Photosynthetic Oxygen Research Articles

Photosynthetic monitoring techniques indicate maximum glycogen accumulation in nitrogen-limited Synechocystis sp. PCC 6803 culture

Chlorophyll fluorescence and oxygen evolution are rapid and non-invasive monitoring techniques to obtain information about the photosynthesis performance of microalgae cultures. These methods may be employed for optimizing the biomass productivity as well as indicate increased production of some valuable compounds.In this work photosynthesis monitoring techniques – saturation pulse analysis of fluorescence quenching, fast fluorescence induction kinetics and photosynthetic oxygen production/respiration – were employed for monitoring of the physiological state of the nitrogen-limited culture of Synechocystis sp. PCC 6803. The correlation between changes of photosynthetic activities, growth and glycogen accumulation was examined in these cultures. The aim was to determine proper point for biomass harvest comparing the changes of photosynthetic activities and the kinetics of glycogen accumulation. In a 4-day trial the highest glycogen accumulation in biomass was found on day 2 showing an inflection point which was accompanied by the reversal of the chlorophyll fluorescence variables, namely maximal photochemical efficiency of PSII Fv/Fm, relative electron transport rate rETR, photochemical efficiency α and photosynthesis saturating irradiance Ik. The rapid increase of cell respiration after day 2 also indicated the relation of these changes with onset of glycogen catabolism.Our laboratory experiments revealed that the three photosynthetic monitoring techniques provided good indication of the physiological changes leading to high glycogen accumulation in biomass of the nitrogen-limited Synechocystis culture. In this way the glycogen accumulation can be manipulated. The photosynthesis measurements can indicate the right time to harvest the culture rich in glycogen in biotechnological applications.

Photosynthetic Microorganisms-Based Biophotothermal Therapy with Enhanced Immune Response.

The production of oxygen by photosynthetic microorganisms (PSMs) has recently attracted interest concerning the in vivo treatment of multiple diseases for their photosynthetic oxygen production in vivo, since PSMs have good biological safety. Here, the first evidence that PSMs can be used as a photothermal source to perform biophotothermal therapy (bio-PTT) is provided. In vitro and in vivo experiments proved that PSMs can generate heat for the direct elimination of tumors and release a series of pathogen-associated molecular patterns and adjuvants for immune stimulation under light irradiation. Bio-PTT enabled a local tumor inhibition rate exceeding 90% and an abscopal tumor inhibition rate exceeding 75%. This strategy also produced a stronger antitumor immune memory effect to prevent tumor recurrence. The bio-PTT strategy provides a novel direction for photothermal therapy as it simultaneously produces local and abscopal antitumor effects.

Incorporating a molecular antenna in diatom microalgae cells enhances photosynthesis

Diatom microalgae have great industrial potential as next-generation sources of biomaterials and biofuels. Effective scale-up of their production can be pursued by enhancing the efficiency of their photosynthetic process in a way that increases the solar-to-biomass conversion yield. A proof-of-concept demonstration is given of the possibility of enhancing the light absorption of algae and of increasing their efficiency in photosynthesis by in vivo incorporation of an organic dye which acts as an antenna and enhances cells’ growth and biomass production without resorting to genetic modification. A molecular dye (Cy5) is incorporated in Thalassiosira weissflogii diatom cells by simply adding it to the culture medium and thus filling the orange gap that limits their absorption of sunlight. Cy5 enhances diatoms’ photosynthetic oxygen production and cell density by 49% and 40%, respectively. Cy5 incorporation also increases by 12% the algal lipid free fatty acid (FFA) production versus the pristine cell culture, thus representing a suitable way to enhance biofuel generation from algal species. Time-resolved spectroscopy reveals Förster Resonance Energy Transfer (FRET) from Cy5 to algal chlorophyll. The present approach lays the basis for non-genetic tailoring of diatoms’ spectral response to light harvesting, opening up new ways for their industrial valorization.

