Present and Future Prospect of Algae: A Potential Candidate for Sustainable Pollution Mitigation

2.1. Pollution Indicator

Due to their unique nutrition requirement and habitat, algae are beneficial in detecting pollutants [13]. Any significant change in air quality can affect algal species. As algae generally use atmospheric air for their growth, any change in the air constituents can be sensed by algal species. Freystein et al. found a specific algal population growth where the particulate matter concentration is 10 [14]. There is significantly less algal population found in a high level of ozone concentration area, which suggests that the concentration of different air pollutants can relate to the occurrence of algal species.

2.2. Algal Mitigation

Algae, as photosynthetic organisms, can use CO₂ as a carbon source and utilise it to produce different metabolites and sustain their growth cycle. Algae can use CO₂ very efficiently to produce biomass, and the algal biomass can be used as different high-value products, hence they have an economic value [15]. Algal biomass and glucose can be used to develop a micropore adsorption system that provides a high surface area for CO₂ capture [16]. Algae can be used in integrated systems to produce biofuels, treat wastewater, and it can be used for CO₂ sequestration [17]. The process simulation of capturing CO₂ by culturing algal biomass followed by oil extraction shows promising results [18]. Chlorella vulgaris grown in catholyte can sequester CO₂ while sustaining power generation in Microbial Carbon Capture Cells (MCCs), implying that algae can be used for both carbon fixation and power generation in MCCs. [19]. Könst et al. explained the integrated production of different high-value products like antioxidants and essential fatty acids with the use of technology of CO₂ capture by algae [20]. Alkaliphilic algal species like Phaeodactylum tricornutum, Chlamydomonas sp. show high levels of CO₂ removal [21].

Algae can achieve high CO₂ sequestration of about 80% to 99% at optimum conditions like pond water [22]. The use of different macro and microalgae that can be used to capture CO² and for the generation of biofuels has been very extensively reviewed [23]. It is estimated that algal ponds sequester CO₂ released from power plants efficiently and convert it as a carbon source for its growth [24]. It has been seen that a high concentration of CO₂ and NO can be tolerated by Scenedesmus dimorphus and showed 75.61% CO₂ utilisation efficiency when there is intermittent sparging of flue gas and pH feedback control system [25]. Algae can be screened based on their growth rate, photosynthetic capacity, environmental stress tolerance capacity, and ability to produce high-value products. It is found that Oscillatoria Blue-green algae can fix about 70 - 80% of the CO₂ at optimal pH, light intensity, and temperature [26]. Chlorella vulgaris was cultured in an airlift photobioreactor and was 80% efficient for removing CO₂ at the optimum temperature [27]. Spirulina sp. shows the highest biomass growth as the addition of CO₂ increased up to 18% in vertical photobioreactor and can be effective for the removal of CO₂ of higher concentration [28]. In all cases, CO₂ is utilised in photosynthesis by algae and is considered the primary mechanism of CO₂ sequestration. Spirulina sp. also sequester SO₂ and NO₂ and use them as a nutrient for their growth; they also have other potential uses [29]. Table 1 shows some of the algae and their air pollutant removal efficiency.

3.1. Petroleum Contaminant

Petroleum hydrocarbons are the primary culprits of groundwater and soil pollution in industrial and urban areas. The significant impact it has is on organic carbon as 75% of carbon in oil is oxidisable, and it eventually causes a decrease in the pH of the soil [31]. The formation of a thick surface layer occurs when these hydrocarbons have contact with the soil, decreasing the exchange of gases between soil and air. This phenomenon leads to the suffocation of the plants, and hence the soil gradually becomes anaerobic, due to which soil microflora suffers a significant loss [32]. Walker and his coworkers showed the crude oil and motor oil-degrading capacity of alga Prototheca zopfii [33]. Phenols and sulfur-containing compounds are the major petrochemical contaminants. Biosorption and degradation of petroleum effluents by a cyanobacterium, Oscillatoria quadripunctulata, remove a significant 40% of TDS (Total Dissolved Solids) [34]. Navicula sp., a diatom, showed effective removal (> 50%) of 4-methyl cyclohexane acetic acid in two weeks [35]. Few algal species, such as Chlorella, Nostoc, and Ulva, are found to be capable of remediating petroleum-derived hydrocarbon under any O₂ condition [36]. Table 2 shows different petroleum-associated contaminants removal by different algal species.

