Photosynthetic electron transport transients in Chlorella vulgaris under fluctuating light

Modulating cultivation regimes of Messastrum gracile SVMIICT7 for biomass productivity integrated with resource recovery via hydrothermal liquefaction

The present study was designed to assess Messastrum gracile SVMIICT7 potential in treating dairy wastewater (autoclaved (ADWW) and raw (DWW)) with relation to nutrient removal, in-vivo Chl-a-based biomass, and bio-oil synthesis. Chlorophyll a fluorescence kinetics revealed improved photochemical efficiency (0.639, Fv/Fm) in M. gracile when grown with DWW. This may be owing to enhanced electron transport being mediated by an effective water-splitting complex at photosystem (PSII) of thylakoids. The increase in ABS/RC observed in DWW can be attributed to the elevated chlorophyll content and reduced light dissipation, as evident by higher values of ETo/RC and a decrease in non-photochemical quenching (NPQ). M. gracile inoculated in DWW had the highest Chl-a-biomass yield (1.8 g L-1) and biomolecules while maximum nutrient removal efficiency was observed in ADWW (83.7% TN and 60.07% TP). M. gracile exhibited substantial bio-oil yield of 29.6% and high calorific value of 37.19 MJ kg-1, predominantly composed of hydrocarbons along with nitrogen and oxygen cyclic compounds. This research offers a thorough investigation into wastewater treatment, illustrating the conversion of algal biomass into valuable energy sources and chemical intermediates within the framework of a biorefinery.

In situ self-induced electrical stimulation to plants: Modulates morphogenesis, photosynthesis and gene expression in Vigna radiata and Cicer arietinum

Specific stimuli to plants influence intracellular and intercellular communications, activation of ion channels, gene expression, growth and development. The functional role of self-induced in situ electrical stimuli at the rhizosphere of the plant by placing electrode assembly in a defined circuit mode was studied on the growth and development of Vigna radiata and Cicer arietinum plants. Experiments were designed with three-circuit mode configurational variations (CC-P, OC-P and SC-P) and compared with the relative performance of control system (non-potential). The plants cultivated under the in situ electrical stimuli (low-current) showed a marked influence on growth and photosynthetic performance of the plants. CC-P operation showed improved vegetative growth, characterized by increased roots, shoots and biomass along with accelerated plant growth from seed germination to vegetation, flowering and pod formation leading towards earlier and more robust flowering compared to control system. Plants also showed higher aquaporin gene expression levels in CC-P operation. The control operation showed 10 days additional maturation time compared to CC-P operation. The strategy can be beneficially applied to augment the bioremediation capacity of complex pollutants with reference to phytoremediation or constructed wetland systems where the plant and its roots are the main enabler apart from agriculture applications specific to nursery-raised or transplanted plants.

Hysteresis light curves: a protocol for characterizing the time dependence of the light response of photosynthesis

Photosynthesis vs. light curves (LCs) have played a central role in photosynthesis research for decades. They are the commonest form of describing how photosynthesis responds to changes in light, being frequently used for characterizing photoacclimation. However, LCs are often interpreted exclusively regarding the response to light intensity, the effects of time of exposure not being explicitly considered. This study proposes the use of 'hysteresis light curves' (HLC), an experimental protocol focused on the cumulative effects of light exposure to obtain information on the time dependence of photosynthetic light responses. HLC are generated by exposing samples to a symmetrical sequence of increasing and decreasing light levels. The comparison of the light-increasing and the light-decreasing phases allows the quantification of the hysteresis caused by high-light exposure, the magnitude and direction of which inform on the activation, and subsequent relaxation of high-light-induced photosynthetic processes. HLCs of the chlorophyll fluorescence indices rETR (relative electron transport rate of photosystem II) and Y(NPQ) (index of non-photochemical quenching) were measured on cyanobacteria, algae, and plants, with the aim of identifying main patterns of hysteresis and their diversity. A non-parametric index is proposed to quantify the magnitude and direction of hysteresis in HLCs of rETR and Y(NPQ). The results of this study show that HLCs can provide additional relevant information on the time dependence of the light response of photosynthetic samples, not obtainable from conventional LCs, useful for phenotyping photosynthetic traits, including photoacclimation state and kinetics of light activation and relaxation of electron flow and energy dissipation processes.