Experimental Study of the Quantitative Impact of Flow Turbulence on Algal Growth

Flow turbulence has been widely accepted as one of the essential factors affecting phytoplankton growth. In this study, laboratory cultures of Microcystis aeruginosa in beakers were carried out under different turbulent conditions to identify the quantitative relationship between the algal growth rate and the turbulent intensity. The turbulent intensity (represented by energy dissipation rate, ε) was simulated with the software FLUENT. Daily measurement of the two parameters (algal biomass and chlorophyll-a concentration) was carried out during the experimental period to represent the algal growth rate. Meanwhile, the rates of photosynthetic oxygen evolution and chlorophyll fluorescence intensity were calculated to investigate the photosynthetic efficiency. The results indicated that the growth rate of Microcystis aeruginosa became higher in the turbulent environment than in the still water environment under the designed experimental conditions. The peak growth rate of Microcystis aeruginosa occurred when ε was 6.44 × 10−2 m2/s3, over which the rate declined, probably due to unfavorable impacts of strong turbulence. In comparison, the maximum rate of photosynthetic oxygen evolution occurred when ε was 0.19 m2/s3. Based on the findings of this study, an exponential function was proposed in order to incorporate the effect of flow turbulence into the existing algal growth models, which usually just consider the impacts of nutrient availability, illumination, and temperature.

In Vivo Evaluation of Oxygenic Photogranules’ Photosynthetic Capacity by Pulse Amplitude Modulation and Phototrophic–Irradiance Curves

The commingled microbial moiety of oxygenic photogranules (OPGs) facilitates aeration-free wastewater treatment. Embedded in an extracellular polymeric substances (EPS) matrix, microbial producers and consumers of oxygen occupying granular niches exchange substrates among themselves and with the bulk fluid. An assessment of the OPG’s phototrophic potential or functional capacity may require combining different photoactivity signals. The photosynthetic capacity was evaluated using photosynthetic oxygen evolution (POE) and chlorophyll fluorescence (rapid light curves, RLC) measurements using OPGs grown at different light intensities.. A maximum oxygen generating capacity for optimal OPGs and reactor conditions was determined to be 284.4 mgO2 gVSS–1 h–1 The OPGs exhibited photoelasticity, with higher photosynthetic capacity in high light (HL) compared to that in low light (LL) adapted samples. Saturation irradiances before the onset of photoinhibition for LL and HL samples were 1000 and 1200 μmol m–2 s–1, respectively using POE signals, and 478 μmol m–2 s–1 and 611 μmol m–2 s–1 using RLC signals. Moreover, HL adapted samples had higher nonphotochemical quenching rates which allude to the OPG’s photoelastic potential. The correlation coefficients (κ) between POE and RLCs were lower than reported values for pure microbial cultures reflecting the enhanced contribution from different photosynthetic clades with a variety of light-harvesting pigments present in OPGs. In an OPG reactor, the photochemical activity can be influenced by the granular size, granular ecology, and reactor operation metrics related to irradiance interactions such as mixing, self-shading, light intensity, photoperiods, and reactor depth. This presents opportunities for design of intensive wastewater resource recovery using phototrophic granular biomass.

Microphytobenthic primary production on exposed coastal sandy sediments of the Southern Baltic Sea using ex situ sediment cores and oxygen optodes

The shallow coastal water zone of the tide-less southern Baltic Sea is dominated by exposed sandy sediments which are typically inhabited by microphytobenthic communities, but their primary production is poorly studied, and hence four stations between 3.0 and 6.2 m depth were investigated. Sediment cores were carefully taken to keep the natural layering and exposed in a controlled self-constructed incubator. Respiratory oxygen consumption and photosynthetic oxygen production were recorded applying planar oxygen optode sensors. We hypothesized that with increasing water depths the effects of wind- and wave-induced erosion and mixing of the upper sediment layer are dampened and expected higher microphytobenthic biomass and primary production in the incubated cores.Our data partly confirm this hypothesis, as cores sampled at the most exposed stations contained only 50% chlorophyll a m−2 compared to the deeper stations. However, primary production was highly variable, probably due to fluctuating sediment-disturbing conditions before the cores were taken. Due to these physical forces sand grains were highly mobile and rounded, and small epipsamic benthic diatoms dominated, which preferentially occurred in some cracks and crevices as visualized by scanning electron microscopy. The data fill an important gap in reliable production data for sandy sediments of the southern Baltic Sea, and point to the ecological importance and relevant contribution of microphytobenthic communities to the total primary production of this marine ecosystem. Oxygen planar optode sensor spots proved to be a reliable, sensitive and fast detection system for ex situ oxygen exchange measurements in the overlying water of intact sediment cores.

