The inhibition effect of recycled Scenedesmus acuminatus culture media: Influence of growth phase, inhibitor identification and removal

Recovery of nutrients from fish sludge to enhance the growth of microalga Chlorella sorokiniana CMBB276

Intensive aquaculture production generates large amounts of sludge. This waste could be considered as a potential source of nutrients that can be recovered and utilized. Little attention has been paid to nutrient recovery from fish sludge. In this study, bioconversion of sludge was evaluated in lab scale under anaerobic (AN), facultative anaerobic (FA) and aerobic (AE) conditions. After 40 days of fermentation, AN recovered the highest values of dissolved total nitrogen (82.7 mg L-1), while AE showed the highest dissolved total phosphorus (11.8 mg L-1) and the highest reduction of total suspended solids (36.0 %). Microbial analysis showed that AN exhibited a distinct bacterial community than that of FA and AE. Furthermore, C. sorokiniana grown in AN effluents collected after 12 days of fermentation achieved the highest biomass production (1.96 g L-1). These results suggest that AN has the best potential to recover nutrients from sludge for production of C. sorokiniana.

Advanced treatment and Resource recovery of brewery wastewater by Co-cultivation of filamentous microalga Tribonema aequale and autochthonous Bacteria

To use unicellular microalgae to remove waste nutrients from brewery wastewater while converting them into algal biomass has been explored but high-cost treatment and low-value biomass associated with current technologies have prevented this concept from further attempts. In this study, a filamentous microalga Tribonema aequale was introduced and the alga can grow vigorously in brewery wastewater and algal biomass concentration could be as high as 6.45 g L-1 which can be harvested by a cost-effective filtration method. The alga together with autochthonous bacteria removed majority of waste nutrients from brewery wastewater. Specifically, 85.39% total organic carbon (TOC), 79.53% total dissolved nitrogen (TN), 93.38% ammonia nitrogen (NH3-N) and 71.33% total dissolved phosphorus (TP) in brewery wastewater were rapidly removed by co-cultivation of T. aequale and autochthonous bacteria. Treated wastewater met the national wastewater discharge quality, and resulting algal biomass contained large amounts of high-value products chrysolaminarin, palmitoleic acid (PLA) and eicosapentaenoic acid (EPA). It is anticipated that reduced cost of algal harvesting coupled with value-added biomass could make T. aequale as a promising candidate for brewery wastewater treatment and resource utilization.

A novel fungal-algal coupling system for slaughterhouse wastewater treatment and lipid production

In this study, a novel fungal-algal coupling system was established for slaughterhouse wastewater treatment with Chlorella sp. DT025 and a new fungus, Penicillium sp. AHP141. With the optimization of cultivation conditions for the fungal-algal coupling system, the harvest efficiency of Chlorella sp. DT025 reached 99.79%. The mechanism of microalgae harvest of the fungal-algal system was revealed to be related to the morphological characteristics, surface charge, and the secretion of humic acid-like compounds and tryptophan on the surface of the fungi cells. For the original slaughterhouse wastewater treatment, the fungal-algal coupling system had a better removal efficiency of COD, TN, and TP than both monoculture systems. In the high-concentration artificial slaughterhouse wastewater, COD removal of the fungal-algal system reached more than 5350 mg/L. The lipid production of the fungal-algal coupling system in the high-concentration artificial slaughterhouse wastewater treatment was improved by 343.33% to 1.33 g/L compared to the microalgae monoculture treatment.

