Megha Thakkar

Cytotoxicity effects of water dispersible oxidized multiwalled carbon nanotubes on marine alga, Dunaliella tertiolecta.

The multiwalled carbon nanotubes (MWNTs) are novel materials with many potential applications. The ecotoxicity of these materials is not well studied, but it is essential for environmental impact assessments. In this study a commercially available MWNT material was carboxylated by microwave assisted acid oxidation. This functionalized MWNT (f-MWNT) material was examined for toxicity effects using unicellular marine green alga Dunaliella tertiolecta. D. tertiolecta was exposed to f-MWNT which had been pre-equilibrated with culture media for 24 h. Substantial growth lag phase was observed at 5 and 10 mgL(-1) f-MWNT, and the resulting 50% effective concentration (EC50) on 96-h growth was 0.82 ± 0.08 mgL(-1). During mid-exponential growth phase cytotoxicity was evidenced at 10 mgL(-1) f-MWNT in 36% reduction in exponential growth rate, 88 mV more positive glutathione redox potential (indicative of oxidative stress), 5% and 22% reduction in photosystem II (PSII) quantum yield and functional cross section respectively, all relative to the control cultures. However, when the large f-MWNT aggregates in the media with 10 mgL(-1) f-MWNT were removed by 0.2 μm filtration, D. tertiolecta did not show significant cytotoxicity effects in any of the above parameters. This suggests that the cytotoxicity effects originated predominantly from the large f-MWNT aggregates. Analysis of the f-MWNT aggregation dynamics suggests active interaction between f-MWNT and algal cells or cell metabolites that promoted f-MWNT aggregation formation. The f-MWNT particles were also found absorbed on algal cell surface. The direct contact between f-MWNT and cell surface was likely responsible for reduced PSII functional cross section and oxidative stress during exponential growth.

Comparative responses of two species of marine phytoplankton to metolachlor exposure.

Metolachlor, a chloroacetanilide herbicide, has been frequently detected in coastal waters. This study examined the growth, photosynthesis, and detoxification responses of chlorophyte Dunaliella tertiolecta (DT) and brown tide alga Aureococcus anophagefferens (AA) upon 5-day exposure to 0.5-5 mg L(-1) metolachlor. Growth was assessed with exponential growth rate, and 5th day in vivo chlorophyll fluorescence, chlorophyll a, b or c, cell density and cell size. The photosynthesis function was assessed with photochemical parameters of photosystem II (PSII) during the mid-exponential growth phase (i.e. 2-4 day metolachlor exposure). The biochemical detoxification was analyzed with glutathione production and metolachlor degradation. Results show that metolachlor caused up to ∼9% inhibition in growth rate in both species and an expected ∼35% and 25% inhibition in chlorophyll based endpoints in DT and AA respectively. DT had an up to 70% inhibition in cell density, but AA a 35% hormesis at 1 mg L(-1) metolachlor and no significant inhibition, as compared to the controls. Both DT and AAs cell sizes were enlarged by metolachlor exposure, but greater in DT (1.2% per mg L(-1)) than in AA (0.68% per mg L(-1)). On PSII photochemistry, maximum quantum yield was not affected in both species; PSII optical cross section and connectivity factor increased in DT but decreased in AA, suggesting species specific impact on PSII function. On detoxification responses, glutathione production, when normalized to total chlorophyll a, was not affected by metolachlor in both species; further, despite of heterotrophic capacity of A. anophagefferens metolachlor was not significantly degraded by this alga during the 5-day incubation. The species specific effects on algal growth have ecological implications of potential selective inhibition of chlorophytes by metolachlor herbicide.

