Adaptation of microalgae to lindane: a new approach for bioremediation

An insights of organochlorine pesticides categories, properties, eco-toxicity and new developments in bioremediation process

Organochlorine pesticides (OCPs) have been used in agriculture, increasing crop yields and representing a serious and persistent global contaminant that is harmful to the environment and human health. OCPs are typically bioaccumulative and persistent chemicals that can spread over long distances. The challenge is to reduce the impacts caused by OCPs, which can be achieved by treating OCPs in an appropriate soil and water environment. Therefore, this report summarizes the process of bioremediation with commercially available OCPs, considering their types, impacts, and characteristics in soil and water sources. The methods explained in this report were considered to be an effective and environmentally friendly technique because they result in the complete transformation of OCPs into a non-toxic end product. This report suggests that the bioremediation process can overcome the challenges and limitations of physical and chemical treatment for OCP removal. Advanced methods such as biosurfactants and genetically modified strains can be used to promote bioremediation of OCPs.

Contrasting sensitivity among oligotrophic marine microbial communities to priority PAHs

Oligotrophic areas represent a large proportion of the oceans, wherein microbial food webs largely determine carbon flux dynamics and biogeochemical cycles. However, little is known regarding the sensitivity of microbial planktonic communities to pollutants in such areas. Organic pollutants such as polycyclic aromatic hydrocarbons (PAH/s) are toxic oil derivatives that occur as complex mixtures and reach marine environments through different sources. Therefore, our study analyzed the PAH tolerance of natural photosynthetic and heterotrophic bacteria and eukaryotes from the oligotrophic Red Sea, which is uniquely susceptible to high oil contamination. Natural communities sampled from the surface layer were exposed to a concentration gradient of a mixture of 16 priority PAHs at in situ conditions for 48 h. The populations of the dominant picocyanobacteria Synechococcus sp., picophytoeukaryotes, and low nucleic acid (LNA) bacteria decreased upon exposure to PAHs in a strong dose-dependent manner. Chlorophyll-a, which was measured as an indicator of the total autotrophic community response, also decreased substantially. High nucleic acid (HNA) bacteria, however, exhibited lower growth inhibition (<50%). The lethal concentration (LC₁₀) thresholds to the 16-PAH mixture demonstrated contrasting sensitivities among the microbial communities studied increasing from picoeukaryotes (5.98 ± 2.08 μg L^-1) < chlorophyll-a (19.51 ± 8.11 μg L^-1) < LNA bacteria (23.63 ± 10.64 μg L^-1) < Synechococcus sp. (26.77 ± 13.34 μg L^-1) < HNA bacteria (97.13 ± 17.28 μg L^-1). The sensitivity of Red Sea Synechococcus and picophytoeukaryotes to the 16-PAH mixture was between 2 and 6.5 times higher compared to single PAH compounds tested previously. However, some populations of HNA bacteria and Synechococcus sp., were highly tolerant, suggesting an adaptation to chronic pollution. Concerningly, the LC₁₀ toxicity thresholds approached the ambient PAH concentrations in the Red Sea, suggesting that environmental oil pollution actively shapes the microbial community structures in the region.

Random Mutagenesis as a Promising Tool for Microalgal Strain Improvement towards Industrial Production

Microalgae have become a promising novel and sustainable feedstock for meeting the rising demand for food and feed. However, microalgae-based products are currently hindered by high production costs. One major reason for this is that commonly cultivated wildtype strains do not possess the robustness and productivity required for successful industrial production. Several strain improvement technologies have been developed towards creating more stress tolerant and productive strains. While classical methods of forward genetics have been extensively used to determine gene function of randomly generated mutants, reverse genetics has been explored to generate specific mutations and target phenotypes. Site-directed mutagenesis can be accomplished by employing different gene editing tools, which enable the generation of tailor-made genotypes. Nevertheless, strategies promoting the selection of randomly generated mutants avoid the introduction of foreign genetic material. In this paper, we review different microalgal strain improvement approaches and their applications, with a primary focus on random mutagenesis. Current challenges hampering strain improvement, selection, and commercialization will be discussed. The combination of these approaches with high-throughput technologies, such as fluorescence-activated cell sorting, as tools to select the most promising mutants, will also be discussed.