Adaptation of a marine diatom to ocean acidification and warming reveals constraints and trade-offs

Ocean acidification and warming are recognized as two major anthropogenic perturbations of the modern ocean. However, little is known about the adaptive response of phytoplankton to them. Here we examine the adaptation of a marine diatom Thalassiosira weissflogii to ocean acidification in combination with ocean warming. Our results show that ocean warming have a greater effect than acidification on the growth of T. weissflogii over the long-term selection experiment (~380 generations), as well as many temperature response traits (e.g., optimum temperatures for photosynthesis, maximal net photosynthetic oxygen evolution rates, activation energy) in thermal reaction norm. These results suggest that ocean warming is the main driver for the evolution of the marine diatom T. weissflogii, rather than oceanacidification. However, the evolution resulting from warming can be constrained by ocean acidification, where ocean warming did not impose any effects at high CO2 level. Furthermore, adaptations to ocean warming alone or to the combination of ocean acidification and warming come with trade-offs by inhibiting photochemical performances. The constrains and trade-offs associated with the adaptation to ocean acidification and warming demonstrated in this study, should be considered for parameterizing evolutionary responses in eco-evolutionary models of phytoplankton dynamics in a future ocean.

Controlling the source of green tides in the Yellow Sea: NaClO treatment of Ulva attached on Pyropia aquaculture rafts

Green macroalgal blooms have occurred in the Yellow Sea for 13 consecutive years since 2007. However, effective control methods have not yet been successfully developed. Ulva prolifera attached on Pyropia aquaculture rafts was identified as the main source of the blooms, thus, eliminating the attached Ulva from the rafts should be the most efficient method to prevent and control these blooms. In this study, we designed a new molecular marker (rps2-trnL) to identify U. prolifera collected from the aquaculture site. Samples identified as U. prolifera were cultured, and different concentrations of sodium hypochlorite (NaClO) solution (0.2, 0.4, 0.6, 0.8, and 1.0%) were used to kill the alga. The measured chlorophyll content, light quantum yield, and photosynthetic oxygen release rate data from the laboratory experiment suggested that the appropriate concentration of NaClO solution should be 0.6–1.0%. To carry out a small-scale field trial, we developed a device that can spray NaClO on Ulva blades and recycle the residual liquid. Results showed that only the 1.0% NaClO solution could quickly kill the Ulva in outdoor experiments. During the treatment, very little NaClO dripped on the tidal flat, which indicated that the recycling device avoided a potential negative impact of residual solution on the marine ecosystem. By the beginning of November 2019, the Pyropia aquaculture area treated by NaClO was about 13,000 ha, covering about one-third of the total area at the coast of Jiangsu Province. Monitoring by an unmanned aerial vehicle showed that very little attached Ulva was present on the ropes after NaClO treatment, which indicated that it was an effective method to control the green tide. Also, the range of residual active chlorine within seawater during the treatment was only from 0.003 to 0.012 mg/L, which complied with Chinese national standards. However, further monitoring of the formation process of green tides should be conducted in future years to evaluate the effect of NaClO treatment.

Effects of Temperature and Light on Growth Rate and Photosynthetic Characteristics of Sargassum horneri

The changing environmental factors exerted great influences on coastal macroalgal communities. To study the responses of the brown seaweed Sargassum horneri to temperature and light, S. horneri was cultured under three temperatures (20, 25 and 30°C) and three light intensities (30, 60, and 120 µmol photonsm−2 s−1) for seven days. The growth rate, chlorophyll a (Chl a) and carotenoids (Car) contents, chlorophyll fluorescence, and photosynthetic oxygen evolution rate were measured. The results show that the highest relative growth rate (RGR), maximal electron transport rate (rETRmax); the net photosynthetic rate (Pn) were observed at the lowest temperature (20°C) and highest light intensity (120 µmol photons m−2 s−1); and the RGR and Pn were significantly inhibited by the highest temperature (30°C), especially at the lowest light intensity (30 µmol photons m−2 s−1) (P <0.05). Additionally, the highest light intensity enhanced the non-photochemical quenching (NPQ) even under the highest temperature (30°C), indicating that the higher light intensity could induce photo-protection reaction of thalli. These results suggest that the higher temperature and lower light intensity exerted negative influences on S. horneri.

