also be a stressor in Dunaliella. In the initial stages of cultures, when the NaCl was increased from 0.5 M (equivalent to seawater) to 1.0 M, lipid increased by 67%; when mid- or late-log phase cultures were subjected to a similar stress, cellular lipid increased to 70%. So while harvesting cells for biotechnological applications, the strain and physiological state of the algae play critical roles in determining output.

In addition to production rates, variations in biochemical profiles must be con­sidered for optimization of harvesting. Carbohydrates, proteins, lipids, and fats are known to vary with the medium used among seven species of marine microalgae (Fernandez-Reiriz et al., 1989) and in sixteen species of microalgae commonly used in aquaculture (Brown, 1991). The medium used (Walne, ES, f/2, and Algal-1) for cultivation also influenced the biochemical profiles of four species (Fernandez — Reiriz et al., 1989).

Through biochemical manipulation, lipid synthesis can be regulated; this involves imposing a physiological stress such as nutrient starvation to channel metabolic pro­cesses toward lipid accumulation. In experiments by Li et al. (2008), cultures of Neochloris oleoabundans were supplied with sodium nitrate, urea, and ammonium bicarbonate as the nitrogen source; only at lower levels of sodium nitrate did cel­lular lipid increase. Co-limitation for inorganic phosphorus and carbon dioxide in Chlamydomonas acidophila Negoro resulted in high photosynthetic rates and also in a mismatch between photosynthesis and growth rates in phosphorus-limited cul­tures (Spijkerman, 2010). In Monodus subterraneus when phosphate was decreased from 175 to 52.5, 17.5, or 0 pM, cellular lipid increased (Khozin-Goldberg, and Cohen, 2006). Limitation of nitrogen in cultures of the green alga Scenedesmus obliquus resulted in an increase of lipid from 12.7% to 43% of cell dry weight (DW) (Mandal and Mallick, 2009); a deficiency of phosphate increased lipid to 29.5% (DW). Lipids in nitrogen-limited Chaetoceros mulleri increased five — to sevenfold compared to nitrogen-replete cultures (McGinnis et al., 1997). Results obtained with Nannochloropsis oculata and Chlorella vulgaris (Converti et al., 2009) con­firmed such an impact of nitrogen limitation. Lipid production is enhanced to 90 kg ha-1d-1 by a two-stage culture system that involves raising high-density cultures under optimal conditions initially and then transferring them to a nitrogen-deficient medium (Rodolfi et al., 2008).

In Dunaliella salina cultures, an increase in CO2 from 2% to 10% increased lipid production by 170% in 7 days (Muradyan et al., 2004). In Chlamydomonas vulgaris, the addition of 1.2 x 10-5 M Fe3+ not only suppressed cell growth initially, but also enhanced the accumulation of lipids up to 56.6% DW. Furthermore, the accu­mulation of lipids occurred earlier during the stationary phase (Liu et al., 2008). To enhance the yield of microalgal biomass, rigorous experiments should be carried out to establish the impact of several micronutrients, such as selenium and boron. Optimized growth of commercial algae should account for the effects of manipula­tions in nutrients, temperature, and chemical composition of media.