Contributions on Lindane Degradation by Microcystis aeruginosa PCC 7806

Cyanobacteria are able to tolerate, and even metabolize, moderate doses of organochlorine pesticides, such as lindane (γ-hexachlorocyclohexane), one of the most persistent and widely used in recent decades. Previous work showed that Microcystis aeruginosa PCC 7806 degrades lindane and that, in the presence of the pesticide, microcystin synthesis is enhanced. In this work, using in silico approaches, we have identified in M. aeruginosa putative homologues of the lin genes, involved in lindane degradation in Sphingobium japonicum UT26S. Real-time RT-PCR assays showed that the putative linC gene was induced in the presence of 7 mg/L of lindane. Additionally, prxA, encoding a peroxiredoxin, and involved in oxidative stress response, was also induced when lindane was present. Taking into account these results, M. aeruginosa PCC 7806 may degrade lindane through a metabolic pathway involving a putative 2,5-dichloro-2,5-cyclohexadiene-1,4-diol dehydrogenase encoded by a linC homologue. However, the low similarity of the other potential lin homologues suggest the existence of an alternative pathway different to that of heterotrophic microorganisms such as S. japonicum.

Recent advances in microalgae-based remediation of industrial and non-industrial wastewaters with simultaneous recovery of value-added products

The ability of microalgae to grow in a broad spectrum of wastewaters manifests great potentials for removing contaminants from effluents of industries and urban areas. Since the post-treatment microalgae biomass is also a significant source of high-value products, microalgae-based wastewater treatment is an economical and sustainable solution to wastewater management. Adding more value, the integration of microalgae with living/non-living materials looks more promising. Microalgae-based treatment technology has certain limitations like high operational costs, problematic harvesting, large land requirements, and hindrance in photosynthesis due to turbid wastewater. These challenges need to be essentially addressed to achieve enhanced wastewater remediation. This review has highlighted the potential applications of microalgae in contaminant removal from wastewaters, simultaneous resource recovery, efficient microalgae-based hybrid systems along with bottlenecks and prospects. This state-of-the-art article will edify the role of microalgae in wastewater remediation, biomass valorization for bio-based products, and present numerous possibilities in strengthening the circular bioeconomy.

Bioremediation of water containing pesticides by microalgae: Mechanisms, methods, and prospects for future research

The application of pesticides reduces the loss of crops while simultaneously increasing crop productivity. However, the frequent use of pesticides can cause serious environmental problems due to their high accumulative and persistent nature. Recently, microalgae technology has received considerable success in the efficient treatment of pesticides pollution. In this review, the metabolic mechanisms responsible for the removal of pesticides are summarized based on previous studies. Different methods used to enhance the ability of microalgae to remove pesticides are critically evaluated. The recycling of microalgae biomass after wastewater treatment for biochar preparation and biodiesel production using the biorefinery approach is also introduced. Furthermore, we present potential future research directions to highlight the prospects of microalgae research in the removal of pesticides along with the production of value-added products.

Occurrence, distribution and risk assessment of pesticides in a river-reservoir system

The water environment from river to reservoir has been considered as a hybrid river-reservoir system due to pronounced environmental properties. This study investigated the distribution and potential environmental behavior of pesticides in river-reservoir system, examining 31 target pesticides in water phase from a key drinking water source (Dongjiang River). The concentrations of ∑₈OCPs, ∑₁₆OPPs and ∑₇SPs with the corresponding occurrence were in the range of 107.57-340.35 ng/L (moderate level), 232.65-1197.95 ng/L (moderate level) and 125.23-245.09 ng/L (low level), respectively. Ecological risk assessments indicated that most of the pesticides posed a high level of risk to the aquatic organisms. Moreover, seasonal agricultural application, rainfall and temperature could influence the levels and compositions of ∑₈OCPs, ∑₁₆OPPs and ∑₇SPs in the river system, while seasonal hydrological processes could only influence their compositions in the reservoir system. In the wet season, the levels of ∑₈OCPs and ∑₇SPs decreased from the river system to reservoir system probably via biogeochemical processes, while the level of ∑₁₆OPPs decreased along the environmental gradient probably via dilution process. Additionally, longer hydraulic retention time of the reservoir system could contribute to the biogeochemical processes for OPPs and SPs exported from the river system, but it might fail to facilitate this process for OCPs. Taken together, this study highlighted that the distributions and environmental behavior of pesticides in river-reservoir system varied seasonally from river system to reservoir system.