Microalgae biomass concentration and reuse of water as new cultivation medium using ceramic membrane filtration

The aim of this study is to advance means for microalgae dewatering with the simultaneous reuse of water as new cultivation medium, specifically through ceramic membrane filtration. Three algae, namely, Spirulina platensis, Scenedesmus obliquus, and Chlorella sorokiniana were tested by filtering suspensions with four ceramic membranes having nominal pore sizes of 0.8 μm, 0.14 μm, 300 kDa, 15 kDa. The observed flux values and organic matter removal rates were related to the membrane pore size and cake layer properties, with some differences in productivity between algae types, likely due to cell size and shape. Interestingly, similar near steady-state fluxes (70-120 L m^-2h^-1) were measured using membranes with nominal pore size above 15 kDa, suggesting the dominance of cake layer filtration independently of the initial flux. Virtually complete algae cells rejections and high nutrient passage (>75%) were observed in all combinations. When the permeate streams were used as media for new growth cycles of the various algae, no or little growth was observed with Spirulina p., while Chlorella s. (permeate from 300 kDa membrane) and especially Scenedesmus o. (permeate from 0.14 μm membrane) showed the fastest growth rates, almost comparable to those observed with ideal fresh media.

Microalgal photoautotrophic growth induces pH decrease in the aquatic environment by acidic metabolites secretion

BACKGROUND

Microalgae can absorb CO₂ during photosynthesis, which causes the aquatic environmental pH to rise. However, the pH is reduced when microalga Euglena gracilis (EG) is cultivated under photoautotrophic conditions. The mechanism behind this unique phenomenon is not yet elucidated.

RESULTS

The present study evaluated the growth of EG, compared to Chlorella vulgaris (CV), as the control group; analyzed the dissolved organic matter (DOM) in the aquatic environment; finally revealed the mechanism of the decrease in the aquatic environmental pH via comparative metabolomics analysis. Although the CV cell density was 28.3-fold that of EG, the secreted-DOM content from EG cell was 49.8-fold that of CV (p-value < 0.001). The main component of EG's DOM was rich in humic acids, which contained more DOM composed of chemical bonds such as N-H, O-H, C-H, C=O, C-O-C, and C-OH than that of CV. Essentially, the 24 candidate biomarkers metabolites secreted by EG into the aquatic environment were acidic substances, mainly lipids and lipid-like molecules, organoheterocyclic compounds, organic acids, and derivatives. Moreover, six potential critical secreted-metabolic pathways were identified.

CONCLUSIONS

This study demonstrated that EG secreted acidic metabolites, resulting in decreased aquatic environmental pH. This study provides novel insights into a new understanding of the ecological niche of EG and the rule of pH change in the microalgae aquatic environment.

Treatment and recycle of harvested microalgal effluent using powdered activated carbon for reducing water footprint and enhancing biofuel production under a biorefinery model

In this study, the suitability of cultivating Monoraphidium sp. KMC4 was exhibited in different effluent based culture (EBC) media concentrations, the latter being treated with powdered activated carbon (PAC) with a loading of 5-50 mg L^-1. The optimum EBC media treated with 30 mg L^-1 PAC enhanced the biomass yield by 21.9% as compared to the untreated one (1.21 g L^-1). A recyclability study performed in five batches resulted in an optimal growth up to three batches with an overall biomass yield of 4.21 g and a total water savings of 30%. Additionally, physico-chemical characterization and FAME profile of the biomass from the recyclability study validated feedstock's energy potential. Moreover, this study proposes a biorefinery model which could recover nutrient rich liquid effluent (3.1 million litres) and solid residue for various applications along with the generation of 5760 kg of biomass followed by 113 L d^-1 biodiesel yield.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

Rewiring the Metabolic Network to Increase Docosahexaenoic Acid Productivity in Crypthecodinium cohnii by Fermentation Supernatant-Based Adaptive Laboratory Evolution