Water defluoridation using a nanostructured diatom–ZrO2 composite synthesized from algal Biomass

Frustules or the rigid amorphous silica cell wall of unicellular, photosynthetic microalgae with unique porous architecture has been used to synthesize a composite by immobilizing ZrO2 on its surface and in the pores. This was effective in water defluoridation. The average diameter of the composite was 80±2 nm and surface area was 140 m(2)/g. The adsorption isotherms followed both Langmuir and Freundlich models, and the composite was regenerable. Adsorption kinetics followed second order model and the adsorption capacity was as high as 11.32 mg/g, while the Langmuir maximum adsorption capacity (q(m)) reached 15.53 mg/g. The research findings highlight the potential of diatoms as hosts for nanomaterials for use in water treatment.

Synthesis of diatom-FeOx composite for removing trace arsenic to meet drinking water standards.

This study presents the synthesis of diatom-FeOx composite as a novel sorbent for arsenic removal from water. The unique porous architecture of the diatom was utilized to immobilize iron oxide to form the composite. The surface area was as high as 70 m(2)/g. The adsorption isotherms for As (III) and As (V) followed the Langmuir, Freundlich and D-R models. Langmuir monolayer adsorption capacity for arsenite (As III) was 10,000 μg/g and arsenate (As V) was 12,500 μg/g. The pseudo-second order rate equation was found to effectively describe the kinetics of arsenic adsorption. This study opens the door for the development of bio derived materials for environmental remediation.

Effects of anodic oxidation of a substoichiometric titanium dioxide reactive electrochemical membrane on algal cell destabilization and lipid extraction.

Efficient algal harvesting, cell pretreatment and lipid extraction are the major steps challenging the algal biofuel industrialization. To develop sustainable solutions for economically viable algal biofuels, our research aims at devising innovative reactive electrochemical membrane (REM) filtration systems for simultaneous algal harvesting and pretreatment for lipid extraction. The results in this work particularly demonstrated the use of the Ti4O7-based REM in algal pretreatment and the positive impacts on lipid extraction. After REM treatment, algal cells exhibited significant disruption in morphology and photosynthetic activity due to the anodic oxidation. Cell lysis was evidenced by the changes of fluorescent patterns of dissolved organic matter (DOM) in the treated algal suspension. The lipid extraction efficiency increased from 15.2 ± 0.6 g-lipidg-algae(-1) for untreated algae to 23.4 ± 0.7 g-lipidg-algae(-1) for treated algae (p<0.05), which highlights the potential to couple algal harvesting with cell pretreatment in an integrated REM filtration process.

Applicability of Hydrogen Peroxide in Brown Tide Control – Culture and Microcosm Studies

Brown tide algal blooms, caused by the excessive growth of Aureococcus anophagefferens, recur in several northeastern US coastal bays. Direct bloom control could alleviate the ecological and economic damage associated with bloom outbreak. This paper explored the effectiveness and safety of natural chemical biocide hydrogen peroxide (H2O2) for brown tide bloom control. Culture studies showed that H2O2 at 1.6 mg L−1 effectively eradicated high density A. anophagefferens within 24-hr, but caused no significant growth inhibition in the diatoms, prymnesiophytes, green algae and dinoflagellates of >2–3 μm cell sizes among 12 phytoplankton species tested over 1-week observation. When applied to brown tide bloom prone natural seawater in a microcosm study, this treatment effectively removed the developing brown tide bloom, while the rest of phytoplankton assemblage (quantified via HPLC based marker pigment analyses), particularly the diatoms and green algae, experienced only transient suppression then recovered with total chlorophyll a exceeding that in the controls within 72-hr; cyanobacteria was not eradicated but was still reduced about 50% at 72-hr, as compared to the controls. The action of H2O2 against phytoplankton as a function of cell size and cell wall structure, and a realistic scenario of H2O2 application were discussed.