Tolerance of cyanobacteria to the toxicity of BDE-47 and their removal ability

Polybrominated diphenyl ethers are ubiquitous and toxic contaminants in aquatic environments. The effect of polybrominated diphenyl ether BDE-47 on five species of cyanobacteria, along with their removal ability was investigated. Four species, namely Synechocystis sp., Oscillatoria planctonica, Microcystis flos-aquae and Nostoc sp., were exposed to BDE-47 at concentrations ranging from 0.05 to 1.0 mg L^-1 for 14 days, while the exposure time for Pseudanabaena sp. was 30 days. The first four species were very tolerant to BDE-47 while growth and photosynthesis of Pseudanabaena were significantly inhibited by BDE-47 at concentrations over 0.1 mg L^-1. However, this species could recover from the toxicity of high concentrations of BDE-47 after 30 days of exposure, indicating the development of some "resistance" after pre-exposure to 1.0 mg L^-1 BDE-47. The "resistant" cells had a higher growth rate, photosynthesis and glutathione S-transferase activity than normal Pseudanabaena cells. The sensitivity of Pseudanabaena to BDE-47 toxicity was affected by its initial filament density, with cultures having a low filament density (2.3 × 10⁶ filaments mL^-1) being up to 14-15 times more sensitive than cultures with a high filament density (13 × 10⁶ filaments mL^-1). All cyanobacteria could remove 70-82% of BDE-47 in their media, with more than 60% of BDE-47 accumulated in cells. This is the first study showing the high tolerance of different cyanobacteria species to BDE-47 toxicity and their removal ability. The study also revealed that the sensitive Pseudanabaena could acquire a "resistance" to BDE-47, which was transferred to the next generation.

Sensitivity of Scenedesmus obliquus and Microcystis aeruginosa to atrazine: effects of acclimation and mixed cultures, and their removal ability

Atrazine is an herbicide frequently detected in watercourses that can affect the phytoplankton community, thus impacting the whole food chain. This study aims, firstly, to measure the sensitivity of monocultures of the green alga Scenedemus obliquus and toxic and non-toxic strains of the cyanobacteria Microcystis aeruginosa before, during and after a 30-day acclimation period to 0.1 µM of atrazine. Secondly, the sensitivity of S. obliquus and M. aeruginosa to atrazine in mixed cultures was evaluated. Finally, the ability of these strains to remove atrazine from the media was measured. We demonstrated that both strains of M. aeruginosa had higher growth rate-based EC₅₀ values than S. obliquus when exposed to atrazine, even though their photosynthesis-based EC₅₀ values were lower. After being exposed to 0.1 µM of atrazine for 1 month, only the photosynthesis-based EC₅₀ of S. obliquus increased significantly. In mixed cultures, the growth rate of the non-toxic strain of M. aeruginosa was higher than S. obliquus at high concentrations of atrazine, resulting in a ratio of M. aeruginosa to total cell count of 0.6. This lower sensitivity might be related to the higher growth rate of cyanobacteria at low light intensity. Finally, a negligible fraction of atrazine was removed from the culture media by S. obliquus or M. aeruginosa over 6 days. These results bring new insights on the acclimation of some phytoplankton species to atrazine and its effect on the competition between S. obliquus and M. aeruginosa in mixed cultures.

Monitoring dihydrodiol polyaromatic hydrocarbon metabolites produced by the freshwater microalgae Selenastrum capricornutum

We found that microalgae exposed to a mixture of polycyclic aromatic hydrocarbons (PAHs) did not show growth inhibition. Thus, we assumed that they could metabolize these compounds. In this study, the dihydrodiol-type PAH metabolites of benzo(a)pyrene (BaP), benzo(a)anthracene (BaA), benzo(b)fluoranthene (BbF) and benzo(k)fluoranthene (BkF) produced by the freshwater microalgae Selenastrum capricornutum were monitored and quantified using high-performance liquid chromatography with fluorescence detection (HPLC-FD) techniques. Exposure bioassays with S. capricornutum were performed using a 266 ng mL(-1) mixture of PAHs at different exposure times (0.75, 1, 3, 8, 16, 24 and 48 h) under controlled temperature (25 °C); the dihydrodiol metabolites formed in the liquid medium and the biomass were quantified. Metabolite identities were confirmed using HPLC-mass spectrometry; most of the metabolites formed were derived from BaA degradation. At 48 h after exposure 5,6-dBaA and 8,9-dBaA/10,11-dBaA were present in the liquid medium at 20% and 67% of the initial mass of BaA, respectively. Three metabolites of BaP were monitored in the liquid medium and biomass and, at 24 h, 4,5-dBaP accounted for 19%; , 7,8-dBaP, 5%; and 9,10-dBaP, 5% relative to the initial BaP mass. Microalgae exposed to BbF showed the presence of 1,2-dBbF and 9,10-dBbF (at 0.3% and 0.1% of the initial BbF mass, respectively) and those exposed to BkF produced 8,9-dBkF (6.5% of the initial BkF mass) in the liquid medium. Seven unknown compounds were formed after exposure; two compounds were identified as the metabolites of BaA and BaP. The results could facilitate the elucidation of the controversial biodegradation mechanism in microalgae.