Docosahexaenoic acid (DHA, 22:6n-3) plays significant roles in enhancing human health and preventing human diseases. The heterotrophic marine dinoflagellate Crypthecodinium cohnii is a good candidate to produce high-quality DHA. To overcome the inhibition caused by the fermentation supernatant in the late fermentation stage of DHA-producing C. cohnii, fermentation supernatant-based adaptive laboratory evolution (FS-ALE) was conducted. The cell growth and DHA productivity of the evolved strain (FS280) obtained after 280 adaptive cycles corresponding to 840 days of evolution were increased by 161.87 and 311.23%, respectively, at 72 h under stress conditions and increased by 19.87 and 51.79% without any stress compared with the starting strain, demonstrating the effectiveness of FS-ALE. In addition, a comparative proteomic analysis identified 11,106 proteins and 910 differentially expressed proteins, including six stress-responsive proteins, as well as the up- and downregulated pathways in FS280 that might contribute to its improved cell growth and DHA accumulation. Our study demonstrated that FS-ALE could be a valuable solution to relieve the inhibition of the fermentation supernatant at the late stage of normal fermentation of heterotrophic microalgae.

Maximizing Energy Content and CO2 Bio-fixation Efficiency of an Indigenous Isolated Microalga Parachlorella kessleri HY-6 Through Nutrient Optimization and Water Recycling During Cultivation

An alternative source of energy and materials with low negative environmental impacts is essential for a sustainable future. Microalgae is a promising candidate in this aspect. The focus of this study is to optimize the supply of nitrogen and carbon dioxide during the cultivation of locally isolated strain Parachlorella kessleri HY-6. This study focuses on optimizing nitrogen and CO₂ supply based on total biomass and biomass per unit amount of nitrogen and CO₂. Total biomass increased from 1.23 to 2.30 g/L with an increase in nitrogen concentration from 15.8 to 47.4 mg/L. However, biomass per unit amount of nitrogen supplied was higher at low nitrogen content. Biomass and CO₂ fixation rate increased at higher CO₂ concentrations in bubbling air, but CO₂ fixation efficiency decreased drastically. Finally, the energy content of biomass increased with increases in both nitrogen and CO₂ supply. This work thoroughly analyzed the biomass composition via ultimate, proximate, and biochemical analysis. Water is recycled three times for cultivation at three different nitrogen levels. Microalgae biomass increased during the second recycling and then decreased drastically during the third. Activated carbon helped remove the organics after the third recycling to improve the water recyclability. This study highlights the importance of selecting appropriate variables for optimization by considering net energy investment in terms of nutrients (as nitrogen) and CO₂ fixation efficiency and effective water recycling.

Monitoring lipids profile, CO2 fixation, and water recyclability for the economic viability of microalgae Chlorella vulgaris cultivation at different initial nitrogen

The economic viability of microalgae as a bioenergy source depends on many factors. High CO₂ fixing rate, improved lipids yield, and minimum water footprint are few key parameters. This study investigates the effect of four initial nitrogen concentrations (1-, 2-, 6- and 10-mM as nitrate) on lipids yield, their classification and composition, CO₂ fixation rate, and water quality for further reuse after first cultivation. The initial 6 mM nitrate was found optimum for the growth and overall lipid productivity of Chlorella vulgaris. The maximum quantum efficiency (as Fv/Fm ratio) for algae decreases along with the cell growth profile and depletion of the initial nitrate concentration. CO₂ fixation rate increased initially and peaked during exponential growth and then declined for the rest of the cultivation period. A higher CO₂ fixation rate was recorded at 6 mM, and an overall fixation rate of CO₂ was high at 6 mM. A higher total organic carbon (TOC) is produced in recycled water at a low nitrogen concentration of 1 and 2 mM. TOC changes during the cultivation period and with each reuse of water. Water was recycled twice successfully, while growth was inhibited during the 3rd cycle. Based on all these investigations, 6 mM of initial nitrogen was found optimal at given growth conditions.

