Melatonin, a phytohormone for enhancing the accumulation of high-value metabolites and stress tolerance in microalgae: Applications, mechanisms, and challenges

Subtleties of tetracycline removal during growth of microalgae-fungi consortia: Mechanistic insights from perspectives of extra- and intracellular metabolites

This study focused on tetracycline (TC) as the target antibiotic and utilized the emerging microbial system microalgae-fungi consortia to treat it. Results indicate that consortia composed of microalgae Chlorella sp. HL and fungi HW12 (Aspergillus caespitosus) (HL-HW12) exhibited the optimum TC removal (93.00 %, residual concentration: 2.73 mg/L) and biomass harvesting efficiency (92.69 %) among the five kinds of constructed microalgae-fungi consortia. Mechanism analysis indicated that outside the cell, microalgae-fungi consortia strengthened TC removal and biomass harvesting by augmenting the contents of proteins, polysaccharides, fulvic acids, and humic acids. While within the cell, microalgae-fungi consortia adjusted the abundance of critical metabolites in the amino acid metabolism, nucleotide metabolism, and other metabolic pathways to cope with the coercion of TC and facilitated its elimination. This study not only provides good TC microbial treatment systems but also comprehensively reveals the TC removal and metabolic response mechanisms by microalgae-fungi consortia.

Advances of microalgae-based enhancement strategies in industrial flue gas treatment: From carbon sequestration to lipid production

The acceleration of industrial development and urban expansion has led to a significant increase in flue gas emissions, posing a significant risk to human health and ecosystems. Recent studies have elucidated the significant potential of microalgae in the domain of sustainable industrial flue gas treatment. However, the inherent multifaceted factors within flue gas exert inhibitory effects on microalgal growth, thereby diminishing the overall system efficacy. Therefore, it is necessary to systematically analyze the flue gas components and propose complete intermediate treatment steps to alleviate their stressful effects on microalgae. Concurrently, to address the intrinsic limitations of the systemic functionality and enhance the applicability of microalgal biotechnology in industrial flue gas treatment, this review proposes a series of innovative solutions and strategies aimed at improving carbon fixation efficiency and lipid productivity of microalgae during flue gas treatment. In addition, the feasibility and potential limitations of these strategies in industrial applications are also discussed. Furthermore, through systematic comparative analysis, the optimal scheme and development trend of industrial flue gas emission reduction technology are explored. This comprehensive review not only establishes a theoretical foundation for the application of microalgae in industrial flue gas treatment, but also offers valuable insights for future research directions in related fields.

Elucidating the pivotal functions of fulvic acid in enhancing Monoraphidium sp. QLZ-3 for cadmium remediation and bioresource recovery

Heavy metal pollution poses substantial challenges to human health and aquatic ecosystems. This study investigates a coupled technology for lipid production and cadmium adsorption utilizing microalgae regulated by fulvic acid (FA). Under the combination of 40 mg L-1 FA and cadmium (Cd) treatment, Monoraphidium sp. QLZ-3 exhibited the highest biomass (3.27 g L-1), lipid content (52.73 %), and lipid productivity (193.26 mg L-1 d-1), which were enhanced by 20.10 %, 15.81 % and 40.27 % respectively compared with the control. Notably, FA application significantly increased cadmium removal efficiency to 100 %. Moreover, the synergistic effect of FA and Cd enhanced the biomass, lipid production, and energy yield (92.38 kJ L-1) by accelerating nitrogen consumption, inhibiting carbohydrate synthesis, and elevating levels of reactive oxygen species and mitogen-activated protein kinase. FA had a minimal impact on fatty acid composition and biodiesel properties. The majority of the biodiesel quality parameters met the specifications for commercial biodiesel. Proteomic analysis revealed that exogenous FA promoted cell growth and lipid accumulation by upregulating the tricarboxylic acid cycle, the nitrogen assimilation pathway, and activating Ca2+ signaling in QLZ-3 under cadmium treatment. Additionally, calcium ion (Ca2+) and reactive oxidative species (ROS) were identified as key factors in promoting cell growth and lipid synthesis under the influence of Cd and FA. These findings collectively indicate that FA can boost both biomass and lipid production, as well as the efficient removal of Cd2+, providing a theoretical foundation for the optimization of microalgal biomass and lipid production and the bioremediation of heavy metal contamination in aquatic environments.

