Acutodesmus Obliquus Research Articles

Response and the detoxification strategies of green alga Acutodesmus obliquus (Chlorophyceae) under lead stress

The responses of Acutodesmus obliquus (Chlorophyceae) cells to the presence of lead (Pb2+) at the range of concentrations 0.01–500μM during 7days of culture were studied. The growth rate was clearly reduced and the changes in the level of low molecular weight metabolites, particularly involved in heavy metal chelation and glutathione synthesis pathway under Pb2+ stress, were observed. External metal induced the accumulation of phytochelatins (PC) precursors (cysteine, glutathione) and stimulated the activity of phytochelatins synthase leading directly to enhanced production of PC2-5 responsible for binding and detoxification of Pb2+. The enhancement of xanthophyll cycle indicates its significant role in algal adaptation to adverse environmental conditions. Another defense mechanisms against metal stress is the disturbances in hormonal homeostasis, as indicated by the increase in the level of abscisic acid with simultaneous decreases in the contents of indole-3-acetic acid and trans-zeatin, representatives of auxins and cytokinins, respectively. Results suggest that the green alga A. obliquus has evolved a complex network to maintain toxic concentrations of Pb2+ within physiological limits and to minimize the harmful effects of this nonessential metal.

Evaluation of the dynamics of microalgae population structure and process performance during piggery wastewater treatment in algal-bacterial photobioreactors

The dynamics of microalgae population during piggery wastewater (PWW) treatment in four open photobioreactors operated at 27days of hydraulic retention time, and inoculated with Chlorella sp. (R1), Acutodesmus obliquus (R2), Oscillatoria sp. (R3) and in the absence of inoculum (R4), were evaluated for 6months. In addition, the algal-bacterial biomass concentration, removal of organic matter, nutrients and heavy metals were also assessed. The results revealed a high diversity and rapid variations in the structure of microalgae populations, Chlorella sp. being dominant in R4 throughout most of the operational period. Steady state average biomass concentration ranged from 2445–2610mg/L in R1–R3 to 3265mg/L in R4. No significant differences were recorded in the removal efficiencies (REs) of total organic carbon (86–87%), inorganic carbon (62–71%), total nitrogen (82–85%) and total phosphorous (90–92%). Finally, Zn-REs accounted for 26% in R3, 37% in R2, and 49% in R1 and R4.

Enhanced biohydrogen production from microalgae by diesel engine hazardous emissions fixation

Microalgae cultivation has gained increased attention from research and industry sectors in recent years, due to the wide variety of applications for the produced biomass, such as biofuels and substances of high economic value. Indirect biophotolysis biohydrogen production from microalgae has been shown recently to be limited by the amount of accumulated biomass during the growth phase. As a result, this study focused on developing a strategy to increase biohydrogen generation via biomass production increase through microalgae cultivation using exhaust gases from diesel engines. In order to achieve that objective, four simultaneous cultures were conducted to compare the growth of microalgae under pure air and emissions injection, in different flow regimes. An indigenous microalgae strain was selected to be robust under different weather conditions and was identified as Acutodesmus obliquus through rDNA sequence analysis. The results indicate an increase in biomass production of about 2.8 times for the best case of cultivation with emissions in comparison to a compressed air condition. Besides the growth analyses, the potential for treating the hazardous emissions injected into the system was investigated and the data demonstrated that the CO2 and NOx content was substantially reduced, showing that no damage to the microalgae is caused by the diesel engine emissions. Numerical simulation results for the H2 production indirect biophotolysis demonstrate that there is an optimal rhythm for maximum time averaged H2 production rate, and that the stoichiometrically limited total H2 production is augmented by a similar factor to microalgae biomass production increase.

The role of starch as transient energy buffer in synchronized microalgal growth in Acutodesmus obliquus