Recycling spent water from microalgae harvesting by fungal pellets to re-cultivate Chlorella vulgaris under different nutrient loads for biodiesel production

Fungal pellet is an emerging material to collect oleaginous microalgae, but rare studies have noticed that harvested water is available resource for the next round of cultivation. To systematically optimize regrowth performances of microalgae Chlorella vulgaris, separated water after harvesting by fungi Aspergillus oryzae was prepared under different N/P ratios. The results showed that chlorophylls and enzymes were significantly affected by the proportion of N and P. Although nutrient deficiency was functioned as a stress factor to restrict carbohydrate and protein synthesis, lipid content was obviously increased by 12.69%. The percentage of saturated fatty acids associated with oxidation stability increased, while this part in fresh wastewater accounted for only 36.96%. The favorable biomass concentration (1.37 g/L) with the highest lipid yield (0.42 g/L) appeared in N/P of 6:1. More strikingly, suitable conditions could save 52.4% of cultivation costs. These experiments confirmed that reusing bioflocculated water could be effectively utilized for biodiesel production.

Effects of sulfate ions on growth and lipid synthesis of Scenedesmus obliquus in synthetic wastewater with various carbon-to-nitrogen ratios altered by different ammonium and nitrate additions

Producing biodiesel from microalgae is a promising strategy to upgrade energy structure. In this study, effects of sulfate (SO₄^2-) on lipid synthesis of Scenedesmus obliquus (S. obliquus) cultivated in synthetic wastewater with different carbon to nitrogen (C/N) ratios regulated by ammonium (NH₄⁺-N) and nitrate (NO₃^--N), separately, were investigated. The results shown that SO₄^2- could dramatically increase cell growth preferring to NH₄⁺-N supply. And SO₄^2- addition could improve its carbon and nitrogen utilization potential for boosting lipid productivity leading α-linolenic acid (C18:3n3) to occupy a dominant component (38.96%) in NH₄⁺-N group at a C/N ratio of 7.5. Additionally, SO₄^2- could enhance the enrichment and expression of up-regulated genes annotated in key enzymes such as GK, GNPAT, CRLS, plc and DEGS involved in glycerolipid, glycerophospholipid and sphingolipid metabolic pathways, resulting in carbon metabolism enhancement and sulfatide accumulation. This study brings a comprehensive view towards nutritional regulation of lipid synthesis in microalgae.

Enhanced microalgae cultivation using wastewater nutrients extracted by a microbial electrochemical system

Cultivating algae using wastewater nutrients is a potential approach to realize resource recovery that can contribute to circular economy. However, growing algae directly in a wastewater has problems such as bacterial contamination and a low biomass density. To address those problems, we investigated microalgal cultivation in a photobioreactor (PBR) fed with the nutrients extracted from wastewater by a microbial nutrient recovery cell (MNRC). With an external voltage of 0.3 V, the MNRC-PBR system removed 96% of COD and recovered 44% of NH₄⁺-N and 39% of PO₄^3--P at a hydraulic retention time of 7.2 h. Microalgae cultivated in the nutrient recovery medium from the MNRC had 8.3-fold biomass density and 1.4-fold lipid contents, versus that cultivated in a food wastewater containing more nutrients. More significantly, 90% of biomass yielded from the MNRC-PBR system was microalgae, much higher than ∼30% in the food wastewater. A liquid exchange ratio of 30% achieved the highest microalgal density of 0.61 ± 0.06 g L^-1, comparable to that in a standard BG11 medium. There was a tradeoff between recycling PBR medium and microalgal growth. The accumulated salinity was observed in the extended operation of the MNRC-PBR system treating an actual food wastewater. The results of this study have demonstrated an effective approach to extract nutrients from wastewater for enhanced microalgal growth and improved biomass quality.

Barriers to microalgal mass cultivation

Economically successful microalgal mass cultivation is dependent on overcoming several barriers that contribute to the cost of production. The severity of these barriers is dependent on the market value of the final product. These barriers prevent the commercially viable production of algal biofuels but are also faced by any producers of any algal product. General barriers include the cost of water and limits on recycling, costs and recycling of nutrients, CO₂ utilization, energy costs associated with harvesting and biomass loss due to biocontamination and pond crashes. In this paper, recent advances in overcoming these barriers are discussed.

