Genetic engineering is a potential method for modifying the genes of microalgae and cyanobacteria to improve the synthesis of desired polymers, such as starch, TAGs, or PHB
[40]. For instance, TAG production in
Neochloris oleoabundans was increased by co-overexpression of lipogenic genes (plastidial lysophosphatidic acid acyltransferase and endoplasmic-reticulum-located diacylglycerol acyltransferase 2). With this transformation,
Neochloris oleoabundans increased 1.6-fold the lipid content and 2.1-fold TAG production
[41]. The researchers also reported a long-term stability of the modified strain since this productivity was maintained for 4 years.
The increase in PHB by using genetic engineering tools has also been reported. In order to induce the production of PHB in
Chlamydomonas reinhardtii, Chaogang et al.
[42] utilized two expression vectors containing the
phbB and
phbC genes from
Ralstonia eutropha, both encoding PHB synthase
[42]. The presence of PHB granules in the cytoplasm of the transgenic cells resulted in a favorable outcome, producing 6 µg g
−1 of PHB compared with no PHB production in the wild-type strain.
Synechocystis sp. PCC6803 also was modified for enhanced PHB production by the overexpression of a heterologous phosphoketolase (
XfpK) from
Bifidobacterium breve, which is used as a strategy to improve acetyl-CoA levels. Using this technique, a PHB production of 232 mg L
−1 (12% w w
−1) was obtained under nitrogen depletion conditions, greater PHB production from
Synechocystis sp. without mutation (1.8% w w
−1)
[43]. Additionally, overexpression of the
phaAB gene in
Synechocystis sp. PCC 6803 enhanced PHB production, obtaining a PHB concentration of 35% w w
−1 growth in nitrogen-deprived medium; also, this technique increased acetyl-CoA levels
[44].
Most algal transgenics now employ constitutive promoters to express the recombinant gene throughout algal biomass synthesis, which might have a detrimental influence on growth due to the increased metabolic burden or a potential toxicity on the cell
[45]. Thus, a preferable technique is to activate the expression of genes near the end of the growth phase by utilizing tightly controlled promoters with a wide dynamic range in conjunction with good codon optimization, boosting the development efficiency and ultimate production of the targeted gene output. Another problem is the urgent need to create effective chloroplast and mitochondria transformation procedures for most useful microalgal species, as these organelles play critical roles in cellular metabolism. Despite the fact that many projects are underway to generate genome, transcriptome, and proteome information for many microalgal species, it is indeed essential to decode the full annotation of genes and the connectivity of biosynthetic processes in order to fully exploit the prospects of microalgae species.
4. Bioplastics from Microalgae and Cyanobacteria Grown in Wastewater
The synthesis of bioplastics from biomass of microalgae and cyanobacteria can be complemented with wastewater treatment. This would allow to grow cellular biomass without requiring synthetic culture medium while also treating wastewater
[46]. Nevertheless, up to date, there are few studies that have evaluated this approach. In a study by López Rocha et al.
[47], blends of microalgae biomass grown in municipal wastewater were prepared with glycerol. The consortium evaluated in the study included
Scenedesmus obliquus,
Desmodesmus communis,
Nannochloropsis gaditana, and
Arthrospira platensis. Following injection molding of the blends at 140 °C, bioplastic materials were obtained. Further characterization of the bioplastics formed showed that they had a high thermal stability with low water absorption
[47]. In another study,
Desmodesmus sp. and
Tetradesmus obliquus biomass grown in municipal wastewater was also evaluated to produce bioplastics
[48]. In that study, microalgae biomass and glycerol were mixed. The results showed that these bioplastics had similar mechanical properties to bioplastics derived from soy and rice proteins
[48].
Most studies on wastewater treatment by microalgae and/or cyanobacteria have focused on PHB production. For instance, PHB from
Botryococcus braunii grown in sewage wastewater obtained a final PHB of 247 mg L
−1 [49].
Synechocystis salina cultivated in digestate from an anaerobic reactor fed with thin stillage was also evaluated for PHB production
[50]. Results showed that at the pilot scale (200 L), 4.8% w w
−1 of PHB accumulated in
Synechocystis, similar to that in control cultures grown in synthetic medium
[50]. Thus, the PHB production by microalgae/cyanobacteria using wastewater as a culture medium could be feasible.