Photosynthetic organisms have evolved to use light efficiently by scheduling their cellular processes, such as growth and cell division, at specific times of the day. During the day, fixed carbon is used for growth and is partially stored as carbohydrates (e.g., starch). It is commonly assumed that this accumulated starch is essential for fuelling up cell division at night. To test this hypothesis, this study investigates growth, cell division and presence of a transitory energy storage (TES) in both the wild-type and starchless mutant strain of Acutodesmus obliquus under light/dark (LD) cycles and nitrogen replete conditions.A. obliquus (formerly known as Scenedesmus obliquus) wild-type utilized light 20% more efficiently under LD regimes compared with continuous light. When exposed to LD regimes, the wild-type scheduled cell division in a 4-hour period starting 2h before ‘sunset’. Starch acted as the major transitory energy storage (TES) compound: it was accumulated during the last part of the light period and was consumed throughout the entire dark period. The slm1 mutant, with a blocked starch synthesis pathway, showed diurnal rhythms in growth and cell division. However, no other carbohydrates nor triacylglycerols took over the role of TES compound in slm1. Therefore, in contrast to what is generally acknowledged, this study shows that neither starch nor any other major alternative TES is required for synchronized growth and cell division in A. obliquus. The starchless mutant did show a reduced growth and cell division rate compared to the wild-type. Starch, thus, plays a major role in efficient harnessing of light energy over LD cycles, likely because the ability to accumulate starch enhances biomass production capacity and accelerates cell division rate in A. obliquus.

Enhanced growth and total fatty acid production of microalgae under various lighting conditions induced by flashing light.

Microalgae are gaining importance as a source of high-value bioproducts. However, data regarding optimization of algal productivity via variation of environmental factors are lacking. Here, we evaluated a novel lighting method for the enhancement of biomass and total fatty acid (TFA) productivities during algal cultivation. We cultivated six different algal strains (Chlorella vulgaris KCTC AG10002, Acutodesmus obliquus KGE18, Uronema sp. KGE03, Micractinium reisseri KGE19, Fragilaria sp., and Spirogyra sp.) under various lighting conditions-continuous light (CL), light-dark cycle (LD), and continuous dark (CD)-with or without additional flashing light. We monitored dry cell weight (DCW) and TFA concentrations during cultivation. For each algal strain, the growth rate showed markedly different responses to the various lighting modes. The growth rates of C. vulgaris KCTC AG10002 (1.34-fold DCW increase, LD with flash), A. obliquus KGE18 (5.16-fold DCW increase, LD with flash), Uronema sp. KGE03 (2.77-fold DCW increase, CL with flash), and M. reisseri KGE19 (1.52-fold DCW increase, CL with flash) markedly increased in response to flashing light. Additionally, in some algal strains cultivated under the LD mode, the flashing light treatment induced increased TFA concentrations (C. vulgaris, 1.19-fold increase; A. obliquus, 2.59-fold increase; and M. reisseri, 3.31-fold increase). Phytohormone analysis of M. reisseri revealed increases in growth rate and TFA concentrations, associated with phytohormone induction via flashing light (e.g. 2.93-fold increase in gibberellic acid); hence, flashing light can promote substantial alterations in algal metabolism.

Effect of salinity stress on growth, lipid productivity, fatty acid composition, and biodiesel properties in Acutodesmus obliquus and Chlorella vulgaris.

Two microalgae strains including Chlorella vulgaris and Acutodesmus obliquus were grown on BG11 medium with salinity stress ranging from 0.06 to 0.4M NaCl. Highest lipid content in C. vulgaris and A. obliquus was 49 and 43% in BG11 amended with 0.4M NaCl. The microalgal strains C. vulgaris and A. obliquus grow better at 0.06M NaCl concentration than control condition. At 0.06M NaCl, improved dry biomass content in C. vulgaris and A. obliquus was 0.92 and 0.68gL-1, respectively. Stress biomarkers like reactive oxygen species, antioxidant enzyme catalase, and ascorbate peroxidase were also lowest at 0.06M NaCl concentration revealing that both the microalgal strains are well acclimatized at 0.06M NaCl concentration. The fatty acid composition of the investigated microalgal strains was also improved by increased NaCl concentration. At 0.4M NaCl, palmitic acid (37%), oleic acid (15.5%), and linoleic acid (20%) were the dominant fatty acids in C. vulgaris while palmitic acid (54%) and stearic acid (26.6%) were major fatty acids found in A. obliquus. Fatty acid profiling of C. vulgaris and A. obliquus significantly varied with salinity concentration. Therefore, the study showed that salt stress is an effective stress that could increase not only the lipid content but also improved the fatty acid composition which could make C. vulgaris and A. obliquus potential strains for biodiesel production.