CO2 biocapture by Scenedesmus sp. grown in industrial wastewater

Greenhouse gases (GHG) emissions are widely related to climate change, triggering several environmental problems of global concern and producing environmental, social, and economic negative impacts. Therefore, global research seeks to mitigate greenhouse gas emissions. On the other hand, the use of wastes under a circular economy scheme generates subproducts from the range of high to medium-value, representing a way to help sustainable development. Therefore, the use of wastewater as a culture medium to grow microalgae strains that biocapture environmental CO₂, is a proposal with high potential to reduce the GHG presence in the environment. In this work, Scenedesmus sp. was cultivated using BG-11 medium and industrial wastewater (IWW) as a culture medium with three different CO₂ concentrations, 0.03%, 10%, and 20% to determine their CO₂ biocapture potential. Furthermore, the concomitant removal of COD, nitrates, and total phosphorus in wastewater was evaluated. Scenedesmus sp. achieves a biomass concentration of 1.9 g L^-1 when is grown in BG-11 medium, 0.69 g L^-1 when is grown in a combination of BG-11 medium and 25% of industrial wastewater; both cases with 20% CO₂ supplied. The maximum CO₂ removal efficiency (8.4%, 446 ± 150 mg CO₂ L^-1 day^-1) was obtained with 10% CO₂ supplied and using a combination of BG-11 medium and 50% IWW (T2). Also, the highest removal of COD was reached with a combination of BG-11 medium and T2 with a supply of 20% CO₂ (82% of COD removal). Besides, the highest nitrates removal was achieved with a combination of BG-11 medium and 75% IWW (T3) with a supply of 10% CO₂ (42% of nitrates removal) and the maximum TP removal was performed with the combination of BG-11 medium and 25% IWW (T1) with a supply of 10% CO₂ (67% of TP removal). These results indicate that industrial wastewater can be used as a culture media for microalgae growth and CO₂ biocapture can be performed as concomitant processes.

A fungal immobilization technique for efficient harvesting of oleaginous microalgae: Key parameter optimization, mechanism exploration and spent medium recycling

To confront with energy crisis, microalgae as the promising feedstock have a great potential in exploring renewable energy field, whereas the high costs related to medium preparation and biomass harvesting are the main bottleneck to hinder the development on a large scale. Though cultivation of filamentous fungi for microalgae harvesting is an efficient, sustainable and emerging method, and the studies on specific mechanisms and spent medium recycling for efficiency improvement as well as resource saving through a co-pelletization mode are urgently needed. Hence, in this study, the harvesting process of autotrophic microalgae Chlorella vulgaris by pre-cultured Aspergillus oryzae pellets was investigated systematically. The highest efficiency (99.23%) was obtained within 5 h under the optimized conditions of 30 °C, 130 rpm and fungi:algae ratio of 1:1 on a dry weight basis without demand for pH adjustment (initial value on 9.68). Charge neutralization was not the main mechanisms involved in fungi-algae aggregations, and the functional group changes on cell surfaces as well as secreted metabolites in medium could be mainly responsible for inducing the bioflocculation process. After harvesting, separated water could also effectively support microalgae re-growth. The biomass concentration in medium with 50% recycling was higher than that in fresh medium, while lipid content was increased from 24.37% to 33.97% in fully recycled medium. These results indicated that the pellet-assisted mode for algal harvesting is a promising way to promote biofuel production and resource recycling.

Water reuse and growth inhibition mechanisms for cultivation of microalga Euglena gracilis

BACKGROUND

Microalgae can contribute to more than 40% of global primary biomass production and are suitable candidates for various biotechnology applications such as food, feed products, drugs, fuels, and wastewater treatment. However, the primary limitation for large-scale algae production is the fact that algae requires large amounts of fresh water for cultivation. To address this issue, scientists around the world are working on ways to reuse the water to grow microalgae so that it can be grown in successive cycles without the need for fresh water.