Algal biomass of Selenastrum capricornutum showed maximum efficiency in removing Benzo (a) pyrene compared to other pollutants. Biodegradation is the most effective mechanism in remediating associated petroleum contaminants.

3.2. Pesticides

Pesticides are being extensively tested with various algal species to establish a concrete relationship between pesticide concentration in water and its effect on the growth and metabolism of alga. Physical adsorption, biodegradation, complexation, and biotransformation are the few mechanisms identified as the true drivers of pesticide removal by the alga. Phycoremediation of a pesticide mixture with Atrazine as a principal constituent is being shown by freshwater algae Chlorella vulgaris and has a promising removal percentage of 98.6% to 98% [42]. Fluroxypyr is one of the most commonly used herbicides. Zhang et al. reported that the unicellular green alga Chlamydomonas reinhardtii removed this herbicide up to 57% through accumulation and degradation [43]. Table 3 shows the removal of pesticides by different algal species and the respective methods that have been adopted.

After comparing various algal species in Table 3, it can be concluded that Bifenthrin is the best-remediated pesticide, and biosorption-biodegradation is the most efficient mechanism of remediation. In the case of remediation of phenol derivatives, the initial concentration beyond 60% (v/v) is a limiting factor as, beyond this concentration, the biosorption process is inhibited.

3.3. Polycyclic Aromatic Hydrocarbons (PAHs)

The US EPA labelled 28 PAH compounds as priority contaminants [49], and the majority of these arise due to incomplete combustion of agricultural remains. Soil concentration of PAH contaminant can reach as high as 300 g kg⁻¹ [50]. PAH are listed amongst the hazardous materials because of their persistent nature. Chlorella sp.MM3 and Rhodococcus wratislaviensis strain 9 showed 100% removal of Phenanthrene, Pyrene, and BaP along with 13 other persistent PAH compounds from soil samples [51]. Selenastrum capricornutum, a green microalga, shows 88% of overall PHE (Phenanthrene) removal percentage in 96 h and 100% for FLA (fluoranthene) and PYR (pyrene) in 24 h [52]. Table 4 shows different algae species for PAH compound degradation.

Benzo [a] pyrene is the best remediated PAH compound with 99% removal efficiency. Adsorption is the most efficient removal mechanism followed by biodegradation. Chlorella sp. has shown the broadest applicability in terms of contaminant remediated.

3.4. Plastics

Polythene is a petroleum-derived product, and plastics are the organic polymers of the high-density polymer. Plastics are used in everyday life in many forms. Because of its ease of production and low cost, it is not commercially replicable. It is composed of toxic chemicals and non-biodegradable substances [58]. It is proven as a critical material for the growth of any developing nation. Its economic importance is evident from the fact that the annual production of plastic exceeds that of steel [59]. Adsorption of microplastics by green microalgae Skeletonema costatum and Chlamydomonas reinhardtii are found to be effective. The main advantage of using microalgae for microplastic treatment is its rapid physical adsorption [60, 61]. The percentage degradation of long density polythene by different microalgae [62] is shown in Fig. (2). Anabaena spiroides remediate Low-density polythene, and its ability to colonise in freshwater bodies and short doubling time makes it the best choice. The only limitation is the slow degradation even under laboratory conditions [62].