Advancements of astaxanthin production in Haematococcus pluvialis: Update insight and way forward

The global market demand for natural astaxanthin (AXT) is growing rapidly owing to its potential human health benefits and diverse industry applications, driven by its safety, unique structure, and special function. Currently, the alga Haematococcus pluvialis (alternative name H. lacustris) has been considered as one of the best large-scale producers of natural AXT. However, the industry's further development faces two main challenges: the limited cultivation areas due to light-dependent AXT accumulation and the low AXT yield coupled with high production costs resulting from complex, time-consuming upstream biomass culture and downstream AXT extraction processes. Therefore, it is urgently to develop novel strategies to improve the AXT production in H. pluvialis to meet industrial demands, which makes its commercialization cost-effective. Although several strategies related to screening excellent target strains, optimizing culture condition for high biomass yield, elucidating the AXT biosynthetic pathway, and exploiting effective inducers for high AXT content have been applied to enhance the AXT production in H. pluvialis, there are still some unsolved and easily ignored perspectives. In this review, firstly, we summarize the structure and function of natural AXT focus on those from the algal H. pluvialis. Secondly, the latest findings regarding the AXT biosynthetic pathway including spatiotemporal specificity, transport, esterification, and storage are updated. Thirdly, we systematically assess enhancement strategies on AXT yield. Fourthly, the regulation mechanisms of AXT accumulation under various stresses are discussed. Finally, the integrated and systematic solutions for improving AXT production are proposed. This review not only fills the existing gap about the AXT accumulation, but also points the way forward for AXT production in H. pluvialis.

Exploiting synergy of dopamine and stressful conditions in enhancing Haematococcus lacustris biomass and astaxanthin yield

Dopamine (DA) has attracted attention because of its effects on Haematococcus lacustris biomass, astaxanthin production, and physiological responses. The alga treated with 25 μM DA combined with 1 g L-1 sodium chloride exhibited 7.63 %, 41.25 %, and 52.04 % increases in biomass (1.41 g L-1), astaxanthin content (32.37 mg/g), and astaxanthin productivity (3.51 mg L-1 d-1) respectively, compared with the salinity stress and high light. Exogenous DA treatment promoted lipid synthesis while reducing carbohydrate and protein contents. Moreover, carotenogenesis and lipogenesis-associated genes were upregulated under DA induction. Inhibition of reactive oxygen species and autophagy, along with mitogen-activated protein kinase activation, promoted astaxanthin accumulation under DA. Furthermore, DA application boosted astaxanthin biosynthesis by regulating the levels of respiratory metabolic intermediates, the γ-aminobutyric acid shunt, and important phytohormones. These findings present a potential and successful biotechnological approach for enhancing biomass and astaxanthin production in H. lacustris under stressful conditions.

A novel stepwise salinity acclimation method to improve the survival of freshwater microalgae Haematococcus lacustris in seawater salinity

Freshwater microalga Haematococcus lacustris rich in astaxanthin, as a supplemental live diet can directly supply natural astaxanthin to the aquaculture organisms, except marine aquaculture organisms, since H. lacustris cannot tolerate seawater salinity. The objective of the present study is to provide a salinity acclimation method that allows H. lacustris to survive and accumulate astaxanthin with the aim of developing a novel supplemental live diet for marine aquaculture organisms. H. lacustris cultured in freshwater was subjected to different stepwise salinity acclimation processes (two-, three-, and four-shift). As the controls, H. lacustris was exposed to five constant salinities conditions (0, 0.05, 0.075, 0.3, and 0.6 M NaCl, respectively). Among the controls, almost all cells in the 0.3 M and 0.6 M NaCl conditions died immediately. In contrast, H. lacustris in the stepwise salinity acclimation processes survived in 0.6 M NaCl (equivalent to seawater salinity of 35 psu), showing the highest living-cell proportion (50.0%) and astaxanthin yield (0.72 mg·L-1) in the four-shift. The present study first demonstrated that H. lacustris tolerated seawater salinity through a stepwise acclimation process, proving a new strategy to supply live microalgal diets rich in natural astaxanthin for marine aquaculture.