Comparative appraisal of nutrient recovery, bio-crude, and bio-hydrogen production using Coelestrella sp. in a closed-loop biorefinery

A closed loop algal-biorefinery was designed based on a three-stage integration of dairy wastewater (DWW) treatment, hydrothermal liquefaction (HTL) of defatted algal biomass, and acidogenic process in a semi-synthetic framework. Initially, Coelestrella sp SVMIICT5 was grown in a 5 L photo-bioreactor and scaled up to a 50 L flat-panel photo-bioreactor using DWW. The microalgal growth showed higher photosynthetic efficiency, resulting in a biomass growth of 3.2 g/L of DCW with 87% treatment efficiency. The biomolecular composition showed 26% lipids with a good fatty acid profile (C12-C21) as well as carbohydrate (24.9%) and protein (31.8%) content. In the second stage, the de-oiled algal biomass was valorized via HTL at various temperatures (150°C, 200°, and 250°C) and reaction atmospheres (N2 and H2). Among these, the 250°C (H2) condition showed a 52% bio-crude fraction and an HHV of ∼29.47 MJ/kg (bio-oil) with a saturated hydrocarbon content of 64.3% that could be further upgraded to jet fuels. The energy recovery (73.01%) and elemental enrichment (carbon; 65.67%) were relatively greater in H2 compared to N2 conditions. Finally, dark fermentation of the complex-structured HTL-AF stream resulted in a total bio-H2 production of 231 ml/g of TOC with a 63% treatment efficiency. Life cycle analysis (LCA) was also performed for the mid-point and damage categories to assess the sustainability of the integrated process. Thus, the results of this study demonstrated comprehensive wastewater treatment and valorization of de-oiled algal biomass for chemical/fuel intermediates in the biorefinery context by low-carbon processes.

Polyhydroxybutyrate production by Chlorella sorokiniana SVMIICT8 under Nutrient-deprived mixotrophy

Polyhydroxybutyrates (PHBs) are naturally occurring biopolymeric compounds that accumulate in a variety of microorganisms, including microalgae as energy and carbon storage sources. The present study was designed to evaluate nature-based PHB production using microalgae (Chlorella sorokiniana SVMIICT8) in biphasic (growth (GP) and stress phase (SP)) nutritional mode of cultivation. Microalgal PHB accumulation was driven by nutrient constraint, with a maximal production of 29.5% of PHB from 0.94 gm L^-1 of biomass. Fluorescence microscopy revealed PHB granules in the cell cytoplasm, while NMR (¹H and ¹³C), XRD and TGA analysis confirmed the structure. The biopolymer obtained was homopolymer of PHB with carbonyl (C=O) stretch of the aliphatic ester moiety. In GC-MS analysis, major peak representing butyric acid methyl ester also confirmed the PHB. Chlorophyll a fluorescence transients inferred through OJIP, exhibited significant variation in photosynthetic process during growth and nutrient limiting conditions. Mining of bio-based products from microalgae cultivation embrace nature-based approach addressing climate change and sustainability inclusively.

Phycoremediation potential ofTetradesmus sp.SVMIICT4in treating dairy wastewaterusingFlat-Panel photobioreactor

Tetradesmus sp. SVMIICT4 was isolated and cultivated mixotrophically in a flat-panel photobioreactor (FP-PBR) for concurrent dairy wastewater treatment, carbon fixation, and biomass production. Integrated wastewater treatment showed good COD and nutrients removal efficiency accounting for biomass with an accumulation of carbohydrate (21.48 mg g^-1) and protein (19.52 mg g^-1). Chlorophyll a fluorescence transients (Fv/Fm, ETo/RC, TRo/RC, and Abs/RC) deduced through OJIP curve fittings, showed consistent improvement in photosynthetic activity throughout the cultivation period. The absorption flux per reaction centre corroborated with increased chlorophyll content (18.94 mg g^-1), resulting in higher electron transport (ET/Rc) and lower non-photochemical quenching (NPQ). The fatty acid profile showed high content of unsaturated, followed by saturated fatty acids, which has multiple applications in food, feed, and fuel industries, enabling a bio-based economy.

