Auxin and cytokinin synergism augmenting biomass and lipid production in microalgae Desmodesmus sp. JS07

Microalgae growth-promoting bacteria for cultivation strategies: Recent updates and progress

Microalgae growth-promoting bacteria (MGPB), both actinobacteria and non-actinobacteria, have received considerable attention recently because of their potential to develop microalgae-bacteria co-culture strategies for improved efficiency and sustainability of the water-energy-environment nexus. Owing to their diverse metabolic pathways and ability to adapt to diverse conditions, microalgal-MGPB co-cultures could be promising biological systems under uncertain environmental and nutrient conditions. This review proposes the recent updates and progress on MGPB for microalgae cultivation through co-culture strategies. Firstly, potential MGPB strains for microalgae cultivation are introduced. Following, microalgal-MGPB interaction mechanisms and applications of their co-cultures for biomass production and wastewater treatment are reviewed. Moreover, state-of-the-art studies on synthetic biology and metabolic network analysis, along with the challenges and prospects of opting these approaches for microalgal-MGPB co-cultures are presented. It is anticipated that these strategies may significantly improve the sustainability of microalgal-MGPB co-cultures for wastewater treatment, biomass valorization, and bioproducts synthesis in a circular bioeconomy paradigm.

Hydroponics with Microalgae and Cyanobacteria: Emerging Trends and Opportunities in Modern Agriculture

The global population is expected to reach 9.5 billion, which means that crop productivity needs to double to meet the growing population's food demand. Soil degradation and environmental factors, such as climate events, significantly threaten crop production and global food security. Furthermore, rapid urbanization has led to 55% of the world's population migrating to cities, and this proportion is expected to increase to 75% by 2050, which presents significant challenges in producing staple foods through conventional hinterland farming. Numerous studies have proposed various sustainable farming techniques to combat the shortage of farmable land and increase food security in urban areas. Soilless farming techniques such as hydroponics have gained worldwide popularity due to their resource efficiency and production of superior-quality fresh products. However, using chemical nutrients in a conventional hydroponic system can have significant environmental impacts, including eutrophication and resource depletion. Incorporating microalgae into hydroponic systems as biostimulants offers a sustainable and ecofriendly approach toward circular bioeconomy strategies. The present review summarizes the plant growth-promoting activity of microalgae as biostimulants and their mechanisms of action. We discuss their effects on plant growth parameters under different applications, emphasizing the significance of integrating microalgae into a closed-loop circular economy model to sustainably meet global food demands.

Enhancing lipid production and sedimentation of Chlorella pyrenoidosa in saline wastewater through the addition of agricultural phytohormones

In this study, the effect of external agricultural phytohormones (mixed phytohormones) addition (1.0, 5.0, 10.0, and 20.0 mg L-1) on the growth performance, lipid productivity, and sedimentation efficiency of Chlorella pyrenoidosa cultivated in saline wastewater was investigated. Among the different concentrations evaluated, the highest biomass (1.00 g L-1) and lipid productivity (11.11 mg L-1 d-1) of microalgae were obtained at 10.0 mg L-1 agricultural phytohormones addition. Moreover, exogenous agricultural phytohormones also improved the sedimentation performance of C. pyrenoidosa, which was conducive to the harvest of microalgae resources, and the improvement of sedimentation performance was positively correlated with the amount of agricultural phytohormones used. The promotion of extracellular polymeric substances synthesis by phytohormones in microalgal cells could be considered as the reason for its promotion of microalgal sedimentation. Transcriptome analysis revealed that the addition of phytohormones upregulated the expression of genes related to the mitogen-activated protein kinase (MAPK)-mediated phytohormone signaling pathway and lipid synthesis, thereby improving salinity tolerance and lipid production in C. pyrenoidosa. Overall, agricultural phytohormones provide an effective and inexpensive strategy for increasing the lipid productivity and sedimentation efficiency of microalgae cultured in saline wastewater.

Effects of staged multiple phytohormones application on capillary-driven attached Chlorella sp. biofilm

Comparing with single phytohormone application, applying multiple phytohormones to microalgae-based wastewater treatment systems can offer more extensive growth-promoting and stress-protecting effects for microalgae, yet the advantage of stress-relieving salicylic acid (SA) under combined phytohormones application scenario has not been exploited. Employing the improved capillary-driven attached microalgae culturing device (CD-PBR) previously used for single phytohormone application, this study compared the effects of mixed and single phytohormone(s) addition under as low as 10-7 M dosage. In order to make the best of SA for its stress-relieving property, postponed SA addition combined with applying other phytohormone(s) at the beginning of microalgae cultivation was also investigated. Combination of 10-6 M 6-benzylaminopurine (6-BA) with 10-7 M SA was sufficient for enhancing growth-promoting effects and anti-oxidative responses for attached Chlorella sp., while indole-3-acetic acid (IAA) addition was unnecessary. Combination of 6-BA addition at the beginning while postponed SA addition on Day 4 could further sustain such beneficial effects, while removing up to 99.7% total nitrogen (TN) and 97.9% total phosphorus (TP) from the bulk liquid. These results provided innovative strategies on mixed phytohormones addition for microalgae.