Evaluating the potential of cytokinins for biomass and lipid enhancement in microalga Acutodesmus obliquus under nitrogen stress

Despite the significant reduction in biomass production, nitrogen stress is the most feasible strategy used for lipid enhancement in microalgae. The present study evaluates the potential for application of exogenous cytokinins to enhance the lipid accumulation and overcome the biomass production constraint of microalgae under nitrogen stressed conditions. This study was undertaken to investigate the effect of exogenously supplied cytokinins viz. kinetin and zeatin at various stages of growth on biomass and lipid productivities of Acutodesmus obliquus under nitrogen stress (ON). The effect of cytokinins on photosynthetic performance and biochemical composition of biomass was also evaluated. Enhanced biomass and lipid productivities were observed upon supplementation of the microalgae with selected cytokinins. Supplementation with kinetin (1mgL−1) and zeatin (0.1mgL−1) resulted in increased biomass productivity up to 50% and 60.7% respectively as compared to ON control. The highest biomass productivity of 176.79mgL−1 d−1 was obtained with the addition of 0.1mgL−1 zeatin (ON-Zi) at the initial log phase. Cytokinin supplementation resulted in improvement in photosynthetic performance and exhibited higher rETR (relative electron transport rate) values as compared to ON control. The addition of kinetin (ON-Km) at mid log phase and Zeatin (ON-Zi) at initial log phase resulted in 64.95% and 63.06% increase in lipid productivity respectively, compared to ON control. Carbohydrate productivity was also found to be increased with zeatin (ON-Zi) supplementation. The supplementation of cytokinins is an easy and scalable strategy for biomass and lipid enhancement in Acutodesmus obliquus under nitrogen stress.

Isolation and Characterization of Native Microalgae from the Peruvian Amazon with Potential for Biodiesel Production

Biodiesel production from microalgae triacylglycerols is growing, because this feedstock is a more sustainable and advantageous alternative. In this study, we isolated and identified fourteen strains of native microalgae from the Peruvian Amazon. These strains showed great heterogeneity in biomass productivity, lipid productivity and lipid content, and thus, three of them (Acutodesmus obliquus, Ankistrodesmus sp. and Chlorella lewinii) were selected for further evaluation under culture of nitrogen-sufficient (+N) and nitrogen-deficient (−N) Chu medium No. 10. These microalgae species showed modifications in biomolecule content (protein, lipid and carbohydrate) with a pronounced increase of lipids and carbohydrate and a decrease of protein content under stress culture. Furthermore, the fatty acid profile was peculiar for each species, and these patterns showed evident changes, particularly in the proportion of saturated and monounsaturated fatty acids. The results of this research suggest that the isolated native microalgae, from the Peruvian Amazon, could be suitable candidates for biodiesel production

Optimization of flocculation with tannin-based flocculant in the water reuse and lipidic production for the cultivation of Acutodesmus obliquus

ABSTRACTThis paper evaluated the optimization of natural flocculant in different conditions in a tubular photobioreactor using swine wastewater effluent as culture media in a pilot-scale microalgae production plant. This study aimed to assess the flocculation efficiency of Tanfloc SG via a central composite design (CCD), with varying Tanfloc SG concentration, biomass concentration and pH. Subsequently, the microalga Acutodesmus obliquus was cultivated in medium obtained from the recycled recovery of microalgae cells via flocculation with Tanfloc SG. Results showed that the recycled medium after flocculation with Tanfloc SG had a positive effect on the biomass and lipid production of A. obliquus.

Microalgal post-treatment of anaerobically digested agro-industrial wastes for nutrient removal and lipids production

The aim of this study was to investigate the effectiveness of cultivating Parachlorella kessleri and Acutodesmus obliquus, in anaerobic digestion effluent (ADE) derived from the co-digestion of end-of-life dairy products with mixtures of agro-industrial wastes. To this end, their performance under sterile and non-sterile conditions and different ADE loadings was evaluated, in terms of biomass and lipid production, nutrient removal efficiency and vitality of the photosynthetic apparatus. 10% (v/v) ADE loading inhibited growth over 9–12days of cultivation, however biomass yields of 1.1 and 1gL−1, 22.7% and 19.5% (w/w) fatty acids concentration, as well as NH3-N assimilation of 49.7mgL−1 and 32.3mgL−1 and TP removal of 84.2% and 84% were recorded for P. kessleri and A. obliquus, respectively. Among all the ADE-based treatments tested, P. kessleri outperformed A. obliquus, with no differences observed between sterilized and non-sterilized ADE.

