concentrations, protein synthesis is low and most of fixed carbon is converted to carbohydrate or lipid.
Thus, concentration of biomass decreases, while accumulation of lipid increases. High lipid production when algae are starved with nitrogen leads to longer duration than the normal time to yield the same amount from non-starved microalgae. The cell division is possibly delayed and the biochemical structure of cell is altered from the original chemical composition due to the stress and converted to oil or starch for survival [48].
The increase in TAG levels from 6.5% up to 39.3% of total lipids has been reported under the condition of phosphorus limitation [179]. Phosphate deficiency changes the
lipid accumulation as reported for
Monodus subterraneus where the cellular contents of digalactosyldiacylglycerol (DGDG) and diacylglyceroltrimethylhomoserine (DGTS) increase sharply, respectively, from 0.29 and 0.19 to 0.60 and 0.38 fg cell−1 [187]. The reduced or oxidized form of Fe ion may also influence cell productivity where cell density increases by adding chelated Fe3+ but behaves negatively with the addition of chelated Fe2+ at high concentrations [18]. Increasing Fe2+ levels from 2.5 to 4 g L-1 enhances the biomass production, while Fe2+ limitation increases lipid content upto 56.6% in C. vulgaris. The marine diatom Amphiprora paludosa accumulates high lipid content of 65.6%, 63.2% and 57.8% at 0.026, 0.023 and 0.009 mM Fe EDTA, respectively [188] while the maximum biomass production of 3.56 g L-1is reported at 18 μmol L-1 iron [189]. On the basis of fluorescence measurements taken over 12 years, it has been demonstrated that iron has a key function in regulating phytoplankton biomass in both high nitrate low-chlorophyll and oligotrophic waters near the Equator and further south. However, the ‘‘bioavailable’’ iron deficiency and some biochemical components such as lipids in response to iron have not been well documented in microalgae [119].
5.3.2 Effects of POME Media
The biomass formation rate (0.144−0.151 g L-1 d-1) and specific growth rates (0.14−0.21 d-1) in the present study were lower than those reported values of
the lipid accumulation at 26−39% were relatively higher than the reported 28.9% total lipid content of the latter, cultured in concentrated municipal wastewater [190].
The higher biomass and lipid production for the 10-15% POME in sea water was likely due to higher and balanced nutrient concentrations. A study on a mixture of green algae and diatoms show increase in biomass from 0.5-0.9 g L-1 and lipid content from 14-29% when the waste water composition is increased from 10-25% [138]. The higher biomass accumulation has also been reported for Chlorella sp. grown on concentrated municipal wastewater [152]. A study on marine Isochrysis sp. utilizing 5% POME-fortified medium achieves maximum biomass of 91.7 mg m-2 day-1 and lipid content of 52.8 ± 2.4% under 10 L outdoor culture system [191]. Another report suggests that algae can grow optimally at 14% POME, followed by 10%, 20% and 30% [192]. Raw POME has been reported previously at 60-65 oC at the time of discharge to collection pit with an acidic pH between 4 to 5 and biological oxidation demand (BOD) between 10250 to 43750 mg L-1 [193, 194]. These may not be suitable for mesophilic methanogens without preconditioning should biogas or biomethane production is the aim. POME contains 50000 mg L-1 chemical oxygen demand (COD), 750 mg L-1 total nitrogen (TN), 95–96% water, 4–5% total solids and 0.6–0.7% oil and grease and typically the oil and grease mean value is 6000 mg L-1 [195].
The more basic pH was most probably due to the absorption of nitrogen and lack of CO2 sparging. The highest growth rate of Scenedesmus obliquus is reportedly achieved at a constant pH of 7 [196]. In microalgal cultivation, pH value usually increases because of the photosynthetic CO2 assimilation and affects the availability of inorganic carbon. Some species such as A. protothecoides UMN280 tolerates high pH in concentrated municipal wastewater suggesting that pH variation may not be the major limiting factor for microalgae in wastewater [197]. The COD removal was enhanced when the POME concentration was increased to 10% and 15% which was in agreement with the 76% COD removal from piggery wastewater associated with microbe in the high rate algal ponds [198]. A study with A. protothecoides UMN280 achieves the removal efficiency of 88% COD and 96% TOC when the algae is grown in concentrated municipal wastewater [190].
Different algal strain could utilize the different organic compounds as carbon sources at different efficiency. The organic substances may function directly as an essential organic nutrient or act as an accessory growth factor.
However microalgae could utilize the available dissolved oxygen to break down organic material. Low COD removal of 41.8% has been reported in the axenic culture condition of Desmodesmus sp.CHX1 [199]. Synechocystis sp. achieves 98% BOD removal from treated wastewater under hydraulic residence time of 24 h [200]. The algae-based sewage treatment plant (STP) has reportedly achieved total BOD removal of 82 % [201]. A three-stage aquaculture of certain macrophytes and algae, such as Eichhornia crassipes, Microcystis aeruginosa, Scenedesmus falcatus, Chlorella vulgaris and Chlamydomonas mirabilis involving a water hyacinth culture in the first stage, followed by an algal culture, and finally a second water hyacinth also achieved BOD reductions around 96.9% when tested in the laboratory conditions [202].
TN is the sum of organic nitrogen, ammonia (NH3), and ammonium (NH4+
) in the chemical analysis of wastewater. Total Kjeldahl nitrogen (TKN) removal of 36%, ammonium N (NH4 -N) removal efficiency of 18%, nitrate (NO3 -N) removal efficiency of 22%, and nitrite (NO2 -N) removal efficiency of 57.8% have been reported for algae-based STP where the predominant algae are euglenoides and chlorophycean [201]. Green Chlorella sp. when grown in different wastewaters from municipal wastewater treatment plant achieve 50.8%– 82.8% TN removal [176]. The relative constancy of uptake, irrespective of nitrogen source, is considered to be due to the saturation of the assimilator to the production of amino groupings for entry into nitrogenous metabolism. Nitrite is generated in the process of nitrate being reduced to ammonium and it is possible that part of the nitrite produced is excreted into the media [203]. When ammonia is utilized as N-source, pH could decrease significantly during active growth because of the release of H+ ions, but with nitrate uptake pH could increase [107].
Oil and grease are poorly soluble in water due to their tendency to separate out from the aqueous phase. Although this characteristic is advantageous in facilitating the separation of oil and grease by the use of floatation devices, it does complicate the
treatment unit, and disposal into receiving waters. The high removal of oil and grease content by the four algal strains suggest big potential to be considered for handling and treatment of the waste material for disposal. These can be co-cultivated with bacterial consortium.
Free Gram-negative bacteria (Pseudomonas sp., P. diminuta and P.
pseudoalcaligenes) are effective for oil and grease removal from contaminated industrial effluents and are able to degrade the palm oil completely utilizing the free fatty acids (FFA) as a carbon source [204]. Besides carbon, nitrogen and phosphorus, other macro-nutrients (e.g potassium, calcium, and magnesium), micro-nutrients (manganese, molybdenum, copper, iron, zinc, boron, chloride and nickel) and some trace elements are important for microalgal cultivation. Many of trace elements are important in enzyme reaction and for the biosynthesis of value-added compounds [205]. Sea water used to make up POME media constituent, itself contains many natural macro and micro nutrients to fulfil microalgal growth requirements. Changes in all chemical parameters of the waste media after the culture of microalgal species, could pave the way for more environmentally-friendly method to treat wastes whilst benefiting from the algal cultivation for value-added products.