Regulation effects of indoleacetic acid on lipid production and nutrient removal of Chlorella pyrenoidosa in seawater-containing wastewater

The utilization of seawater supplemented with wastewater nutrients for microalgae cultivation represents a promising and cost-effective approach that combines the benefits of wastewater treatment and microalgal resource recovery. However, the high salt content in seawater poses a significant challenge, hindering microalgal growth and reducing the removal of nitrogen and phosphorus on a large scale. The phytohormone indoleacetic acid (IAA) was used in this study to enhance stress resistance and lipid production of Chlorella pyrenoidosa grown in seawater-wastewater medium. Compared to the control groups involving regular wastewater and seawater-containing wastewater without IAA, Chlorella pyrenoidosa cultivated in the seawater-containing wastewater supplemented with IAA exhibited remarkable outcomes. Specifically, microalgae in IAA-enhanced seawater-containing wastewater achieved the highest lipid productivity (22.67 mg L-1 d-1) along with impressive nitrogen (99.3 %) and phosphorus (97.3 %) removal rates. Moreover, their cell sedimentation ratio reached 76.6 %, indicating enhanced settling properties. Additionally, the physiological mechanism changes after exposure to seawater stress and IAA were revealed based on the changes in antioxidant enzymes, endogenous hormones, and fatty acid saturation. Furthermore, the transcriptomic analysis elucidated the molecular mechanisms underlying microalgal lipid synthesis and their response to antioxidant stress when exposed to seawater. The supplementation of IAA under seawater stress stimulated energy metabolism and the antioxidant response in microalgal cells, effectively mitigating the adverse effects of seawater stress and promoting overall algal lipid productivity. Overall, this study unveiled the potential of exogenous plant hormones, particularly IAA, in enhancing stress resistance and lipid productivity of microalgae grown in seawater-wastewater medium, which significantly contributed towards the efficient use of seawater resources for microalgae cultivation and biofuel production.

Phytohormones: novel strategy for removing emerging contaminants and recovering resources

Microalgae have the ability to remove emerging contaminants (ECs) from wastewater and to recover resources, but this is limited by oxidative damage caused by the contaminants. Recently, phytohormones have been found to improve the tolerance of microalgae under oxidative stress, to promote the removal of ECs, and to enhance the synthesis of metabolites.

Hormone released by the microalgae Neochlorisaquatica and alkalinization influence growth of terrestrial and floating aquatic plants

The microalgae Neochloris aquatica were previously evaluated as a potential biological control agent and source of bioactive compounds against immature stages of Culex quinquefasciatus. Larvae reared on microalgae suspension showed mortality or drastic effects with morphological alterations and damage in the midgut. N. aquatica have nutritional and toxic effects, resulting in delayed life cycle and incomplete adult development. Given the possibility of its use as a biological control agent, in this work we evaluate the effect of microalgae on other organisms of the environment, such as plants. Arabidopsis thaliana, a terrestrial plant, and Lemna sp., a floating aquatic plant, were selected as examples. Interaction assays and compound evaluations showed that the microalgae release auxins causing root inhibition, smaller epidermal cells and hairy root development. In Lemna sp., a slight decrease in growth rate was observed, with no deleterious effects on the fronds. On the other hand, we detected a detrimental effect on plants when interactions were performed in a closed environment, in a medium containing soluble carbonate, in which microalgae culture rapidly modifies the pH. The experiments showed that alkalinization of the medium inhibits plant growth, causing bleaching of leaves or fronds. This negative effect in plants was not observed when plants and microalgae were cultured in carbonate-free media. In conclusion, the results showed that N. aquatica can modify plant growth without being harmful, but the rapid alkalinization produced by carbon metabolism of microalgae under CO₂-limiting conditions, could regulate the number of plants.