4.1. Heavy Metals

Industrialisation and urbanisation have worked hand in hand to gradually kill the remaining healthy water bodies near human settlements. Combustion of solid and wet waste, metallurgic activities, mining, maritime transportation, and extensive use of fertilisers to meet the food demand are a few of the causes of increased organic and heavy metal pollutants in the soil. Recently, researchers explored the potential of marine microalgae in the removal of heavy metal pollutants from water. Travieso et al. worked with two microalgae for demonstrating heavy metal removal. With the use of two different cell immobilisation methods, they did a comparative study. Predominantly, three heavy metals, cadmium, zinc, and chromium, weretargeted. With Kappa-carrageenan as an immobilisation method, the amount of heavy metal (Cd, Cr, and Zn) removal gradually increased compared to polyurethane foam in 24 h and 48 h period for both the algal species Chlorella vulgaris LAM-C30 and Scenedesmus acutus. However, in S. acutus, the difference in removal percentages for heavy metals of the two immobilisation methods is minute. C. vulgaris showed the average removal percentages of 61.5%, 81.5%, and 41% for Cd, Zn, and Cr, respectively. S. acutus showed a better removal percentage with 71%, 87.5%, and 33.5% for Cd, Zn, and Cr, respectively [63]. Table 5 shows heavy metals removal by various algal species along with their removal percentages.

By comparing the heavy removal efficiencies of various algal species, the study concluded that Zn is the best-remediated contaminant and Scenedesmus obliquus is the most efficient amongst them all. C. vulgaris is found to have the broadest range of applicability in bioremediation. Within a wide spectrum of mechanisms of heavy metal removal, biosorption is the most adopted one.

4.2. Organic Pollutants

Organic pollutants are of two types, biodegradable and Persistent Organic Pollutants (POPs). Tributyltin (TBT), Dimethyl Phthalate (DMP), p-nitrophenol, polychlorinated dibenzofurans, etc., are the few classes of POPs mainly contributing to the increasing amount of these compounds in the surface and groundwater reserves. Their persistent nature is due to the almost negligible or no solubility in water . The primary mechanism of action adopted by the majority of algal species is bioaccumulation, with few exceptions for bio adsorption and biosorption. Chlorella sp., Chlamydomonas sp., and Oscillatoria are the most promising organic pollutant removing agents [69]. The removal of bisphenol A by freshwater green algae Monoraphidium braunii with a maximum removal efficiency of 48% [70] turned out to be the early indicators of the hidden potential of microalgae in organic pollutant remediation. Selanastrum capricornatum and Cyanobacteria are used for the degradation of benzene, toluene, naphthalene, phenanthrene, pyrene in the contaminated water and soil [71]. Chlorinated hydrocarbons removal by 11 marine phytoplankton species [72] showed great potential for organic pollutant removal by freshwater microalgae as they have comparatively better accessibility and easy isolation.

4.3. Wastewater Treatment

Twenty-three metro cities contribute almost 60% of wastewater in India. Domestic sewage constitutes a large part of urban wastewater, and 80% of this wastewater is still discharged to water bodies untreated. It consists of lipids, fats, cellulose, heavy metals, several simple salts, and several pathogenic microbes. Biological Oxygen Demand (BOD) is the universally accepted norm for estimating contaminants in water bodies. Many blue-green algae, flagellated algae, and diatoms are predominantly residing in polluted rivers and drains. Hence the ability of several algal species to remove the water contaminants is extensively explored. Table 6 shows a list of algal species used in wastewater contaminant removal. Heavy metals contaminants discussed in section 4.1 hold a significant part in wastewater effluents from the industries. Manufacturing units and municipal facilities have toxic heavy metal pollutants in their effluent stream. An advantage of immobilised systems is that the biocatalyst and product may be separated easily, enabling effective product recovery (downstream processing). Another advantage is that the biocatalyst can be reused. The biological catalyst would be more stable in immobilised form.

Phosphorus-containing compounds were completely remediated from wastewater samples using algal biomass. Domestic sewage was efficiently remediated as compared to other effluent streams. Freshwater algae are efficient in remediating ammonia and have better applicability than any other algae.

