Microalgae mediated bioremediation of polycyclic aromatic hydrocarbons: Strategies, advancement and regulations

Polycyclic aromatic hydrocarbons (PAHs) are pervasive in the atmosphere and are one of the emerging pollutants that cause harmful effects in living systems. There are some natural and anthropogenic sources that can produce PAHs in an uncontrolled way. Several health hazards associated with PAHs like abnormality in the reproductive system, endocrine system as well as immune system have been explained. The mutagenic or carcinogenic effects of hydrocarbons in living systems including algae, vertebrates and invertebrates have been discussed. For controlling PAHs, biodegradation has been suggested as an effective and eco-friendly process. Microalgae-based biosorption and biodegradation resulted in the removal of toxic contaminants. Microalgae both in unialgal form and in consortium (with bacteria or fungi) performed good results in bioaccumulation and biodegradation. In the present review, we highlighted the general information about the PAHs, conventional versus advanced technology for removal. In addition microalgae based removal and toxicity is discussed. Furthermore this work provides an idea on modern scientific applications like genetic and metabolic engineering, nanomaterials-based technologies, artificial neural network (ANN), machine learning (ML) etc. As rapid and effective methods for bioremediation of PAHs. With several pros and cons, biological treatments using microalgae are found to be better for PAH removal than any other conventional technologies.

Current strategies on algae-based biopolymer production and scale-up

The craving for an alternative to the existing plastic products gives rise to the concept of algae-based bioplastic production, which appears to be excellently biodegradable and cost-effective. The significant assortment of algal biopolymers draws great attention to stop the surge of plastic waste and to mitigate the burning problems of environmental pollution. The polyhydroxyalkanoates (PHA) are naturally-occurring biopolymers found in the form of esters accumulated within a number of microbes, which provides the pillar for several biomolecules. This review summarizes the global scenario as well as the precise technique of algae-based PHA extraction and bioplastic production. In addition, different techniques for valorisation of PHA production, its biodegradability and its commercial applications are also taken into consideration.

Co-Cultivation of Leptolyngbya tenuis (Cyanobacteria) and Chlorella ellipsoidea (Green alga) for Biodiesel Production, Carbon Sequestration, and Cadmium Accumulation

The co-cultivation approach using cyanobacteria-Leptolyngbya tenuis and green alga-Chlorella ellipsoidea demonstrated in the present study showed additive and synergistic effects on biomass yield, biomass productivity, lipid yield, lipid productivity, CO₂ fixation, and cadmium bioremediation efficiency. The results of co-culture in batch mode revealed about 2-3 times increase in biomass and two times increase in total lipid, when compared to the pure culture batches. The results revealed that co-cultures exhibited significantly high CO₂ fixation rate of 2.63  ±  0.09 g/L/d, which is 1.5-2 times better than monocultures (P < 0.05). To explore the bioaccumulation of cadmium by co-cultures and pure cultures, different concentrations of cadmium nitrate was used in flask trials. Cadmium accumulation was observed in the order: co-culture (74%, 0.37 mg/L) > Chlorella (58%, 0.29 mg/L) > Leptolyngbya (50%, 0.25 mg/L) (P < 0.05). In addition, fatty acid composition, CHNS analysis, biodiesel characterization, and biochemical compositions were also determined using co-culture method. The maximum biomass yield, productivity, lipid content, and CO₂ fixation rate in cadmium induced co-culture were 3.95  ±  0.13 g/L, 258.88  ±  15.75 mg/L/d, 41.43  ±  0.71%, and 3.21  ±  0.20 g/L/d, respectively which is 1.2, 1.3, 2.3, and 1.2 times higher than the control (P < 0.05). Cadmium induced changes in growth and lipid yield using co-culture suggests cost-effective and eco-friendly production of biodiesel and carbon mitigation.

Indole-3-acetic-acid-induced phenotypic plasticity in Desmodesmus algae

Phenotypic plasticity is the ability of a single genotype of an organism to exhibit variable phenotypes in response to fluctuating environments. It plays a crucial role in their evolutionary success. In natural environments, the importance of interactions between microalgae and other microorganisms is generally well appreciated, but the effects of these interactions on algal phenotypic plasticity has not been investigated. In this study, it revealed that indole-3-acetic acid (IAA), the most common naturally occurring plant hormone, can exert stimulatory at low concentrations and inhibitory effects at high concentrations on the growth of the green alga Desmodesmus. The morphological characteristics of Desmodesmus changed drastically under exposure to IAA compared with the algae in the control environment. The proportion of Desmodesmus unicells in monocultures increased with the IAA concentration, and these unicells exhibited less possibility of sedimentation than large cells. Furthermore, we discovered that lipid droplets accumulated in algal cells grown at a high IAA concentration. Results also demonstrated that the presence of algal competitor further stimulated inducible morphological changes in Desmodesmus populations. The relative abundance of competitors influenced the proportion of induced morphological changes. The results indicate that phenotypic plasticity in microalgae can be a response to fluctuating environments, in which algae optimize the cost–benefit ratio.