Focus on the role of synthetic phytohormone for mixotrophic growth and lipid accumulation by Chlorella pyrenoidosa

Synthetic phytohormone (SP) is regarded as an attractive candidate for microalgae cultivation due to its potential for high-value microalgae biomass production. Herein, α-naphthylacetic acid (NAA), indomethacin (IN) and 2,4-dichlorophenoxyacetic acid (2,4-D) were used for the mixotrophic cultivation of Chlorella pyrenoidosa with mariculture wastewater (MW) acidogenic fermentation effluent. The growth and lipid accumulation of Chlorella pyrenoidosa added with SP were enhanced, given their high bioavailability of the nutrients. Among these three SPs, IN was optimal for Chlorella pyrenoidosa growth, with the maximum optical density of 1.81. NAA exhibited the best performance for lipid production and the proportion of lipid reached 50.24%. Furthermore, the energy of Chlorella pyrenoidosa cultured with SP preferentially allocated to lipogenesis. To understand the mechanism of lipid accumulation in Chlorella pyrenoidosa in response to SP, the enzyme activities involved in carbon metabolism were determined. The malic enzyme (ME) and acetyl-CoA carboxylase (ACCase) were positively correlated with lipid accumulation. Phosphoenolpyruvate carboxylase (PEPC) was the negative feedback enzyme for lipid synthesis. The findings could provide valuable information for regulation mechanism of lipid accumulation and value-added products recovery by microalgae.

Improving the content of high value compounds in Nordic Desmodesmus microalgal strains

Nordic Desmodesmus microalgal strains (2-6) and (RUC-2) were exposed to abiotic stress (light and salt) to enhance lipids and carotenoids. The biomass output of both strains increased by more than 50% during light stress of 800 μmol m^-2 s^-1 compared to control light. The biomass of Desmodesmus sp. (2-6) contained most lipids (15% of dry weight) and total carotenoids (16.6 mg g^-1) when grown at moderate light stress (400 μmol m^-2 s^-1), which further could be enhanced up to 2.5-fold by salinity stress. Desmodesmus sp. (RUC-2) exhibited maximal lipid (26.5%) and carotenoid (43.8 mg L^-1) content at light intensities of 400 and 100 μmol m^-2 s^-1, respectively. Salinity stress stimulated lipid accumulation by 39%. Nordic Desmodesmus strains therefore are not only able to tolerate stress conditions, but their biomass considerably improves under stress. These strains have high potential to be used in algal bio-factories on low-cost medium like Baltic seawater.

Enhanced production of microalgal biomass and lipid as an environmentally friendly biodiesel feedstock through actinomycete co-culture in biogas digestate effluent

In this study, an innovative approach to enhance the production of microalgal biomass and lipid as a promising sustainable feedstock for biodiesel was proposed using an actinomycetes co-culture with microalgae in the biogas digestate effluent (BDE) that can be employed as an environmentally friendly and cost-effective strategy. Among tested actinomycete isolates, Piscicocus intestinalis WA3 produced indole-3-acetic acid and siderophores as algal growth promoting agents and showed effective lipid accumulation with satisfying fatty acids composition. During co-cultivation of P. intestinalis WA3 with microalga Tetradesmus obliquus AARL G022 in the BDE, biomass production, chlorophyll a content, and lipid productivity were significantly increased by 1.30 folds, 1.39 folds, and 1.55 folds, respectively, compared to microalgae monoculture. The accumulated lipids contained long-chain fatty acids with better fuel properties that could potentially be used as biodiesel feedstock. The overall results evidenced that actinomycete co-culture would contribute greatly to the cost-effective production of environmental-friendly microbial-based biofuel.

Algae-induced taste and odour problems at low temperatures and the cold stress response hypothesis

The existence of taste and odour (T&O) in drinking water is one of the principal causes of consumer complaints and is commonly related to algae growth. Numerous studies have confirmed the existence of algal blooms emerging specifically in low-temperature periods, herein referred to as "cold algae"; these include chrysophytes, cryptophytes, dinoflagellates and diatoms. In addition, the adaption mechanisms of these "cold algae" involve high flexibility in their nutrient intake and to the hydrological characteristics of the waters and their high contents of intracellular polyunsaturated fatty acids (PUFAs). Like algae proliferating in higher temperature waters, cold algae can also produce offensive odours. The potential dominant T&O compounds of low-temperature algae probably include saturated/unsaturated aldehydes and even some terpenoids. Among these, the polyunsaturated aldehydes (PUAs), the derivatives of polyunsaturated fatty acids, are the dominant T&O compounds and are probably synthesized during cell rupture. It was found that, for cold algae, low temperature may have a favourable effect on the generation of algae-induced T&O compounds. Furthermore, to better understand the internal mechanisms of algal T&O production, the stress response theory is introduced, which provides ideas for T&O control in raw water and in water treatment. Finally, implications for T&O management are given based on this review. KEY POINTS: • Like algae proliferating in higher temperature waters, cold algae can produce offensive odours. • Low temperatures may have a favourable effect on the generation of algae-induced T&O compounds. • The stress response theory can help to better understand the internal mechanisms of algal T&O production.