4.4. Explosives

The most used explosive in the modern world is 2,4,6-trinitrotoluene (TNT). It is amongst the most significant marine pollutants due to numerous military installations and the testing of explosives-driven weapons along the seashore. TNT is relatively more soluble in water than other explosive chemical components [77]. Marine water pollutants are not well recorded and as significant as compared to other types of pollutants. TNT is highly toxic for the marine environment [78]. Three microalgae, Acrosiphonia coalita (green), Portieria hornemannii (red), and Pyropia yezoensis (red), are observed to have 100% TNT removal potential from seawater. 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene are the two leftover compounds of TNT degradation and constitute around 20% or less than the initial TNT amount. Hwang et al. worked with the microalga Scenedesmus obliquus to understand its biotransformation capabilities against hexahydro-1,3,5-trinity-1,3,5-triazine (RDX) [79]. The maximum transformation percentage was found to be 60%. Additional studies with green algae Pediastrum biwae showed up to 90% RDX transformation. Microalgae in the remediation of explosives have a considerable scope of research in the future, but currently, the literature is limited. A. coalita is considered the best choice for remediating TNT contaminants in water samples as it is distributed in a wide geographical area and can be easily found and isolated.

5.1. Current Mitigation Strategies

Natarajan et al. critically reviewed the physical methods (incineration, distillation, evaporation, dumping), chemical methods (chemical precipitation, wet oxidation, acid digestion, immobilisation of radioactive waste), biological methods (microbial bioremediation, plant bioremediation, phytoextraction, phytovolatilisation, phytostabilisation, rhizofiltration, phytodegradation) for the treatment of radioactive waste [80].

5.2. Algal Removal of Radionuclides

Algae can remove radionuclides from the environment by the biosorption or bioaccumulation method. Algae can also be used with other composite materials to form adsorbent material for the adsorption of radioactive materials. Some of the algae are reported to have resistance to high ionising radiations like α-rays, making them a potential candidate for removing radionuclides. Algae use different survival techniques to withstand the high ionising radiation (active DNA repairing) and use different mechanisms (ion channels, complex formation, binding of specific domains) to uptake radioactive nucleotides.

Krejci et al. reported that Closterium moniliferum, a freshwater Charophyceae of desmid order having a crescent shape, can bioaccumulate the strontium from the water and store it in its subcellular location, i.e., in the vacuole in crystal form. Strontium, due to its similarity of atomic size and properties with calcium, is very dangerous especially, Sr-90 (a radioisotope of strontium) due to its long half-life (30 years) and can enter into the human body and cause cancer. Closterium moniliferum accumulates barium and calcium and co-accumulates the strontium due to its structural similarity with calcium and eventually precipitates to form crystals. Hence, by exploiting this property, Sr-90 accumulation can be increased [82, 83]. Rivasseau et al. isolated Coccomyxa actinabiotis (CCAP 216-25), an extremophile unicellular freshwater eukaryotic green microalga, which can tolerate massive ionising radiation doses with a value of about 20 000 Gy by possibly utilising its specific resistance and repair mechanisms. They also found that C. actinabiotis can uptake and accumulate different types of radionuclides, having 100% removal efficiency of Ag-110, Zn-65, Cs-137, and up to 95% uptake in the case of U-238. It also shows an uptake rate of about 90 - 91% in the case of Co-60, Co-58, and Mn-54 [84, 85]. Jha et al. found and evaluated free-floating algal species' ability to accumulate Ra-226 in water bodies near radioactive mining areas. Chara sp., Nitella sp. are filamentous algae, and aquatic plants like Pistia sp., Jussia sp., Eichornia sp., Hydrilla shows a high level of Ra-226 accumulation. The activity concentration ratio of Ra-226 in filamentous algae and aquatic plants to the activity concentration in water shows excellent value, which varies from 1.1×10³ up to 8.6 ×10⁴ [86]. Zalewska and Saniewski studied the bioaccumulation activity of Polysiphonia fucoides and Furcellaria lumbricalis, and Rhodophyta found in the Baltic and found that Polysiphonia fucoides show greater bioaccumulation of Cr-51, Zn-65, Ag-110m, Sn-113, Cs-137, and Am-241. Mn-54, Co-57, Co-60 show greater activity concentration in Furcellaria lumbricalis, and hence both of these show promising features for the bioaccumulation of radionuclides [87]. Veliscek Carolan et al. reported that during a dose assessment study, Ulva sp. and Ecklonia radiata that grew near a sewage treatment plant showed concentration factors 176 and 526 bioaccumulation of I-131 [88]. Radiation tolerance study with the cyanobacterium Chroococcidiopsis reported that these algae could survive x-rays of radiation up to the value of 15 kGy as compared to the extremophilic bacterium Deinococcus radiodurans which shows the highest ionising radiation survival rate [89, 90]. The mechanism adopted by Chroococcidiopsis is that it can repair the damaged DNA very fast and efficiently, which also happens during the DNA damage when there is dehydration [91].