Batch vs continuous-feeding operational mode for the removal of pesticides from agricultural run-off by microalgae systems: A laboratory scale study

Microalgae-based water treatment technologies have been used in recent years to treat different water effluents, but their effectiveness for removing pesticides from agricultural run-off has not yet been addressed. This paper assesses the effect of microalgae in pesticide removal, as well as the influence of different operation strategies (continuous vs batch feeding). The following pesticides were studied: mecoprop, atrazine, simazine, diazinone, alachlor, chlorfenvinphos, lindane, malathion, pentachlorobenzene, chlorpyrifos, endosulfan and clofibric acid (tracer). 2L batch reactors and 5L continuous reactors were spiked to 10 μg L(-1) of each pesticide. Additionally, three different hydraulic retention times (HRTs) were assessed (2, 4 and 8 days) in the continuous feeding reactors. The batch-feeding experiments demonstrated that the presence of microalgae increased the efficiency of lindane, alachlor and chlorpyrifos by 50%. The continuous feeding reactors had higher removal efficiencies than the batch reactors for pentachlorobenzene, chlorpyrifos and lindane. Whilst longer HRTs increased the technology's effectiveness, a low HRT of 2 days was capable of removing malathion, pentachlorobenzene, chlorpyrifos, and endosulfan by up to 70%. This study suggests that microalgae-based treatment technologies can be an effective alternative for removing pesticides from agricultural run-off.

Effect of the pesticide lindane on the biomass of the microalgae Nannochloris oculata

This study assesses the growth of the microalgae Nannochloris oculata in the presence of lindane and the ability of N. oculata to remove lindane from media. Algal biomass increased with 0.1 and 0.5 mg L^-1 of lindane, and lindane concentrations in the media decreased. N. oculata removed 73% and 68.2% of lindane in the 0.1 and 0.5 mg L^-1 media concentrations, respectively. Algal biomass decreased to the level of the control at lindane concentrations greater than 2.5 mg L^-1, probably due to toxicity. N. oculata removed lindane from the media at concentrations lower than 1.0 mg L^-1. Thus, N. oculata may be useful for lindane bioremediation in contaminated aquatic systems.

Biodegradation of benzo(a)pyrene by two freshwater microalgae Selenastrum capricornutum and Scenedesmus acutus: a comparative study useful for bioremediation

A comparative evaluation of the removal of benzo(a)pyrene (BaP) by sorption and degradation by two microalgal species, Selenastrum capricornutum and Scenedesmus acutus was performed. The monitoring of the amount of BaP remaining in the liquid culture media and the biomass along with the appearance of three metabolites (4,5 dihydrodiol-BaP; 7,8-dihydrodiol-BaP; and 9,10 dihydrodiol-BaP) at short time periods (from 0.25 to 72 h) in cultures exposed to BaP was made by high-performance liquid chromatography (HPLC) with fluorescence and UV detection. Complete removal of BaP was achieved by the two live microalgal species: S. capricornutum at 15 h of exposure (99%) and S. acutus at 72 h of exposure (95%). Sorption is an important phenomenon for BaP removal by S. capricornutum but biodegradation is the principal means of removing BaP in live cells. The formation of metabolites by S. capricornutum is rapid and seems to be proportional to the amount of the BaP added to cultures. In contrast, in these bioassays, most of the BaP removal of S. acutus is due to sorption rather than degradation. The appearance of metabolites in the cultures is very slow and at a low amount compared to cultures of S. capricornutum. The similarities and differences existing between the two microalgae are important for the establishment of the conditions for bioremediation.

