Salicylic acid and aspirin stimulate growth of Chlamydomonas and inhibit lipoxygenase and chloroplast desaturase pathways

Allelopathic effect of benzoic acid (hydroponics root exudate) on microalgae growth

Hydroponic effluent (HE) contains a reasonable amount of residual nutrients. Therefore, HE could be used as a low-cost growth media for microalgae mediated resource recovery and water recycling. However, the presence of root exudates (particularly, benzoic acid) may lead to toxicity in microalgae.In the present study, the allelopathic effects of benzoic acid on microalgal growth was tested. During 96 h batch growth, Chlorella pyrenoidosa showed the highest biomass concentration (0.064-0.037 g.L^-1) compared to Chlorella sorokiniana (0.09-0.26 g.L^-1) at the tested benzoic acid doses. Moreover, both the species showed growth stimulation and growth inhibition up to certain benzoic acid doses. Hence, both the microalgal species showed allelopathic behaviour at different doses of benzoic acid. Further, the observed half effective concentration (96 h EC₅₀) were 65.10 mg.L^-1 and 105.27 mg.L^-1, respectively, for Chlorella pyrenoidosa and C. sorokiniana with 95% confidence limits. Further, Haldane's model best fitted with experimental data of both the microalgae (r ∼ 0.99). Overall, the study reveals that the HE with low benzoic acid dose may serve as a suitable growth media for microalgae. However, further in-depth research interventions using real HE are desirable to determine its real-world applicability.

Lipid ROS- and Iron-Dependent Ferroptotic Cell Death in Unicellular Algae Chlamydomonas reinhardtii

The phenomenon of heat stress leading to ferroptosis-like cell death has recently been observed in bacteria as well as plant cells. Despite recent findings, the evidence of ferroptosis, an iron-dependent cell death remains unknown in microalgae. The present study aimed to investigate if heat shock could induce reactive oxygen species (ROS) and iron-dependent ferroptotic cell death in Chlamydomonas reinhardtii in comparison with RSL3-induced ferroptosis. After RSL3 and heat shock (50 °C) treatments with or without inhibitors, Chlamydomonas cells were evaluated for cell viability and the induction of ferroptotic biomarkers. Both the heat shock and RSL3 treatment were found to trigger ferroptotic cell death, with hallmarks of glutathione-ascorbic acid depletion, GPX5 downregulation, mitochondrial dysfunction, an increase in cytosolic calcium, ROS production, lipid peroxidation, and intracellular iron accumulation via heme oxygenase-1 activation (HO-1). Interestingly, the cells preincubated with ferroptosis inhibitors (ferrostatin-1 and ciclopirox) significantly reduced RSL3- and heat-induced cell death by preventing the accumulation of Fe²⁺ and lipid ROS. These findings reveal that ferroptotic cell death affects the iron homeostasis and lipid peroxidation metabolism of Chlamydomonas, indicating that cell death pathways are evolutionarily conserved among eukaryotes.

A ménage à trois: salicylic acid, growth inhibition, and immunity

Salicylic acid (SA) is a plant hormone almost exclusively associated with the promotion of immunity. It is also known that SA has a negative impact on plant growth, yet only limited efforts have been dedicated to explain this facet of SA action. In this review, we focus on SA-related reduced growth and discuss whether it is a regulated process and if the role of SA in immunity imperatively comes with growth suppression. We highlight molecular targets of SA that interfere with growth and describe scenarios where SA can improve plant immunity without a growth penalty.

Reprogramming the metabolism of Synechocystis PCC 6803 by regulating the plastoquinone biosynthesis

Cyanobacteria can utilize CO2 or even N2 to produce a variety of high value-added products efficiently. Plastoquinone (PQ) is an important electron carrier in both of the photosynthetic and respiratory electron transport chain. Although the content of PQ, as well as their redox state, have an important effect on physiology and metabolism, there are relatively few studies on the synthesis of PQ and its related metabolic regulation mechanism in photosynthetic microorganisms. In this study, the strategies of overexpression of Geranyl diphosphate: 4-hydroxybenzoate geranyltransferase (lepgt) and addition of 4-hydroxybenzoate (4-HB) as the quinone ring precursor were adopted to regulate the biosynthesis of PQ in Synechocystis PCC 6803. Combined with the analysis the photosystem activity, respiration rate and metabolic components, we found the changes of intracellular PQ reprogrammed the metabolism of Synechocystis PCC 6803. The results showed that the overexpression of lepgt reduced PQ content dramatically, by 22.18%. Interestingly, both of the photosynthesis and respiration rate were enhanced. In addition, the intracellular lipid and protein contents were significantly increased. Whereas, the addition of low concentrations of 4-HB enhanced the biosynthesis of PQ, and the intracellular PQ contents were increased by 14.76%-70.86% in different conditions. Addition of 4-HB can regulate the photosystem efficiency and respiration and reprogram the metabolism of Synechocystis PCC 6803 efficiently. In a word, regulating the PQ biosynthesis provided a novel idea for promoting the reprogramming the physiology and metabolism of Synechocystis.

Key Targets for Improving Algal Biofuel Production

A number of technological challenges need to be overcome if algae are to be utilized for commercial fuel production. Current economic assessment is largely based on laboratory scale up or commercial systems geared to the production of high value products, since no industrial scale plant exits that are dedicated to algal biofuel. For macroalgae (‘seaweeds’), the most promising processes are anaerobic digestion for biomethane production and fermentation for bioethanol, the latter with levels exceeding those from sugar cane. Currently, both processes could be enhanced by increasing the rate of degradation of the complex polysaccharide cell walls to generate fermentable sugars using specifically tailored hydrolytic enzymes. For microalgal biofuel production, open raceway ponds are more cost-effective than photobioreactors, with CO2 and harvesting/dewatering costs estimated to be ~50% and up to 15% of total costs, respectively. These costs need to be reduced by an order of magnitude if algal biodiesel is to compete with petroleum. Improved economics could be achieved by using a low-cost water supply supplemented with high glucose and nutrients from food grade industrial wastewater and using more efficient flocculation methods and CO2 from power plants. Solar radiation of not <3000 h·yr−1 favours production sites 30° north or south of the equator and should use marginal land with flat topography near oceans. Possible geographical sites are discussed. In terms of biomass conversion, advances in wet technologies such as hydrothermal liquefaction, anaerobic digestion, and transesterification for algal biodiesel are presented and how these can be integrated into a biorefinery are discussed.