Optimization and mass culture of Acutodesmus obliquus RDS01 under open phototrophic pond cultivation for enhancing biodiesel production

ABSTRACTIn the optimization of medium for open phototrophic pond cultivation of microalgae, Acutodesmus obliquus RDS01, response surface methodology (RSM) was used and lipid, biomass and biodiesel production were analyzed. The experimental investigation showed that a good achievement and regression coefficient was determined highly (R2 = 0.999) in analysis of variance (ANOVA). The effect of six different macro-elements (NaNO3, KH2PO4, K2HPO4, MgSO4, CaCl2 and NaCl) on growth of Acutodesmus obliquus RDS01 was premeditated in bold basal medium (BBM). The absolute concentration of lipid was 220.07 mg mL−1, biomass 14.49 g L−1 and biodiesel 3.9 mL g−1. The results achieved in phototrophic open pond cultivation using optimized medium were approximately 2 times higher than in the control medium. Fourier transform infrared (FTIR) analysis confirmed the presence of a lipid profile functional group similar to biodiesel and major fatty acid methyl ester (FAME) profiling viz., tetradecanoic acid (C14:0), hexadecanoic acid (C16:0), octadecanoic acid (C18:0), (Z)-Octadec-9-enoic acid (C18:1), linoleic acid (C18:2), alpha-linolenic acid (C18:3), cis-11-eicosenoic acid (C20:5), 13-cis-docosenoic acid (C22:6) on gas chromatography and mass spectrometric (GC-MS) analysis. A high productivity of lipid, biomass and biodiesel from isolated freshwater microalgae was found.

Associated effects of storage and mechanical pre-treatments of microalgae biomass on biomethane yields in anaerobic digestion

The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH4) yields during anaerobic digestion. This study investigated the combined effects of two different biomass storage methods and physical pre-treatments on the anaerobic digestion for three different microalgae species. Acutodesmus obliquus, Chlorella vulgaris and Chlorella emersonii were cultivated in 80 L sleevebag photobioreactors (batch mode), and then subjected to different storage (cooling and freezing) and pre-treatment methods prior to anaerobic digestion using the biochemical methane potential (BMP) test. A. obliquus was selected to evaluate pre-treatment methods for further experimentation. Significantly higher CH4 yields of cooled (4 °C) A. obliquus biomass were achieved through ultrasonication (+53% CH4) and wet-milling (+51% CH4). These methods were then applied in follow-up experiments to cooled (4 °C) biomass of C. emersonii and A. obliquus. Ultrasonication again led to significantly higher CH4 yields for A. obliquus biomass (323 dm3 kg−1 CH4 yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH4), and C. emersonii biomass (308 dm3 kg−1; +35% CH4). In a third experiment series, frozen A. obliquus and C. vulgaris biomass were thawed prior to pre-treatment and BMP-testing. Among all BMP tests, the highest CH4 yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm3 kg−1); pre-treatment did not enhance CH4 yields for C. vulgaris, but for A. obliquus (ultrasonication +20%). Pre-treatment was more effective for cooled than freeze-thawed microalgal biomass and combined effects acted strain dependently.

Microalga, Acutodesmus obliquus KGE 30 as a potential candidate for CO2 mitigation and biodiesel production.

In this study, the effect of flue gas CO2 on growth, lipid production, and fatty acid composition of a green microalga Acutodesmus obliquus KGE 30 was investigated. The highest growth rate (0.46gL(-1) and μmax = 1.09day(-1)), total inorganic carbon removal (95.9mgL(-1)), and lipid productivity (20.1mgL(-1) day L(-1)) was obtained at 14.1% CO2 after 4days of cultivation. In a semicontinuous batch reactor, the highest biomass production (1.19gL(-1)) was achieved after 12days with continuous injection of flue gas CO2. Compared with synthetic CO2, fatty acid methyl ester analysis showed that the amount of unsaturated fatty acid increased by 19.2% with 14.1% flue gas CO2. The application of flue gas CO2 improved biomass production and lipid productivity in A. obliquus. The current investigation demonstrated that the use of flue gas CO2 could reduce the cost of microalgae biomass production for better biofuel generation.