It has been reported that along with the indicator of trace elements, red alga Jania longifurca found and isolated from the coast of Syria shows the bioaccumulation of Pb-210 (26.01 ± 2.60 Bq kg^-1) and J. longifurca shows the bioaccumulation of Po-210 (27.43 ± 0.58 Bq kg^-1). Brown algae like Cystoseira sp. and P. pavonia show the bioaccumulation activity for Po-210 (26.40 ± 1.10 Bq kg^-1 and 24.43 ± 0.66 Bq kg^-1, respectively), but Cs-137 accumulation was less in all cases [92]. Farhi et al. performed ionising-radiation tolerance studies on green micro-alga Chlorophyceae followed by Spectroscopic investigation and found that alga can tolerate high γ -irradiation up to 6 kGy. Spectroscopic investigations show the release of amino acids, which shows that the probability of intense DNA repair activity was due to radiation [93]. Bioaccumulation studies of Cs⁺ and Cs-137 by nine strains of algae were conducted and found that Desmodesmus armatus can remove Cs⁺ by bioaccumulation. The process of bioaccumulation is followed by a separation process where magnetic nanoparticles coated with polyethyleneimine are used for the separation of the algae, which has accumulated Cs-137. Adding organic substrates could improve Cs⁺ uptake by algae. On the other hand, if K+ ion concentration is more than 10 mg/l, it will competitively decrease the rate of Cs⁺ absorption by algae, possibly because both the ions use the same transport channel for the transportation of ions across the boundary of the cell. It is also reported that the bioaccumulation activity is high at higher pH (8-9) and also increases with an increase in initial Cs concentration [94]. Alissa et al. reported that Spirogyra, which is a filamentous charophyte, can accumulate uranium with a concentration factor of a maximum of 67 and have an average concentration factor of about 22. Cladophora sp., which is a filamentous Ulvophyceae, can accumulate uranium with a concentration factor of a maximum of 280 and have an average concentration factor of about 250 [95]. It is found that Ulva sp. and Na bentonite composite shows biosorption/adsorption of uranium ions from aqueous solutions with a high adsorption rate under optimum pH, temperature, and initial uranium concentration, etc [96]. Lee et al. reported that Haematococcus pluvialis red cyst shows a high value of biosorption and removal of almost 95% radioactive Cs-137 in 48 h, which is higher than Chlorella vulgaris and Anabaena sp. The mechanism involved for Cs-137 accumulation involves uptake through a potassium transport channel, binding with caesium binding domain on the red cyst surface, and complex formation with a functional group [97]. Khani et al. studied the biosorption of uranium by Cystoseria indica in a batch system. It is reported that C. indica shows maximum absorption with optimum temperature and pH conditions. Absorption increases with an increase in initial uranium concentration with an adsorption value in the range of 198 mg/g to 233 mg/g [98]. A freshwater Chlorophyta, Graesiella emersonii isolated from hot spring, shows maximum sorption capacities of Ra-226 and U-238 under optimum pH and initial cell concentration, which affects the biosorption system considerably and hence plays an essential role in scaling up the process [99].