RESULTS

In this study, we present the results when we cultivate microalgae with cultivation water that is purified and reused. Specifically, we purify the cultivation water using an ultrafiltration membrane (UFM) treatment and investigate how this treatment affects: the biomass and biochemical components of the microalgae; characteristics of microalgae growth inhibitors; the mechanism whereby potential growth inhibitors are secreted (followed using metabolomics analysis); the effect of activated carbon (AC) treatment and advanced oxidation processes (AOPs) on the removal of growth inhibitors of Euglena gracilis. Firstly, the results show that E. gracilis can be only cultivated through two growth cycles with water that has been filtered and reused, and the growth of E. gracilis is significantly inhibited when the water is used a third time. Secondly, as the number of reused water cycles increases, the Cl^- concentration gradually increases in the cultivation water. When the Cl^- concentration accumulates to a level of fivefold higher than that of the control, growth of E. gracilis is inhibited as the osmolality tolerance range is exceeded. Interestingly, the osmolality of the reused water can be reduced by replacing NH₄Cl with urea as the source of nitrogen in the cultivation water. Thirdly, E. gracilis secretes humic acid (HA)-which is produced by the metabolic pathways for valine, leucine, and isoleucine biosynthesis and by linoleic acid metabolism-into the cultivation water. Because HA contains large fluorescent functional groups, specifically extended π(pi)-systems containing C=C and C=O groups and aromatic rings, we were able to observe a positive correlation between HA concentration and the rate of inhibition of E. gracilis growth using fluorescence spectroscopy. Moreover, photosynthetic efficiency is adversely interfered by HA, thereby reductions in the synthetic efficiency of paramylon and lipid in E. gracilis. In this way, we are able to confirm that HA is the main growth inhibitor of E. gracilis. Finally, we verify that all the HA is removed or converted into nutrients efficiently by AC or UV/H₂O₂/O₃ treatments, respectively. As a result of these treatments, growth of E. gracilis is restored (AC treatment) and the amount of biomass is promoted (UV/H₂O₂/O₃ treatment).

CONCLUSIONS

These studies have important practical and theoretical significance for the cyclic cultivation of E. gracilis and for saving water resources. Our work may also provide a useful reference for other microalgae cultivation.

Chlorella vulgaris cultivation in simulated wastewater for the biomass production, nutrients removal and CO2 fixation simultaneously

Chlorella vulgaris (C. vulgaris) was promising microalgae to simultaneously achieve biomass production, carbon dioxide (CO₂) fixation, nutrients removal and proteins production especially under different conditions of CO₂ gas and wastewaters. Results presented that maximal specific growth rate of C. vulgaris was 0.21-0.35 d^-1 and 0.33-0.43 d^-1 at 0.038% and 10% CO₂ respectively, and corresponding maximal CO₂ fixation rate was attended with 4.51-14.26 and 56.26-85.72 mg CO₂·L^-1·d^-1. C. vulgaris showed good wastewater removal efficiency of nitrogen and phosphorus at 10% CO₂ with 96.12%-99.61% removal rates. Nitrogen fixation amount achieved 41.86 mg L^-1 when the initial NH₄Cl concentration was set at 60 mg L^-1 at 10% CO₂. Improved total protein (25.01-365.49 mg) and amino acids (24.56-196.44 mg) contents of C. vulgaris biomass was observed with the increasing of added CO₂ and ammonium concentrations. Moreover, the developed kinetic function of C. vulgaris growth depends on both phosphorus quota and nitrogen quota with correlation coefficient (R²) ranged from 0.68 to 0.97. Computed maximal consumed nutrients concentrations (ΔC_max) based on Logistic function are positively related to initial NH₄⁺-N concentrations, which indicated that adding ammonium could stimulate the utilization of both phosphorus and nitrogen.