Two-stage cultivation of N-rich and N-deprived Acutodesmus obliquus biomass: Influence of cultivation and dewatering methods on microalgal biomass used in anaerobic digestion

Microalgae can be manipulated to accumulate certain cellular compounds of interest without the need for genetic modification - simply by controlling growth parameters such as nitrogen (N). Therefore, A. obliquus was batch-cultivated in a sleevebag photobioreactor system in N-deprived and N-rich medium to test the effect of N-status on CH4 yield under anaerobic digestion. Two different dewatering methods, i.e., centrifugation and sedimentation were applied to each resulting biomass. For the N-deprived biomass, cellular protein content dropped by 42–49%, and lipids increased up to 20% relative to the N-rich biomass. The highest CH4 yields were achieved with N-deprived, sedimented biomass (391Nm3t−1 VS=normalized gas volume in m3 corrected to norm temperature and pressure per unit volatile solids), followed by N-rich sedimented biomass (361Nm3t−1 VS). Centrifugation led to lower CH4 yields, where N-deprived biomass achieved 280 compared to 200Nm3t−1 VS for N-rich microalgae. Our data indicate that valuable organic material was lost to the supernatant during the centrifugation step. We conclude that not only the N-status of cultivation, but also the biomass dewatering method has an instrumental effect on CH4 yield of microalgal biomass in anaerobic digestion.

Characterization and classification of highly productive microalgae strains discovered for biofuel and bioproduct generation

This paper describes the characteristics of microalgal strains that originated out of an isolation and screening project included within the National Alliance for Advanced Biofuels and Bioproducts (NAABB). The project's goal was to identify new potential platform strains with high growth rates and/or lipid productivities. To classify the best performing strains, we conducted a combined microscopic and phylogenetic analysis. Among the best performing strains were many coccoid green algae. Several strains belong to the species Acutodesmus (Scenedesmus) obliquus and to the species Chlorella sorokiniana, thus expanding on existing germplasm. Identified at the genus level were some Desmodesmus strains and one Ankistrodesmus strain. Several strains were classified as belonging to the genus Coelastrella, a taxon reported for the first time for North America. Multiple additional strains had ambiguous identities, with some strains possibly representing novel species. Reporting on the above strains, some of which have been tested successfully in outdoor ponds and most of which are deposited at the University of Texas Culture Collection of Algae, is a step forward in expanding the biological resources available for algae biofuel production.

Utilization of gaseous emissions from a methanol plant for cultivation of Acutodesmus obliquus

This study aimed to culture the green alga Acutodesmus obliquus utilizing the gaseous emissions containing a high concentration of CO2 (99.13 %) from a methanol plant and study the tolerance of microalgae. The effect of CO2 concentration, aeration rate, inoculum concentration, intermittent sparging, and nitrogen sources on the growth of A. obliquus was examined. Acutodesmus obliquus also was cultivated in a 500-L pilot outdoor tubular photobioreactor (OTP) to advance the laboratory scale system to outdoor scale-up applications. The results showed that A. obliquus could tolerate high CO2 concentrations of 50 %, and a maximum biomass of 0.935 g L−1 (dry weight) was achieved at 20 % CO2. An aeration rate of 500 mL min−1, inoculum concentration (optical density at 680 nm [OD680] = 0.3), and intermittent sparging of 10 min per 2 h enhanced growth to the optimum and influenced culture pH and photosynthesis. Urea as a nitrogen source was shown to be more beneficial to cell growth. A urea concentration of 0.3 g L−1 and an N/P ratio of 15 led to maximum biomass accumulation thus enhancing the gaseous emission utilization efficiency. In conclusion, this work demonstrated that gaseous emissions containing high concentration of CO2 from a methanol plant could be directly introduced into A. obliquus cultures and that A. obliquus was suitable well for large-scale outdoor cultivation in a tubular photobiorecator.

Photosynthetic sensitivity of phytoplankton to commonly used pharmaceuticals and its dependence on cellular phosphorus status.