It is found that the biosorption of uranium by non-living biomass of the Sargassum fluitans, a Phaeophyceae, after protonation shows bioaccumulation of uranyl ion from aqueous solution at optimum pH. High uranium adsorption at high pH (4.0) happens due to the presence of hydrolysed uranyl ions [100]. It is reported that Padina sp., a brown alga is a potential species that shows biosorption of uranium. The algae show an adsorption capacity of 434.8 mg/g at 10°C with a defined uranium concentration [101]. Bampaiti et al. reported that a brown alga Dictyopteris polypodioides shows biosorption of U (VI) ions in batch technique under optimum conditions; they also reported that biosorption under defined experimental conditions like pH, initial uranium concentration, biomass concentration, temperature show accumulation efficiency up to 94% [102]. Decontamination study of removal of low-level Co-60 and Cs-137 reported that Haematococcus sp. (freshwater species of Chlorophyta) showed 62.3% removal efficiency for low-level Co-60 concentration in 2 days and also showed the removal of Cs-137 with an efficiency of 88% in 48 hrs [103]. Extremophilic Arthrospira platensis, a filamentous cyanobacterium, shows the removal of radionuclides by the biosorption and bioaccumulation process. A. platensis showed high bioaccumulation and biosorption capacity for Pu-239, Sr-90, and Am-241 and showed an increased uptake of U-233 when polyphosphates were added to the cultivation medium [104, 105]. Dabbagh et al. reported that Oscillatoria homogenea filamentous cyanobacterium could remove Sr-90 and stable strontium from the aqueous solution at an optimum pH after ten days of incubation. Biovolume of cyanobacteria, illumination, initial Sr concentration, and incubation times shows a considerable effect on the bioaccumulation and biosorption of Sr [106]. Table 7 shows a list of different algal strains, radioactive contamination list, and mechanisms involved in the removal.

Fukuda et al. performed a global search for the potential algal and plant strains to determine their ability to bioaccumulate accidentally released radionuclides from the nuclear power plant in Fukushima, which involves the radionuclides like Cs-137, Sr-90, as these radionuclides due to their analogy with potassium and calcium, respectively can accumulate inside a living organism, for example, accumulation of Sr-90 in the bone. They found that out of 188 strains of algae and plant strains, some algae strain shows a very good efficiency in the accumulation of Sr-90, Cs-137, and I-125. Stigonema ocellatum, Oedogonium sp., Egeria densa show the bioaccumulation of Sr-90 with an efficiency of 41.3%, 36.3%, and 33.9%, respectively. Algal species like Nostoc commune, Scytonema javanicum, Stigonema ocellatum, Ophiocytium sp. show the bioaccumulation of I-125 with an efficiency of 65.9%, 61.9%, 48.5%, and 41.6%, respectively. A freshwater Eustigmatophyceae strain nak-9 which was later identified as Vacuoliviride crystalliferum shows up to 90% bioaccumulation of Cs-137 in 1 hr. Other than that, Batrachospermum virgato-decaisneanum, Chloroidium saccharophilum also shows the bioaccumulation of Cs-137 with an efficiency of 37.9% 22.4%, respectively. Stigonema ocellatum shows the elimination of both Sr-85 and I-125. Lemna aoukikusa shows the elimination of both Cs-137 and I-125, respectively [107-109]. Another extremophilic unicellular red alga (Rhodophyta), Galdieria sulphuraria, shows the bioaccumulation of Cs-137 from a medium deprived of potassium-containing caesium concentration of 30 µg/l with a removal efficiency of 52 ± 15% in mixotrophic growth condition with a culture period of 10 days [110]. Data's efficiency of different algae strains is shown in the table. Table 8 shows the bioaccumulation efficiency of different algae, their type, mechanism of removal, and the concentration of radionuclides.

Cs-134, Am-214, Sr-85, Ra-226, can be adsorbed by cyanobacteria biomass. Nostoc corneum, Nostoc insulare, Oscillatoria geminate, Spirulina laxissima, and waste product from alginate production, which contain algal species like Laminaria digitata and Laminaria japonica, have shown different uptake efficiency for different radionuclides. It is also reported that phosphorylated cyanobacterial biomass shows improved efficiency of radionuclide adsorption [111]. Some of the removal efficiency of phosphorylated and non-phosphorylated cyanobacterial strain was shown in Table 9.

Most of the studies suggest that algae found in extreme habitats show a higher ability to remove radionuclides from the environment. However, their limitation involves providing optimum conditions to grow the algae so that they can show maximum efficiency to remove the contaminants. Algae can be a potential candidate for the mitigation of radioactive wastes. Bioaccumulation and biosorption of radionuclides by algae highly depend on types of culture operation, pH, media constituents, contact time, initial biomass load, initial radionuclide concentration, light intensity, and adsorbent material. Hence, further research in the above areas and isolation and screening of new algae can be the potential area of research.