γ-Lindane Increases Microcystin Synthesis in Microcystis aeruginosa PCC7806

HCH factories, and the waste dumpsites associated to its production, have become a global environmental concern, and their runoff could pollute ground and surface waters with high levels of the pollutant. In this study, the influence of lindane (γ-HCH) on microcystin production has been investigated in Microcystis aeruginosa PCC7806. This toxic cyanobacterium is highly tolerant to γ-lindane (20 mg/L), and produces more toxin (microcystin) in the presence of the pollutant. Microcystis degrades γ-lindane and presence of γ-lindane induces genes involved in its own degradation (nirA). RT-PCRsq has been used to monitor changes in levels of transcripts encoded by the mcy operon (mcyD, mcyH and mcyJ), responsible for the microcystin synthesis machinery, as well as other genes involved in its transcriptional regulation, such as ntcA and fur family members. The presence of lindane in the culture media induces mcyD expression, as well as ntcA gene transcription, while other genes, such as mcyH, (putative ABC transporter), are downregulated. The amount of microcystin found in the cells and the culture media is higher when M. aeruginosa is treated with γ-lindane than in control cells. The results suggest that in a lindane polluted environment, Microcystis toxic strains may enhance their microcystin synthesis.

Bioremediation of the oil spill polluted marine intertidal zone and its toxicity effect on microalgae

Custom-designed devices with 0.6 m (L) × 0.3 m (W) × 0.4 m (H) and a microbial consortium were applied to simulate bioremediation on the oil spill polluted marine intertidal zone. After the bioremediation, the removal efficiency of n-alkanes and polycyclic aromatic hydrocarbon homologues in crude oil evaluated by GC-MS were higher than 58% and 41% respectively. Besides, the acute toxicity effects of crude oil on three microalgae, i.e. Dicrateria sp., Skeletonema costatum and Phaeodactylum tricornutum, varied with concentration. The effects of microbe and surfactant treated water on the three microalgae followed a decreasing order: the microbial consortium plus Tween-80 > the microbial consortium > Tween-80. During 96 h, the cell densities of the three microalgae in treated seawater increased from 4.0 × 10(5), 1.0 × 10(5) and 2.5 × 10(5) cells per mL to 1.7 × 10(6), 8.5 × 10(5) and 2.5 × 10(6) cells per mL, respectively, which illustrated that the quality of seawater contaminated by crude oil was significantly improved by the bioremediation.

Analytical investigations on the lindane bioremediation capability of the demosponge Hymeniacidon perlevis

Lindane is an organochlorine pesticide that has been widely used to treat agricultural pests. It is of particular concern because of its toxicity, persistence and tendency to bioaccumulate in terrestrial and aquatic ecosystems. In this context, we investigated the ability of the demosponge Hymeniacidon perlevis to bioremediate lindane polluted seawater during in vitro experimentation. Lindane was extracted by solid-phase micro-extraction (SPME) and determined by gas chromatography-mass spectrometry (GC-MS). Furthermore, we assessed the role exerted in lindane degradation by bacteria isolated from the sponge. Sponges showed low mortality in experimental conditions (lindane concentration 1 μg/L) and were able to remove about 50% of the lindane content from seawater in 48 h. Bacteria isolated from sponges showed a remarkable remediating capacity (up to 97% of lindane removed after 8-days). A lindane metabolite was identified, 1,3,4,5,6-pentachloro-cyclohexene. The results obtained are a prelude to the development of future strategies for the in situ bioremediation of this pollutant.

Photosynthesis at the forefront of a sustainable life

The development of a sustainable bio-based economy has drawn much attention in recent years, and research to find smart solutions to the many inherent challenges has intensified. In nature, perhaps the best example of an authentic sustainable system is oxygenic photosynthesis. The biochemistry of this intricate process is empowered by solar radiation influx and performed by hierarchically organized complexes composed by photoreceptors, inorganic catalysts, and enzymes which define specific niches for optimizing light-to-energy conversion. The success of this process relies on its capability to exploit the almost inexhaustible reservoirs of sunlight, water, and carbon dioxide to transform photonic energy into chemical energy such as stored in adenosine triphosphate. Oxygenic photosynthesis is responsible for most of the oxygen, fossil fuels, and biomass on our planet. So, even after a few billion years of evolution, this process unceasingly supports life on earth, and probably soon also in outer-space, and inspires the development of enabling technologies for a sustainable global economy and ecosystem. The following review covers some of the major milestones reached in photosynthesis research, each reflecting lasting routes of innovation in agriculture, environmental protection, and clean energy production.