3D-Bioprinted Constructs that Breathe

3D-Bioprinted Constructs that Breathe

Peatland water conservation by agroforestry system

Peat swamp forest have a high economic value and are potential for agricultural development. However, peatlad are fragile ecosystems because they are easily damaged and difficult to restore. The main problem of all peat soil types is their irreversible drying. In this condition the peat soil is easy burning, unable to store water, and if the groundwater is far from the surface, the plants will be stressed from lack of water. Therefore to support sustainable agricultural development on peatlands it requires careful planning, application of appropriate technology and suitable management. One of the land management systems that can be applied on peatlands is combining agricultural crop and forest plant (agroforestry). Forest plant and seasonal agriculture crops can increase CO2 absorption so that photosynthetic efficiency and oxygen production increase. These proses increase the production of biomass which can maintain soil organic matter and prevent erosion. Organic matter enhance the effectiveness of rewetting and water retention capacity. In addition, this system also affects the microclimate, such as the soil becoming more humid because it gets shade from the vegetation above it, so reduce temperature and elevated air moisture. Indirectly, agroforestry can reduce excessive evaporation and maintains the ground water level, especially during the dry season. This review aim to explain the role of intercropping system in peatland water conservation.

Quantification of RuBisCO Expression and Photosynthetic Oxygen Evolution in Cyanobacteria.

Phototrophic microorganisms are frequently engineered to regulate the expression and the activity of targeted enzymes of interest for specific biotechnological and agricultural applications. This protocol describes a method to evaluate the expression of RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) in the model cyanobacterium Synechococcus elongatus PCC 7942, at both the transcript and protein levels by quantitative PCR and Western blot, respectively. We further describe an experimental method to determine photosynthetic activity using an oxygen electrode that measures the rate of molecular oxygen production by cyanobacterial cultures. Our protocol can be utilized to assess the effects of RuBisCO engineering at the metabolic and physiological levels.

Calcium phosphate engineered photosynthetic microalgae to combat hypoxic-tumor by in-situ modulating hypoxia and cascade radio-phototherapy.

Rationale: Hypoxia is one of the crucial restrictions in cancer radiotherapy (RT), which leads to the hypoxia-associated radioresistance of tumor cells and may result in the sharp decline in therapeutic efficacy.Methods: Herein, living photosynthetic microalgae (Chlorella vulgaris, C. vulgaris), were used as oxygenators, for in situ oxygen generation to relieve tumor hypoxia. We engineered the surface of C. vulgaris (CV) cells with calcium phosphate (CaP) shell by biomineralization, to form a biomimetic system (CV@CaP) for efficient tumor delivery and in-situ active photosynthetic oxygenation reaction in tumor.Results: After intravenous injection into tumor-bearing mice, CV@CaP could remarkably alleviate tumor hypoxia by continuous oxygen generation, thereby achieving enhanced radiotherapeutic effect. Furthermore, a cascade phototherapy could be fulfilled by the chlorophyll released from photosynthetic microalgae combined thermal effects under 650 nm laser irradiation. The feasibility of CV@CaP-mediated combinational treatment was finally validated in an orthotropic breast cancer mouse model, revealing its prominent anti-tumor and anti-metastasis efficacy in hypoxic-tumor management. More importantly, the engineered photosynthetic microalgae exhibited excellent fluorescence and photoacoustic imaging properties, allowing the self-monitoring of tumor therapy and tumor microenvironment.Conclusions: Our studies of this photosynthetic microsystem open up a new dimension for solving the radioresistance issue of hypoxic tumors.

Investigating the impact of light quality on macromolecular composition of Chaetoceros muelleri.

Diatoms (Bacillariophyceae) are important to primary productivity of aquatic ecosystems. This algal group is also a valuable source of high value compounds that are utilised as aquaculture feed. The productivity of diatoms is strongly driven by light and CO2 availability, and macro- and micronutrient concentrations. The light dependency of biomass productivity and metabolite composition is well researched in diatoms, but information on the impact of light quality, particularly the productivity return on energy invested when using different monochromatic light sources, remains scarce. In this work, the productivity return on energy invested of improving growth rate, photosynthetic activity, and metabolite productivity of the diatom Chaetoceros muelleri under defined wavelengths (blue, red, and green) as well as while light is analysed. By adjusting the different light qualities to equal photosynthetically utilisable radiation, it was found that the growth rate and photosynthetic oxygen evolution was unchanged under white, blue, and green light, but it was lower under red light. Blue light improved the productivity return on energy invested for biomass, total protein, total lipid, total carbohydrate, and in fatty acids production, which would suggest that blue light should be used for aquaculture feed production.