Although not many studies have considered the production of bioplastics from other macromolecules besides PHB from wastewater-grown microalgae and/or cyanobacteria, it is possible discuss their potential based on the carbohydrate (including starch and glycogen) and lipid (TAGs) content reported in the literature. For instance,
Chlorella vulgaris grown in aquaculture wastewater obtained a cell density of 3.2 g L
−1 with a high accumulation of carbohydrates (39% w w
−1)
[51].
Isochrysis galbana reached a cell density of 3.2 g L
−1 with an accumulation of 37% w w
−1 carbohydrates grown in aquaculture wastewater
[51], and
Desmodesmus spp. grown in landfill leachate and municipal wastewater accumulated 41% w w
−1 of carbohydrates and 20% w w
−1 of lipids
[52]. Additionally, in that study, it was determined that low concentrations of nitrogen enhance starch production of microalgae culture growth in treated wastewater.
Chlorella sp. and
Scenedesmus sp. grown in domestic wastewater were able to achieve cell growth of 1.78 g L
−1 and accumulated 34 % w w
−1 of lipids
[53].
Tetraselmis sp. grown in municipal wastewater achieved 1.57 g L
−1 of microalgae biomass with 38% w w
−1 of lipids
[54]. Additionally,
Chlorella sorokiniana accumulated 43% w w
−1 of lipids when grown in aquaculture wastewater
[55].
Chlorella sp. grown in swine wastewater (wastewater characterized for having a high organic load) increased lipid production, including triacylglycerols (2.5 higher times compared with standard medium)
[56]. In addition, as it was mentioned before, the used grease wastewater as culture medium enhanced lipid content in microalgae biomass
[57]; hence, the use of this wastewater for microalgae culture can also increase the production of bioplastics from these cultures.
5. Environmental Impact of Bioplastics
The environmental impacts of plastics are extremely important and have become a scientific, social, and political issue. Common plastics of petrochemical origin are widely used in different applications due to their low price, durability, and strength. In the last years, derived from their high demand and incorrect disposal, environmental problems have risen due to their accumulation and persistence in terrestrial and aquatic ecosystems. Bioplastics have emerged as an alternative to conventional plastics. They can be produced from materials of biological origin and have a lower impact on the environment
[58][59]. Bioplastics are classified in three categories: (1) those that are biobased and biodegradable, (2) fossil-based and biodegradable, and (3) biobased and not biodegradable. Standard plastics (i.e., fossil-based and nonbiodegradable) are not bioplastics. Biodegradable bioplastics can be decomposed by the environment and microorganisms and are thus reintegrated into the ecosystem. For instance, starch-based bioplastics, PLA, and PHA/PHB can be easily degraded in small fragments that are digested by microorganisms. Production of bioplastics is, however, a relatively recent development, and there are still some constrains to be solved, as discussed below.
Land plant crops, such as corn, are currently used for bioplastic production. However, the use of these biomass feedstocks is controversial. They require large areas of cultivation, time, water, fertilizers, and pesticides. These grains are no longer used as food source but in the production of bioplastics and biofuels (e.g., ethanol). In fact, estimations report that a quarter of the cultivated land is currently used to produce biofuels and bioplastics, which has generated a marked increase in the prices of basic foods
[60]. Walker and Rothman
[60] compared bioplastics from PHB and PLA with plastics of petrochemical origin and determined that the production of bioplastics was more polluting due to the use of fertilizers and pesticides in crops
[60]. Nevertheless, Elsawy et al.
[61] reported that bioplastics, such as PLA, which are biodegradable and compostable, produce 70% less greenhouse gas emissions during their manufacturing compared with conventional plastics
[61]. Therefore, the use of renewable biomass or organic waste can be a strategy to produce ecological bioplastics with lower greenhouse gas emissions
[62].
The production of bioplastics using the biomass of microalgae produced from wastewater generates at least two positive impacts on the environment. Microalgae can reduce more than 80% of the nitrogen and COD present in the wastewater; likewise, different types of wastewaters can be used in this process, which demonstrates the versatility of the process
[20][21]. Additionally, microalgae could be produced in established wastewater treatment plants, which would not mean the use of arable land for this purpose
[63]. On the other hand, the cultivation of microalgae can help reduce the concentration of CO
2 in the atmosphere, since for every kg of biomass of microalgae produced, 1.8 kg of CO
2 can be captured in the process
[64]. In addition, as stated above, the rate of decomposition of a bioplastic is lower than that of conventional plastics
[65], so its use would reduce the production of garbage and reduce the use of land for landfills.