Mixotrophic cultivation of isolated Messastrum gracile SVMIICT7: Photosynthetic response and product profiling

The isolated Messastrum gracile SVMIICT7 was mixotrophically cultivated in flat panel photobioreactor (FP-PBR) towards understanding the photosynthetic transient and product profile. Biomass productivity attained a maximum of 45 mg L^-1d^-1, with COD, nitrate and phosphate removal of 83.3%, 84.05%, and 74.98% respectively. Messastrum sp. showed good assimilation of proteins (124 mg g^-1) (w/w), carbohydrates (119 mg g^-1) (w/w) and lipids (26%) (w/w). The myristoleic acid (C14:1-39.1%) and heptadecanoic acid (C17:0-29.1%) are abundant fatty acids with therapeutic, food and feed applications. The cellular ultrastructure studies revealed facile arrangement of chloroplast and starch covered pyrenoids supporting increased carbohydrate accumulation. Photosystem II (PSII) [Y(II), ETR(II), Y(NPQ), and Y(NO)] and photosystem I (PSI) [Y(I), ETR(I), Y(NA), and Y(ND)] transients showed improved photosynthetic efficiency directing microalgae growth and biomass productivity. Higher Fv/Fm values indicates relatively good water splitting and carbon fixation at PSII and PSI facilitating improved photosynthetic electron transport and synthesis of value-added products thereby enabling bioeconomy.

Investigation of the photosynthetic response of Chlorella vulgaris to light changes in a torus-shape photobioreactor

An efficient use of light is essential to achieve good performances in microalgae cultivation systems. This can be challenging particularly under solar conditions where light is highly dynamic (e.g., day/night cycles, rapid changes in wind and weather conditions). Microalgae display different mechanisms to optimize light use efficiency. In the short term, when high light is encountered, several processes of photoprotection can be involved to avoid cell damages (e.g., xanthophyll cycle). In the long term, when cells are exposed to a different light intensity, pigment content changes, i.e., photoacclimation. The purpose of this study is to investigate the photosynthetic response of Chlorella vulgaris cultures grown in closed lab-scale, torus-shape photobioreactor under well-controlled light conditions, namely, constant and dynamic light transitions. Experiments were conducted in continuous mode with detailed characterization of the light attenuation conditions for each condition, as represented by the mean rate of photon absorption (MRPA), so as to characterize the time responses of the photosynthetic cells toward light changes. This enables to observe short-term and long-term responses with their own characteristic times. The mechanisms involved were found to be different between increasing and decreasing light transitions. Furthermore the MRPA was found a valuable parameter to relate the effect of light to biological responses (i.e., pigment changes) under constant light and dynamic light conditions.Key points• MRPA proved valuable to relate C. vulgaris responses to light changes.• A linear evolution was found between pigment content and MRPA in continuous light.• A rising PFD step induced fast protection and acclimation mechanisms.

Photosynthetic transients in Chlorella sorokiniana during phycoremediation of dairy wastewater under distinct light intensities

The present study is aimed to understand the photosynthetic transients of Chlorella sorokiniana SVMBIOEN2 during treatment of dairy wastewater under different light intensities (100, 150, and 200 µmol m^-2s^-1) in mixotrophic mode. Light intensities showed marked influence on photosystem behavior, lipid profile, and organic pollutant removal. Analysis of Chlorophyll a fluorescence transient including Fv/Fm, ETo/RC, TRo/RC, and Abs/RC showed better photosystem efficiency at 100 µmol m^-2s^-1 operations. OJIP curve fitting depicted a positive L-band at 150 µmol m^-2s^-1 indicating lower kinetic energy of photosystem II (PSII) reaction centres at high light intensities. Better photosynthetic activity at 100 µmol m^-2s^-1 operations resulted in good assimilation of biomass (2.3 g L^-1), carbohydrates (10.2 mg g^-1), and proteins (14 mg g^-1) with a significant reduction in chemical oxygen demand (85%). Phycoremediation of dairy wastewater accumulates predominantly monounsaturated fatty acids followed by polyunsaturated fatty acids showing the application of C. sorokiniana in nutraceutical and food industries.