Interactive effects of increasing atmospheric CO2 and copper exposure on the growth and photosynthesis in the young sporophytes of Sargassum fusiforme (Phaeophyta)

Little attention has been given to the combined effects of elevated atmospheric CO2-induced ocean acidification (OA) and heavy metal pollution on marine macroalgae at the young stage. This study investigated the mutual effects of copper (Cu) and elevated CO2 on the young sporophytes of brown macroalgae Sargassum fusiforme. A matrix of four copper concentrations, 0, 0.025, 0.075 and 0.15 mg‧L−1, and two levels of CO2 (ambient CO2: 400 μatm; elevated CO2: 1,000 μatm) were used. High concentration of copper exposure greatly depressed photosynthesis and growth of the young sporophytes of S. fusiforme by reducing the apparent photosynthetic efficiency (ɑ), maximum net photosynthetic oxygen evolution rate (Pmax), maximum photochemical quantum yield (Fv/Fm) and pigments content (Chl a and Car). While elevated CO2 alone had obscure impact on this alga. However, the inhibition of Cu stress on Fv/Fm was weakened by elevated CO2, which also decreased the light compensation point (Ic). Meanwhile, the Cu2+-induced ascent in the dark respiration rate (Rd) and superoxide dismutase (SOD) activity was mitigated under the growth with elevated CO2, suggesting an alleviated oxidative stress. Overall, we propose that, under CO2 enrichment condition, the young sporophytes of S. fusiforme may increase photosynthesis efficiency and synthesize less enzymatic antioxidants in face of increasing Cu stress.

Cell size influences inorganic carbon acquisition in artificially selected phytoplankton.

Cell size influences the rate at which phytoplankton assimilate dissolved inorganic carbon (DIC), but it is unclear whether volume-specific carbon uptake should be greater in smaller or larger cells. On the one hand, Fick's Law predicts smaller cells to have a superior diffusive CO2 supply. On the other, larger cells may have greater scope to invest metabolic energy to upregulate active transport per unit area through CO2 -concentrating mechanisms (CCMs). Previous studies have focused on among-species comparisons, which complicates disentangling the role of cell size from other covarying traits. In this study, we investigated the DIC assimilation of the green alga Dunaliella tertiolecta after using artificial selection to evolve a 9.3-fold difference in cell volume. We compared CO2 affinity, external carbonic anhydrase (CAext ), isotopic signatures (δ13 C) and growth among size-selected lineages. Evolving cells to larger sizes led to an upregulation of CCMs that improved the DIC uptake of this species, with higher CO2 affinity, higher CAext and higher δ13 C. Larger cells also achieved faster growth and higher maximum biovolume densities. We showed that evolutionary shifts in cell size can alter the efficiency of DIC uptake systems to influence the fitness of a phytoplankton species.

Recycling nutrients from soy sauce wastewater to culture value-added Spirulina maxima

Soy sauce is one of the most common condiments in East Asian cuisines. Its manufacturing process generates a large amount of organic-rich soy sauce wastewater, which is difficult to treat through traditional processes. In this study, Spirulina NCU-Sm was used to treat soy sauce wastewater and simultaneously produce value-added biomass. The results showed that NCU-Sm grew well in 100% (undiluted) raw soy sauce wastewater (RSW). The maximum biomass yield of NCU-Sm reached 1.984 g/L, and the removal efficiencies of NH4+-N, TN, COD, chromaticity, and salinity in RSW reached 93.86, 81.76, 84.08, 40.93, and 63.08%, respectively. High COD removal efficiency in 100% RSW proved NCU-Sm grew in mixotrophic mode. pH and light spectrum, two major factors affecting growth of soy sauce wastewater-grown Spirulina NCU-Sm, were further studied. When the initial pH of RSW was adjusted to 8, NCU-Sm biomass yield was increased by 11.11%, and the contents of protein, chlorophyll-a, and carotenoid were increased by 8.69, 37.14, 40.40%, and reached 65.57%, 7.57 and 2.78 mg/g, respectively. The removal efficiencies of NH4+-N, TN, and chromaticity in RSW with pH 8 were improved to 99.34, 88.29, 43.18%, respectively. Red light enhanced the photosynthetic oxygen release efficiency of NCU-Sm and increased the carotenoid content of NCU-Sm by 117.95%, reaching 5.95 mg/g. Blue light was adverse to the growth and reduced the protein content of NCU-Sm but significantly increased its chlorophyll-a content to up to 10.99 mg/g. These results suggest that cultivation of Spirulina in soy sauce wastewater serves dual roles in soy sauce wastewater treatment, as well as value-added algal biomass (such as high-quality protein and natural pigments raw materials) production.