Unveiling significant roles of phytohormone 6-benzylaminopurine in empowering Phaeodactylum tricornutum for high-salinity wastewater treatment and bioresource recovery

High-salinity presents significant challenges in microalgal wastewater treatment and bioresource recovery due to salinity stress. This study explored the use of salt-tolerant microalgae in conjunction with phytohormone regulation. 1 µM 6-benzylaminopurine increased the biomass of Phaeodactylum tricornutum by 38.3 % and enhanced lipid production by 36.8 %. 6-benzylaminopurine significantly improved the removal of inorganic carbon, total nitrogen, and total phosphorus by 85.2 %, 27.4 %, and 31.9 %. Specifically, 6-benzylaminopurine improved K+ transportation by 71.0 %, increased the activity of Ca2+ transport ATPase and Ca2+ sensors by 49.0 %-83.0 %, optimized osmotic balance, and alleviated salt-induced damage. The contents of proline and extracellular polymers increased by 34.8 % and 35.5 %. A 38.4 % reduction in reactive oxygen species indicated that high-salinity stress was mitigated. The analysis of Sustainable Development Goals showed a 56.2 % improvement in Affordable and Clean Energy. Overall, these findings further highlighted the promising application of the phytohormone 6-benzylaminopurine in microalgal high-salinity wastewater treatment and lipid production.

Construction and transcriptomic analysis of salinity-induced lipid-rich flocculent microalgae

The lack of cost-effective nutrient sources and harvesting methods is currently a major obstacle to the production of sustainable biofuels from microalgae. In this study, Chlorella pyrenoidosa was cultured with saline wastewater in a stirred photobioreactor, and lipid-rich flocculent microalgae particles were successfully constructed. As the influent salinity of the photobioreactor increased from 0% to 3%, the particle size and sedimentation rate of flocculent microalgae particles gradually increased, and the lipid accumulation of microalgae also increased gradually. Transcriptome analysis showed that the number of differentially expressed genes (DEGs) in microalgae increased as the salinity of wastewater increased from 1% to 3%, and the number of up-expressed genes was greater than that of down-expressed genes in microalgae at different salinity levels. The enrichment analysis of DEGs showed that the up-expressed genes under salt stress mainly involved in fatty acid biosynthesis and other metabolic processes, which initially revealed the mechanism of the lipid accumulation of microalgal particles in saline wastewater. In addition, the expression and functions of genes involved in lipid and EPS synthesis pathway in microalgae were analyzed, and the key genes involved in salinity affecting lipid and EPS synthesis in microalgae were preliminarily identified. The results could provide novel insight for genetic engineering to regulate the construction of lipid-rich flocculent microalgae particles.

Enhancing lead tolerance in spirulina using glycine betaine under high CO2 and salinity for sustainable carbon capture

Microalgae-based carbon capture and utilization (CCU) offers a promising negative emission technology that absorbs CO2 and generates valuable biomass. However, heavy metal pollutants in industrial flue gases can compromise carbon sequestration efficiency and bioproduct quality. Spirulina was investigated as a model organism for CO2 sequestration, using a modified Zarrouk's medium to integrate salt stress and an osmoprotectant to boost biomass yield under 15 % CO2. This approach enhances both biomass yield （380.83 mg L-1 d-1）and resistance to lead toxicity. Analytical assessments revealed that glycine betaine (GB) supplementation drastically reduced lead accumulation, decreasing the extracellular and intracellular contents by 39.7 % and 60.7 %, respectively. A notable decrease in extracellular dissolved organic matter was also observed. Furthermore, transcriptomic analyses confirmed that GB treatment strengthened osmotic stress responses and suppressed metal ion transport. These findings enhanced the feasibility of microalgae-based CCU technologies, marking significant progress in sustainable algal biotechnology.

Synergistic effects of exogenous IAA and melatonin on seed priming and physiological biochemistry of three desert plants in saline-alkali soil

To investigate the synergistic effect of IAA and melatonin (MT) on three plants to alleviate the effects of salt damage on plants, we aim to determine the optimal concentrations of exogenous hormone treatments that improve salinity resistance for each species. In this experiment, three desert plants, Sarcozygium xanthoxylon, Nitraria tangutorum, and Ammopiptanthus mongolicus, which are common in Wuhai City, were used as plant materials. Two time periods (12 h,24 h) of exogenous hormone IAA (100 μmol/L) and exogenous melatonin concentration (0, 100, 200, 300 μmol/L) were used to treat the three desert plants in saline soil under different conditions of exogenous IAA and exogenous melatonin. The results indicate that under different concentrations of exogenous IAA and melatonin, the germination rate and vigor of the three desert plant species in saline-alkaline soil improved. However, as the concentration of melatonin increased, the germination rate and vigor of these desert plants were inhibited. Whereas, plant height, root length, leaf length, fresh weight, dry weight, and root vigor of the three desert plants were alleviated under different conditions of exogenous IAA and exogenous melatonin. under the action of two exogenous hormones, the low concentration of melatonin decreased their malondialdehyde content and increased their proline content. As melatonin levels increased, the activity of antioxidant enzymes also rose initially, followed by a subsequent decline. This study highlights the synergistic effects of two exogenous hormones on the critical role of cell osmomodulators and antioxidant enzyme activity in combating salinity damage in three desert plants.

