Oleaginous microorganisms are able to convert numerous agro-industrial and municipal wastes into storage lipids (single cell oil (SCO)) and are therefore considered as potential biofuel producers. While from an environmental and technological point of view the idea to convert waste materials into fuels is very attractive, the production cost of SCO is not currently competitive to that of conventional oils due to the low productivity of oleaginous microorganisms in combination with the high fermentation cost. Current strategies used to optimize the lipid-accumulating capacity of oleaginous microorganisms include the overexpression of genes encoding for key enzymes implicated in fatty acid and triacylglycerol synthesis, such as ATP-dependent citrate lyase, acetyl-CoA carboxylase, malic enzyme, proteins of the fatty acid synthase complex, glycerol 3-phosphate dehydrogenase and various acyltransferases, and/or the inactivation of genes encoding for enzymes implicated in storage lipid catabolism, such as lipases and acyl-CoA oxidases. Furthermore, blocking, even partially, pathways competitive to lipid biosynthesis (e.g., those involved in the accumulation of storage polysaccharide or organic acid and polyol excretion) can also increase lipid-accumulating ability in oleaginous microorganisms. Methodologies, such as adaptive laboratory evolution, can be included in existing workflows for the generation of strains with improved lipid accumulation capacity. In our opinion, efforts should be focused in the construction of strains with high carbon uptake rates and a reprogrammed coordination of the individual parts of the oleaginous machinery that maximizes carbon flux towards lipogenesis.

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Complex biochemical mechanisms are being involved in oleaginous microorganisms during storage lipid and polysaccharide metabolism. Detailed biochemical analyses and monitoring of key enzymes involved in carbon metabolism were performed in Yarrowia lipolytica and Umbelopsis isabellina, which are often used as model oleaginous microorganisms. It was found that during the early oleaginous phase, the carbon source (glucose) was channeled to lipid accumulation, but also to polysaccharide biosynthesis. However, during transition from the early to the late oleaginous phase, glucose was exclusively converted to lipids, while in U. isabellina, but not in Y. lipolytica, an additional conversion of cellular polysaccharides into lipids was observed. After glucose depletion in the growth medium, cellular storage material was degraded either for generating maintenance energy or for supporting further microbial growth, depending on the availability of essential nutrients in the growth medium. We demonstrated that in both microorganisms, reserve lipids were exclusively used as an intra-cellular carbon source in order to generate energy for maintenance purpose. When cellular storage material degradation was related to new cell mass production, a bioconversion of lipids into new lipid-free material, consisting of polysaccharides and proteins, was observed in Y. lipolytica, while new lipid-free material in U. isabellina was richer in proteins. Lipid and polysaccharide remodeling may occur in some cases in both microorganisms. This study revealed some new biochemical features of oleaginous microorganisms that may be crucial for the design of new biotechnological processes, such as the production of bio-molecules of industrial, technological, and medical interest.

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Poultry litter extract (PLE) was treated using a microbial consortium dominated by the filamentous cyanobacterium Leptolyngbya sp. in synergy with heterotrophic microorganisms of the poultry waste. Laboratory- and pilot-scale experiments were conducted under aerobic conditions using suspended and attached growth photobioreactors. Different dilutions of the extract were performed, leading to different initial pollutant (nitrogen, phosphorus, dissolved chemical oxygen demand (d-COD), total sugars) concentrations. Significant nutrient removal rates, biomass productivity, and maximum lipid production were determined for all the systems examined. Higher d-COD, nitrogen, phosphorus, and total sugars removal were recorded in the attached growth reactors in both laboratory- (up to 94.0%, 88.2%, 97.4%, and 79.3%, respectively) and pilot-scale experiments (up to 82.0%, 69.4%, 81.0%, and 83.8%, respectively). High total biomass productivities were also recorded in the pilot-scale attached growth experiments (up to 335.3 mg L−1d−1). The produced biomass contained up to 19.6% lipids (w/w) on a dry weight basis, while the saturated and monounsaturated fatty acids accounted for more than 70% of the total fatty acids, indicating a potential biodiesel production system. We conclude that the processing systems developed in this work can efficiently treat PLE and simultaneously produce lipids suitable as feedstock in the biodiesel manufacture.

Fish farm effluents may be used as culture media for marine microalgae, the cell mass of which constitute an excellent fish feed rich in bioactive compounds. In the current investigation different fish farm effluents were tested as culture media for Nannochloropsis strains. Among them, Nannochloropsis gaditana grew well on the effluent released from the sedimentation tank (EST), which is the final step of the wastewater treatment. Mono‐algal but non‐aseptic cultures were conducted in two types of photo‐bioreactors, namely stirred tank reactor (STR) and open pond simulating reactor (OPSR) working under various photoperiods. N. gaditana grew well under full illumination mode on phosphate rich EST in the STR, producing 847.0 mg/L of dry cell mass containing 7.8%, w/w lipids, while when cultivated on phosphate limited EST, cell mass production was slightly lower but lipid biosynthesis was favored, with the lipid content reaching 24.7%, w/w in dry cell mass. In all trials, Nannochloropsis cell mass contained significant quantities of proteins and polysaccharides. Neutral lipids were predominant over polar lipids. Both glycolipid and phospholipid fractions were rich in polyunsaturated fatty acids, especially in eicosapentaenoic acid. We conclude that fish farm wastewaters can be re‐used as microalgae growth media, which is of financial and environmental importance.

Aquaculture plays an important role in human nutrition and economic development but is often expanded to the detriment of the natural environment. Several research projects, aimed at cultivating microalgae in aquaculture wastewaters (AWWs) to reduce organic loads and minerals, along with the production of microalgal cell mass and metabolic products, are underway. Microalgal cell mass is of high nutritional value and is regarded as a candidate to replace, partially at least, the fish meal in the fish feed. Also, microalgal cell mass is considered as a feedstock in the bio-fuel manufacture, as well as a source of high-added value metabolic products. The production of these valuable products can be combined with the reuse of AWWs in the light of environmental concerns related with the aquaculture sector. Many research papers published in the last decade demonstrate that plenty of microalgae species are able to efficiently grow in AWWs, mainly derived from fish and shrimp farms, and produce valuable metabolites reducing the AWW pollutant load. We conclude that bio-remediation of AWWs combining with the production of microalgae cell mass and specific metabolites is probably the most convenient and economical solution for AWWs management and can contribute to the sustainable growth of the aquaculture.