Effect on Growth, Photosynthesis, and Oxidative Stress of Single Walled Carbon Nanotubes Exposure to Marine Alga Dunaliella tertiolecta

Single walled carbon nanotubes were carboxylated by microwave assisted acid oxidation (f-SWCNTs) and examined for their ecotoxicity on marine alga chlorophyte Dunaliella tertiolecta. Toxicity was evaluated based on growth, photosynthetic activities, oxidative stress, and intracellular glutathione in the concentration range of 0.1-20 mg/L f-SWCNT. Physical interactions between the f-SWCNT and alga were examined using light microscopy and scanning electron microscope. Increasing the nanotube concentration increased the toxic effects where growth inhibition was as high as 30%, photosynthetic yield decreased by as much as 18%, and intracellular glutathione reduction reached 95%. The results from f-SWCNTs were somewhat different when compared to our previous study using the same algae and functionalized multiwalled carbon nanotubes, where exposure led to longer lag phase and higher growth rate inhibition.

Effect of algal growth phase on Aureococcus anophagefferens susceptibility to hydrogen peroxide

A cells growth phase could affect its susceptibility to a biocide in microbial control. This study examines the growth phase dependent susceptibility of a brown tide bloom alga Aureococcus anophagefferens to microbial biocide hydrogen peroxide (H2O2). Test cultures of A. anophagefferens cells in exponential and stationary growth phase and similar initial cell density (1.6×10(6) cells mL(-1)) were exposed to 0.4-1.6 mg L(-1) H2O2. Changes in algal growth (in vivo fluorescence, total chlorophyll a, and cell density), cell physiology (maximum quantum yield of photosystem II, and total intracellular non-protein thiols), and H2O2 decomposition were quantified. Results show that the stationary phase cells are more susceptible to H2O2 than the exponential phase cells, and this is attributed to the weaker ROS (reactive oxygen species) scavenging system and consequently greater cell damage in stationary phase cells. The stationary phase cells potentially require 30-40% less H2O2 to reach 90% removal within 12 h of treatment as compared to the exponential phase cells. The results have practical implications in brown tide bloom control with respect to the timing and the dosage of H2O2 application.

Effect of Carbon Nanotube-Metal Hybrid Particle Exposure to Freshwater Algae Chlamydomonas reinhardtii

We demonstrate for the first time the toxicity of carbon nanotube (CNT) metal hybrids on freshwater algae. Carbon nanotube-silver (CNT-Ag) and platinum hybrids (CNT-Pt) were synthesized and exposed to Chlamydomonas reinhardtii (C. reinhardtii), and their toxicity was compared to the pure metal salts. Interactions between CNT-metal and algae were studied using electron microscopy and it was observed that while outer membrane of the algal cell was damaged as a result of Ag+ toxicity from pure Ag, the CNT-Ag only caused the distortion of the cell wall. It was also observed that the CNT-Ag particles could be internalized and enclosed in internal vesicles in the algal cells. Long-term exposure of the CNT-metals showed delay in algal growth. CNT-Ag at a concentration of 5.0 mg/L showed 90% growth inhibition and also showed a significant effect on photosynthetic yield with a 21% drop compared to the control. It was observed that pure silver was more toxic compared with CNT-Ag for both growth and photosynthesis in the 96-hour exposure. In general, CNT-Pt showed significantly less toxic effects on the algae than CNT-Ag. Based on this study, it is postulated that the CNT suppressed the release of Ag+ from CNT-Ag hybrids, thus reducing overall toxicity.

Novel diatom-FeOx composite as highly active catalyst in photodegradation of Rhodamine-6G

Abstract We report the study of nanoporous silica-iron oxide composite generated from diatom frustules as a highly active catalyst for the photodegradation of the dye Rhodamine-6G. The unique architecture and high surface area of diatoms were utilized to immobilize iron oxide on their surface to form the composite. Photodegradation was carried out under 365-nm radiation and was observed using the absorption spectrum of the dye. The reaction was found to follow pseudo-first-order kinetics. The results were compared with commercially available granular iron oxide. The rate constant K (min−1) for photodegradation by the diatom composite was found to be as high as 0.0584 min−1 for diatom-FeOx composites, which is 52% higher than 0.0273 min−1 for granular FeOx at a dye concentration of 0.02 mm. The unique structural morphology and the synthetic strategy have led to the composites showing superior activity in the degradation of the dye Rhodamine-6G.