Synergistic interplay between melatonin and hydrogen sulfide enhances cadmium-induced oxidative stress resistance in stock (Matthiola incana L.)

Ornamental crops particularly cut flowers are considered sensitive to heavy metals (HMs) induced oxidative stress condition. Melatonin (MLT) is a versatile phytohormone with the ability to mitigate abiotic stresses induced oxidative stress in plants. Similarly, signaling molecules such as hydrogen sulfide (H2S) have emerged as potential options for resolving HMs related problems in plants. The mechanisms underlying the combined application of MLT and H2S are not yet explored. Therefore, we evaluated the ability of individual and combined applications of MLT (100 μM) and H2S in the form of sodium hydrosulfide (NaHS), a donor of H2S, (1.5 mM) to alleviate cadmium (Cd) stress (50 mg L-1) in stock (Matthiola incana L.) plants by measuring various morpho-physiological and biochemical characteristics. The results depicted that Cd-stress inhibited growth, photosynthesis and induced Cd-associated oxidative stress as depicted by excessive ROS accumulation. Combined application of MLT and H2S efficiently recovered all these attributes. Furthermore, Cd stress-induced oxidative stress markers including electrolyte leakage, malondialdehyde, and hydrogen peroxide are partially reversed in Cd-stressed plants by MLT and H2S application. This might be attributed to MLT or H2S induced antioxidant plant defense activities, which effectively reduce the severity of oxidative stress indicators. Overall, MLT and H2S supplementation, favorably regulated Cd tolerance in stock; yet, the combined use had a greater effect on Cd tolerance than the independent application.

Exogenous melatonin delays leaves senescence and enhances saline and alkaline stress tolerance in grape seedlings

Saline and alkaline stress is one of the major abiotic stresses facing agricultural production, which severely inhibits the growth and yield of plant. The application of plant growth regulators can effectively prevent crop yield reduction caused by saline and alkaline stress. Exogenous melatonin (MT) can act as a signaling molecule involved in the regulation of a variety of physiological processes in plants, has been found to play a key role in enhancing the improvement of plant tolerance to abiotic stresses. However, the effects of exogenous MT on saline and alkaline tolerance of table grape seedlings and its mechanism have not been clarified. The aim of this study was to investigate the role of exogenous MT on morphological and physiological growth of table grape seedlings (Vitis vinifera L.) under saline and alkaline stress. The results showed that saline and alkaline stress resulted in yellowing and wilting of grape leaves and a decrease in chlorophyll content, whereas the application of exogenous MT alleviated the degradation of chlorophyll in grape seedling leaves caused by saline and alkaline stress and promoted the accumulation of soluble sugars and proline content. In addition, exogenous MT increased the activity of antioxidant enzymes, which resulted in the scavenging of reactive oxygen species (ROS) generated by saline and alkaline stress. In conclusion, exogenous MT was involved in the tolerance of grape seedlings to saline and alkaline stress, and enhanced the saline and alkaline resistance of grape seedlings to promote the growth and development of the grape industry in saline and alkaline areas.

Selective synergistic effects of oxalic acid and salicylic acid in enhancing amino acid levels and alleviating lead stress in Zea mays L

Lead is one of the major environmental pollutants which is highly toxic to plants and living beings. The current investigation thoroughly evaluated the synergistic effects of oxalic acid (OA) and salicylic acid (SA) on Zea mays L. plants subjected to varying durations (15, 30, 30, and 45 days) of lead (Pb) stress. Besides, the effects of oxalic acid (OA) combined with salicylic acid (SA) for different amino acids at various periods of Pb stress were also investigated on Zea mays L. The soil was treated with lead nitrate Pb (NO3)2 (0.5 mM) to induce Pb stress while the stressed plants were further treated using oxalic acid (25 mg/L), salicylic acid (25 mg/L), and their combination OA + SA (25 mg/L each). Measurements of protein content, malondialdehyde (MDA) levels, guaiacol peroxidase (GPOX) activity, catalase (CAT) activity, GSH content, and Pb concentration in maize leaves were done during this study. MDA levels increased by 71% under Pb stress, while protein content decreased by 56%, GSH content by 35%, and CAT activity by 46%. After treatment with SA, OA, and OA+SA, there was a significant reversal of these damages, with the OA+SA combination showing the highest improvement. Specifically, OA+SA treatment led to a 45% increase in protein content and a 39% reduction in MDA levels compared to Pb treatment alone. Moreover, amino acid concentrations increased by 68% under the Pb+OA+SA treatment, reflecting the most significant recovery (p < 0.0001).