Recently pharmaceuticals have become significant environmental pollutants in aquatic ecosystems, that could affect primary producers such as microalgae. Here we analyzed the effect of pharmaceuticals on the photosynthesis of microalgae commonly found in freshwater-two species of Chlorophyceae and a member of the Eustigmatophyceae, via PAM fluorometry. As pharmaceuticals, three medicines often consumed in households were chosen: (i) fluoxetine, an antidepressant, (ii) propranolol, a β-blocker and (iii) ibuprofen, an anti-inflammatory and analgesic medicine. The EC50 for the quantum yield of photosystem II in phytoplankton acclimated to inorganic phosphorus (Pi)-replete and Pi-limited conditions was estimated. Acute toxicity experiments over a 5h exposure revealed that Nannochloropsis limnetica was the least sensitive to pharmaceuticals in its photosynthetic yield out of all species tested. Although the estimation of sub-lethal effects can be vital in contrast to that of LC50s, the EC50 values in all species and for all medicines were orders of magnitude higher than concentrations found in polluted surface water. Chlamydomonas reinhardtii was the most sensitive to fluoxetine (EC50 of 1.6mgL(-1)), and propranolol (EC50 of 3mgL(-1)). Acutodesmus obliquus was most sensitive to ibuprofen (EC50 of 288mgL(-1)). Additionally, the sensitivity to the pharmaceuticals changed under a Pi-limitation; the green algae became less sensitive to fluoxetine and propranolol. In contrast, Pi-limited algal species were more sensitive to ibuprofen. Our results suggest that the sensitivity of algae to pharmaceuticals is (i) highly compound- and species-specific and (ii) dependent on the cellular P status.

Treating high-strength saline piggery wastewater using the heterotrophic cultivation of Acutodesmus obliquus

This study combined an algae treatment with post-ozonation for the robust treatment of piggery effluent in which high levels of organic constituents, inorganic nutrients, color, and salts remained. Due to a nearly complete light limitation resulting from a high level of color, the algae treatment was conducted with continuous O2 supplementation instead of using the combination of high lighting and CO2 injection. The microalga Acutodesmus obliquus KGE-17 showed tolerance to high salinity (up to 5.2% as chloride) and was capable of utilizing dissolved organics (1923mg-COD/(g-cell)(day)) under the heterotrophic growth conditions. The use of A. obliquus for remediation of piggery effluent also resulted in an operational simplicity in the simultaneous removal of nitrogen and phosphorus in a single treatment process, in contrast to conventional biological nutrient removal processes. Subsequent ozonation was successful in decolorizing the algae-treated wastewater and in improving the biodegradability of residual organics in post-treatment processes.

Unique mitochondrial genome structure of the green algal strain YC001 (Sphaeropleales, Chlorophyta), with morphological observations

:We characterized the complete sequence of the mitochondrial genome of the freshwater green algal strain YC001, known as ‘Ettlia sp.’, which is of great interest for biofuel production. The size of the mitogenome was 52,489 base pairs (bp), with 49.0% GC content. It consisted of 49 mitochondrial genes typical of the Chlorophyceae, including 16 protein-coding genes, 27 transfer RNA genes and six ribosomal RNA genes as well as three introns within cox1 (6832 bp), rnl4 (1805 bp) and cob (1107 bp). We determined that YC001 is a species of Coelastrella on the basis of molecular phylogenetic analyses and microscopic observations. A unique feature of this mitogenome is the presence of three different types of inverted repeats between trnL and trnI and between nad4 and cox1 as well as the presence of orf215, orf755 and one additional copy of the trnM gene, all of which enable a comparison with an available sister taxon, Acutodesmus obliquus.

Responses of photosystems I and II of Acutodesmus obliquus to chemical stress caused by the use of recycled nutrients.

Nutrients derived from hydrothermal gasification of Acutodesmus obliquus were tested on its biological compatibility to support growth of the same microalgae. Photosynthetic parameters of photosystems I and II (PS I and PS II) were investigated to study physiological effects on the microalgal cell. The nutrients were collected as liquid residues. Dilutions of 1:500 showed no effect on both photosystems. Lower dilutions affected PS II initially and later also PS I. Cyclic electron flow around PS I compensated for loss of electrons due to partially inhibited PS II. The highest tested concentration of liquid residue erased any photosynthetic activity of PS II after 28min and onwards. In contrast, PS I remained active. The results suggest that PS I is less susceptible than PS II and that the mixture of chemicals in the liquid residue did not directly affect PS I but PS II. The toxicants in the residues seemed to interfere with linear electron flow of PS II even though light-driven formation of radicals and subsequent damage to one of the photosystems can be excluded as demonstrated in darkness. Lowered photosynthetic activity of PS I during actinic irradiation was caused due to lack of supply of electrons from PS II. The cyclic electron flow might play a key role in delivering the energy needed to restore PS II activity and to biodegrade the toxicants when linear electron flow failed. These negative effects of liquid residue towards microalgal cells require a remediation step for direct application of the liquid residue to substitute commercial fertilizers in microalgal mass cultures.