Extracellular and intracellular phenol production by microalgae during photoautotrophic batch cultivation

Understanding the mechanisms of phenol production by microalgae can contribute to the development of microalgal biorefinery processes with higher economic and environmental sustainability. However, little is known about how phenols are produced and accumulate during microalgal cultivation. In this study, both extracellular and intracellular phenol production by two microalgal strains (Tetradesmus obliquus and Chlorella sp.) were investigated throughout a conventional photoautotrophic batch cultivation. The highest intracellular phenol content (10-25 mg g^-1) and productivity (12-18 mg L^-1 d^-1) were attained for both strains in the first part of the batch, indicating a positive relation with nutrient availability and biomass productivity. Extracellular phenol production was 2-20 fold lower than intracellular phenols, but reached up to 27 mg L^-1 for T. obliquus and 13 mg L^-1 for Chlorella sp. The latter finding highlights relevant issues about the management of the exhausted culture medium, due to likely antimicrobial effects.

Harvesting of Microcystis flos-aquae using chitosan coagulation: Influence of proton-active functional groups originating from extracellular and intracellular organic matter

Algogenic organic matter (AOM) produced by Microcystis cells inhibits coagulation harvesting; however, the harvesting inhibitory mechanisms at the functional groups level remain to be determined. This study fractionated extracellular organic matter (EOM) and intercellular organic matter (IOM) from Microcystis flos-aquae into five different hydrophilic and hydrophobic fractions and investigated their inhibition of chitosan coagulation harvesting. The proton-active functional groups in the inhibitory fractions were further analysed by potentiometric titration, and the interaction between these functional groups and chitosan was elucidated. The results showed that the harvesting inhibition of M. flos-aquae cells was dominated by HPI in AOM due to its high charge density, which resulted in greater consumption of coagulant. Potentiometric titration results suggested that the proton-active functional groups of both HPI_EOM and HPI_IOM consist mainly of phosphodiester, carboxylic, phosphoryl and amine/hydroxyl functional groups, and the harvesting inhibition of HPI on M. flos-aquae cells at pH 6.5 was mainly due to the deprotonation of phosphodiester and carboxylic functional groups. Moreover, carboxylic functional groups with stronger polarity could enhance the intermolecular interaction between HPI and chitosan more effectively than phosphodiester at pH 6.5. Preventing the deprotonation of carboxylic functional groups by adjusting the pH to 4.3 could effectively alleviate the harvesting inhibition caused by HPI. These findings revealed the inhibition mechanism of AOM on the coagulation harvesting of M. flos-aquae cells from the perspective of deprotonation of proton-active functional groups, which may provide important insights for assessing the role of AOM in the coagulation harvesting of Microcystis cells.

Carbon Mass Balance in Arthrospira platensis Culture with Medium Recycle and High CO2 Supply

Medium recycling combined with CO2 recovery helps sustainable use of the alkaline medium in Arthrospira culture. However, high CO2 supply may cause inorganic carbon accumulation and pH reduction, which could result in low CO2 recovery and reduced algal growth. This study aimed to elucidate the effect of medium recycling and high CO2 supply through carbon mass balance analysis in Arthrospira culture. In all CO2 supply conditions, carbon supply was higher than Arthrospira carbon assimilation, which accounted for 30–58% of supply. However, CO2 recovery of nearly 100% and 63% for lower (0.20 and 0.39 gC L−1 d−1) and higher (0.59 gC L−1 d−1) CO2 supply rates were achieved, respectively, because of the high concentration of the alkaline agent. The excess carbon accumulated in the medium and ultimately escaped from the system in a form of dissolved inorganic carbon (DIC). Dissolved organic carbon (DOC) contributed to 16–24% of the total photosynthetically assimilated carbon, and the final concentration reached 260–367 mgC L−1, but there was no significant growth reduction caused by DIC and DOC accumulation. This study demonstrated the stability of the medium-recycling process even at high CO2 supply rates although a balanced supply is recommended for longer operations.