Lipidomics analysis of microalgal lipid production and heavy metal adsorption under glycine betaine-mediated alleviation of low-temperature stress

Heavy metal pollution in the cold region is serious, affecting human health and aquatic ecology. This study investigated the ability of microalgae to remove heavy metals (HMs) and produce lipid at low temperature. The removal efficiency of different HMs (Cd2+, Cu2+, Cr3+ and Pb2+), cell growth and lipid synthesis of microalgae were analyzed at 15 °C. Moreover, addition of glycine betaine (GB) further enhanced the productivity of microalgae in treating HMs and lipid production, and simultaneously increased the antioxidant capacity of microalgae against environmental stresses. The results showed that the highest lipid productivity of 100.98 mg L-1 d-1 and the removal efficiency of 85.8 % were obtained under GB coupled with Cr3+. The highest glutathione content of 670.34 nmol g-1 fresh alga was achieved under GB coupled with Pb2+. In addition, lipidomics showed that GB was able to up-regulate the triglyceride and diglyceride content, influenced fatty acid composition to regulate the microalgal metabolism, and mediated lipid accumulation under 15 °C mainly through the regulation of glycerol ester metabolism. This study provided a new perspective on microalgal lipid production and the removal of HMs in cold regions and provided evidence for the use of phytohormones to improve the algal environmental resistance.

Antioxidants alleviated low-temperature stress in microalgae by modulating reactive oxygen species to improve lipid production and antioxidant defense

The aim of this study was to investigate the effects of various concentrations of antioxidants, including butyl hydroxy anisd (BHA), butylated hydroxytoluene (BHT), fulvic acid (FA), melatonin (MT), glycine betaine (GB) and putrescine (Put), on growth and lipid synthesis of microalgae under low-temperature (15 ℃). Changes in biochemical indicators, reactive oxygen species (ROS) level, glutathione (GSH) content and antioxidant enzyme activities were also studied. The results indicated that the maximum biomass concentration (1.3 g/L) and lipid productivity (75.3 ± 5.8 mg/L d-1) were achieved under 100 μM MT and 1 μM GB, respectively. Moreover, antioxidants were able to increase the GSH and antioxidant enzymes activities in algal cells under low-temperature stress. This study was enlightening for the utilization of antioxidants to improve the resistance to low-temperature stress and lipid production in microalgae, and provided a theoretical basis for the application of microalgae for lipid accumulation in cold regions.

Gamma-aminobutyric acid as a regulator of astaxanthin production in Haematococcus lacustris under salinity: Exploring physiology, signaling, autophagy, and multi-omics landscape

Haematococcus lacustris-derived natural astaxanthin has significant commercial value, but stressful conditions alone impair cell growth and reduce the total productivity of astaxanthin in industrial settings. This study used gamma-aminobutyric acid (GABA) to increase biomass, astaxanthin productivity, and tolerance to salinity. GABA under NaCl stress enhanced the biomass to 1.76 g/L, astaxanthin content to 30.37 mg g-1, and productivity to 4.10 mg/L d-1, outperforming the control. Further analysis showed GABA enhanced nitrogen assimilation, Ca2+ level, and cellular GABA content, boosting substrate synthesis, energy metabolism, osmoregulation, autophagy, and antioxidant defenses. GABA also activated signaling pathways involving phytohormones, cAMP, cGMP, and MAPK, aiding astaxanthin synthesis. The application of biomarkers (ethylene, salicylic acid, trans-zeatin) and an autophagy inhibitor cooperated with GABA to further enhance the total astaxanthin productivity under NaCl stress. Combining GABA with 25 μM salicylic acid maximized astaxanthin yield at 4.79 mg/L d-1, offering new strategies for industrial astaxanthin production.