Rapid adaptation of some phytoplankton species to osmium as a result of spontaneous mutations

To understand the vulnerability of individual species to anthropogenic contamination, it is important to evaluate the different abilities of phytoplankton to respond to environmental changes induced by pollution. The ability of a species to adapt, rather than its initial tolerance, is the basis for survival under rapidly increasing levels of anthropogenic contamination. High doses of osmium (Os) cause massive destruction of diverse phytoplankton groups. In this study, we found that the coastal chlorophyte Tetraselmis suecica and the continental chlorophyte Dictyosphaerium chlorelloides were able to adapt to a lethal dose of Os. In these species, Os-resistant cells arose as a result of rare spontaneous mutations (at rates of approximately 10(-6) mutants per cell division) that occurred before exposure to Os. The mutants remained in the microalgal populations by means of mutation-selection balance. The huge size of phytoplankton populations ensures that there are always enough Os-resistant mutants to guarantee the survival of the population under Os pollution. In contrast, we observed that neither a haptophyte species from open ocean regions nor a cyanobacterium from continental freshwater were able to adapt to the lethal Os dose. Adaptation of phytoplankton to Os contamination is relevant because industrial activities are leading to a rapid increase in Os pollution worldwide.

Rapid adaptation of microalgae to bodies of water with extreme pollution from uranium mining: an explanation of how mesophilic organisms can rapidly colonise extremely toxic environments

Extreme environments may support communities of microalgae living at the limits of their tolerance. It is usually assumed that these extreme environments are inhabited by extremophile species. However, global anthropogenic environmental changes are generating new extreme environments, such as mining-effluent pools of residual waters from uranium mining with high U levels, acidity and radioactivity in Salamanca (Spain). Certain microalgal species have rapidly adapted to these extreme waters (uranium mining in this area began in 1960). Experiments have demonstrated that physiological acclimatisation would be unable to achieve adaptation. In contrast, rapid genetic adaptation was observed in waters ostensibly lethal to microalgae by means of rare spontaneous mutations that occurred prior to the exposure to effluent waters from uranium mining. However, adaptation to the most extreme conditions was only possible after recombination through sexual mating because adaptation requires more than one mutation. Microalgae living in extreme environments could be the descendants of pre-selective mutants that confer significant adaptive value to extreme contamination. These "lucky mutants" could allow for the evolutionary rescue of populations faced with rapid environmental change.

Multigenerational exposure of the microalga Tetraselmis suecica to diuron leads to spontaneous long-term strain adaptation

To investigate the ability of microalgae to develop stable, long-term resistance to herbicides, the marine microalga Tetraselmis suecica was exposed to the herbicide diuron (5 μg/L) for a 43-generation exposure period followed by a 12-generation depuration phase. During the first 25 generations, diuron-exposed cultures showed doubling times ranging from 1.95 to 2.6 days, which was 2 to 2.5-fold longer than control cultures. Between generations 25 and 38, during diuron exposure, two out of the three exposed cultures exhibited a spontaneous drop in doubling time. These results provided evidence of culture adaptation to diuron. To assess persistence of the diuron adaptation observed on growth performance, one of the adapted cultures (D3) was maintained for 12 months in unexposed conditions and then tested by a second, short-term exposure to diuron 5 μg/L, in parallel with a control culture (C1) for six generations. Flow cytometry analyses were used to monitor cell density, viability, morphology, relative chlorophyll content and intracellular reactive oxygen species (ROS) level. Under these conditions, diuron induced a strong increase of doubling time in exposed-C1 cultures (2.5-fold longer than unexposed-C1 cultures), but no significant increase occurred in exposed D3-cultures compared with unexposed D3- and unexposed C1-cultures, showing the persistence of adaptation in the previously-exposed strain D3. Intracellular ROS level showed the same trend. Significant differences were observed between these strains, with weaker effects of diuron on strain D3 compared with strain C1: forward scatter (FSC), representing relative cell size, decreased in exposed cultures (67.8% and 95% of the controls for C1 and D3, respectively), whereas FL3 as relative chlorophyll content increased in exposed cultures (115.6% and 108.6% of the controls for C1 and D3, respectively). Results of second exposure to diuron revealed that the adaptation of strain D3 had persisted after 12 months of depuration, as no growth impairment was observed. This study demonstrates the possible appearance of stable diuron resistance in microalgae in cases of strong, multigenerational chronic exposure to this herbicide in polluted environments.