Diclofenac and atrazine restrict the growth of a synchronous Chlamydomonas reinhardtii population via various mechanisms

Non-steroidal anti-inflammatory drug diclofenac (DCF) is commonly found in freshwater bodies and can have adverse effects on non-target organisms. Among the studies on DCF toxicity, several ones have reported its harmful effects on plants and algae. To gain a better understanding of the mechanisms of DCF toxicity towards green algae, we used a synchronous Chlamydomonas reinhardtii cc-1690 culture and compared DCF (135 mg/L) effects with effects caused by atrazine (ATR; 77.6 μg/L), an herbicide with a well-known mechanism of toxic action. To achieve our goal, cell number and size, photosynthetic oxygen consumption/evolution, chlorophyll a fluorescence in vivo, H2O2 production by the cells, antioxidative enzymes encoding genes expression were analyzed during light phase of the cell cycle.We have found, that DCF and ATR affect C. reinhardtii through different mechanisms. ATR inhibited the photosynthetic electron transport chain and induced oxidative stress in chloroplast. Such chloroplastic energetics disruption indirectly influenced respiration, the intensification of which could partially mitigate low efficiency of photosynthetic energy production. As a result, ATR inhibited the growth of single cell leading to limitation in C. reinhardtii population development. In contrast to ATR-treated algae, in DCF-treated cells the fraction of active PSII reaction centers was diminished without drastic changes in electron transport or oxidative stress symptoms in chloroplast. However, significant increase in transcript level of gene encoding for mitochondria-located catalase indicates respiratory processes as a source of H2O2 overproduced in the DCF-treated cells. Because the single cell growth was not strongly affected by DCF, its adverse effect on progeny cell number seemed to be related rather to arresting of cell divisions. Concluding, although the DCF phytotoxic action appeared to be different from the action of the typical herbicide ATR, it can act as algal growth-inhibiting factor in the environment.

Symbiotic Photosynthetic Oxygenation within 3D-Bioprinted Vascularized Tissues

In this study, we present the photosynthetic oxygen (O2) supply to mammalian cells within a volumetric extracellular matrix-like construct, whereby a three-dimensional (3D)-bioprinted fugitive pattern encapsulating unicellular green algae, Chlamydomonas reinhardtii (C. reinhardtii), served as a natural photosynthetic O2-generator. The presence of bioprinted C. reinhardtii enhanced the viability and functionality of mammalian cells while reducing the hypoxic conditions within the tissues. We were able to subsequently endothelialize the hollow perfusable microchannels formed after enzymatic removal of the bioprinted C. reinhardtii-laden patterns from the matrices following the initial oxygenation period, to obtain biologically relevant vascularized mammalian tissue constructs. The feasibility of co-culture of C. reinhardtii with human cells, the printability and the enzymatic degradability of the fugitive bioink, as well as the exploration of C. reinhardtii as a natural, eco-friendly, cost-effective, and sustainable source of O2 would likely promote the development of engineered tissues, tissue models, and food for various applications.

Rapid assessment of heavy metal-induced toxicity in water using micro-algal bioassay based on photosynthetic oxygen evolution

Toxicity assessment using microalgae adopted various endpoint measurements like mortality rate, photosynthetic activity, chlorophyll content, enzymatic activity, cell density, &lt;sup&gt;14&lt;/sup&gt;C assimilation, and phosphate uptake. These algal toxicity tests usually require 3-4 days of exposure time and laborious work to measure these endpoints parameters. In the present study, we described a simple and rapid toxicity assessment procedure using photosynthetic oxygen evolution as an endpoint measurement to determine heavy metal-induced toxicity. Oxygen evolution in gaseous phase was measured over a 12 h exposure time. The toxicity of six selected heavy metals was assessed. Concentrations of 1.02, 1.91, 0.46, 3.2, 7.5, and 65.6 mg/L were obtained as EC50 values for mercury, silver, cyanide, copper, cadmium, and hexavalent chromium, respectively, for microalgae (Chlorella vulgaris). Our results demonstrated that hexavalent chromium is less sensitive depending on the conditions (low exposure time, pH 6, and high initial biomass). Toxicities in decreasing order of strength were cyanide &gt; mercury &gt; silver &gt; copper &gt; cadmium &gt; hexavalent chromium. Results obtained in the current and earlier studies demonstrate that photosynthetic oxygen evolution offers an alternative endpoint measurement to assess toxicity since it allows sensitive and rapid detection of heavy metal-induced toxicity in water.