Effect of phytohormones on the carbon sequestration performance of CO2 absorption-microalgae conversion system under low light restriction

Microalgae have the potential to fix CO2 into valuable compounds. Low photosynthetic efficiency caused by low light was one of the challenges faced by microalgae carbon sequestration. In this study, Melatonin (MT) and indole-propionic acid (IPA) were used to alleviate the growth inhibition of Spirulina in CAMC system under low light restriction. The results showed that MT and IPA increased biomass and carbon fixation capacity. 10 mg/L IPA group achieved the maximum biomass and carbon fixation capacity, which were 17.11% and 21.46% higher than control. MT and IPA promoted the synthesis of chlorophyll, which in turn captured more light energy for microalgae growth. The increase of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) activities enhanced the resistance of microalgae to low light stress. MT and IPA promoted the secretion of extracellular polymeric substances (EPS) which was benefit to protect cells. The maximum phycocyanin content and yield was found in 10 mg-IPA group, which was 20.67% and 46.67% higher than control. MT and IPA improved the synthesis of carbohydrates and proteins and increased carbohydrates and proteins yield. This indicated that adding phytohormones was an effective method to alleviate the growth of microalgae restricted by low light stress, which provided a theoretical guidance for the application of CAMC system in CO2 capture and resource utilization.

Integrating bicarbonate-based microalgal production with alkaline sewage for ocean negative carbon emissions

Using sewage (wastewater) for ocean alkalinity enhancement (OAE) has been considered as one promising ocean negative carbon emissions (ONCE) approach due to its high carbon sequestration efficiency and low environmental risk. To make this process more profitable and sustainable, this perspective proposes to integrate bicarbonate-based microalgal production and sewage alkalinity enhancement for ONCE. In this concept, the spent aqueous alkaline bicarbonate-based microalgal medium is cheap or even free for OAE, while the produced microalgae with high value-added compositions make this process more profitable. To make the proposed idea more efficient and sustainable, the prospects for its future development are also discussed in this opinion article. This perspective provides a novel and practical idea for achieving efficient carbon neutralization and high economic value simultaneously.

Continuous selenite biotransformation and biofuel production by marine diatom in the presence of fulvic acid

In this study, the biochemical response of Phaeodactylum tricornutum to varying concentrations of inorganic selenium (Se) was investigated. It was observed that, when combined with fulvic acid, P. tricornutum exhibited enhanced uptake and biotransformation of inorganic Se, as well as increased microalgal lipid biosynthesis. Notably, when subjected to moderate (5 and 10 mg/L) and high (20 and 40 mg/L) concentrations of selenite under fulvic acid treatment, there was a discernible redirection of carbon flux towards lipogenesis and protein biosynthesis from carbohydrates. In addition, the key parameters of microalgae-based biofuels aligned with the necessary criteria outlined in biofuel regulations. Furthermore, the Se removal capabilities of P. tricornutum, assisted by fulvic acid, were coupled with the accumulation of substantial amounts of organic Se, specifically SeCys. These findings present a viable and successful approach to establish a microalgae-based system for Se uptake and biotransformation.

Dual sources of melatonin and evidence for different primary functions

This article discusses data showing that mammals, including humans, have two sources of melatonin that exhibit different functions. The best-known source of melatonin, herein referred to as Source #1, is the pineal gland. In this organ, melatonin production is circadian with maximal synthesis and release into the blood and cerebrospinal fluid occurring during the night. Of the total amount of melatonin produced in mammals, we speculate that less than 5% is synthesized by the pineal gland. The melatonin rhythm has the primary function of influencing the circadian clock at the level of the suprachiasmatic nucleus (the CSF melatonin) and the clockwork in all peripheral organs (the blood melatonin) via receptor-mediated actions. A second source of melatonin (Source # 2) is from multiple tissues throughout the body, probably being synthesized in the mitochondria of these cells. This constitutes the bulk of the melatonin produced in mammals and is concerned with metabolic regulation. This review emphasizes the action of melatonin from peripheral sources in determining re-dox homeostasis, but it has other critical metabolic effects as well. Extrapineal melatonin synthesis does not exhibit a circadian rhythm and it is not released into the blood but acts locally in its cell of origin and possibly in a paracrine matter on adjacent cells. The factors that control/influence melatonin synthesis at extrapineal sites are unknown. We propose that the concentration of melatonin in these cells is determined by the subcellular redox state and that melatonin synthesis may be inducible under stressful conditions as in plant cells.