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- Metal uptake by microalgae: underlying mechanisms and practical applicationsPublication . Monteiro, Cristina M.; Castro, Paula M. L.; Malcata, F. XavierMetal contamination of a few aquatic, atmospheric, and soil ecosystems has increased ever since the industrial revolution, owing to discharge of such elements via the effluents of some industrial facilities. Their presence to excessive levels in the environment will eventually lead to serious health problems in higher animals owing to accumulation throughout the food web. Current physicochemical methods available for recovery of metal pollutants (e.g., chemical precipitation, oxidation/reduction, or physical ion exchange) are either expensive or inefficient when they are present at very low concentrations. Consequently, removal of toxic metals by microorganisms has emerged as a potentially more economical alternative. Microalgae (in terms of both living and nonliving biomass) are an example of microorganisms suitable to recover metals and able to attain noteworthy percent removals. Their relatively high metal-binding capacities arise from the intrinsic composition of their cell walls, which contain negatively charged functional groups. Consequently, microalgal cells are particularly efficient in uptake of those contaminants when at low levels. Self-defense mechanisms developed by microalgal cells to survive in metal-containing media and environmental factors that affect their removal (e.g., pH, temperature, and biomass concentration) are reviewed here in a comprehensive way and further discussed in attempts to rationalize this form of remediation vis-a-vis with conventional nonbiological alternatives
- Biosorption of zinc ions from aqueous solution by the microalga Scenedesmus obliquusPublication . Monteiro, Cristina M.; Castro, Paula M. L.; Malcata, F. XavierAquatic environments are often exposed to toxic heavy metals, which gain access to the food chain via microalgae and may cause severe problems at higher trophic levels. However, such a metabolic specificity can be taken advantage of in bioremediation strategies. The potential of a novel wild strain of Scenedesmus obliquus, previously isolated from a heavy metal-contaminated site in northern Portugal, to remove Zn from aqueous solutions was thus studied, using several initial concentrations. The removal extent reached its maximum by 1 day: 836.5 mg Zn/g biomass, at the initial concentration of 75 mg/L, mainly by adsorption onto the cell surface. Comparative studies encompassing a commercially available strain of the same microalgal species led to a maximum removal extent of only 429.6 mg Zn/g biomass, under identical conditions. Heat-inactivated cells permitted a maximum removal of 209.6 mg Zn/g biomass, at an initial concentration of 50 mg Zn/L. The maximum adsorption capacity of Zn, estimated via Langmuir's isotherm, was 330 mg Zn/g biomass. Finally, Zn removal was highest at pH 6.0-7.0. It was proven, for the first time, that such a wild microalga can uptake and adsorb Zn very efficiently, which unfolds a particularly good potential for bioremediation. Its performance is far better than similar (reference) species, especially near neutrality, and even following heat-treatment.
- Cadmium removal by two strains of desmodesmus pleiomorphus cellsPublication . Monteiro, Cristina M.; Castro, Paula M. L.; Malcata, F. XavierThe capacity of microalgae to accumulate heavy metals has been widely investigated for its potential applications in wastewater (bio)treatment. In this study, the ability of Desmodesmus pleiomorphus (strain L), a wild strain isolated from a polluted environment, to remove Cd from aqueous solutions was studied, by exposing its biomass to several Cd concentrations. Removal from solution reached a maximum of 61.2 mg Cd g−1 biomass by 1 day, at the highest initial supernatant concentration used (i.e., 5.0 mg Cd L−1), with most metal being adsorbed onto the cell surface. Metal removal by D. pleiomorphus (strain ACOI 561), a commercially available ecotype, was also assessed for comparative purposes; a removal of 76.4 mg Cd g−1 biomass was attained by 1 day for the same initial metal concentration. Assays for metal removal using thermally inactivated cells were also performed; the maximum removal extent observed was 47.1 mg Cd g−1 biomass, at the initial concentration of 5 mg Cd L−1. In experiments conducted at various pH values, the highest removal was achieved at pH 4.0. Both microalga strains proved their feasibility as biotechnological tools to remove Cd from aqueous solution.
- Metabolic relationships between macro- and micronutrients, and the eicosapentaenoic acid and docosahexaenoic acid contents of Pavlova LutheriPublication . Carvalho, Ana P.; Pontes, Isabel; Gaspar, Hugo; Malcata, F. XavierThe use of specific microalgae as sources of polyunsaturated fatty acids (PUFA), for incorporation in either aquaculture or human diets, has been receiving increasing interest. The fatty acid profile of intracellular fats in a given species can, within certain limits, be modulated via manipulation of the culture growth conditions, namely nutrient availability. In attempts to shed further light on the effects of chemical parameters upon microalgal metabolism, correlations between the components of a medium and the resulting biomass yield, as well as the eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contents are presented, using Pavlova lutheri as model system. Although several studies of individual nutrients have been published previously, the effects of variation of all medium components upon fatty acid production are systematically presented here, apparently for the first time. The optimum formulation to optimise EPA and DHA contents should consider the buffering agent, since some of them are toxic to the cells; P. lutheri was able to use several nitrogen sources, yielding equivalent amounts of EPA and DHA; synthesis of EPA was induced in culture media under low salinity, whereas a trend could be noticed between decreasing yields of EPA and increasing concentrations of N, at 0.15 g L−1 of P. Regarding the micronutrient composition, a culture medium lacking sulphur was unable to sustain cellular growth, whereas cultures deprived of either calcium, iron or manganese revealed a statistically significant decrease in cellular yields relative to those possessing such minerals; conversely, those cultures without boron produced higher cell numbers than those containing this element. Media deprived of boron, molybdenum or copper led to increases of the relative amounts of both EPA and DHA, whereas calcium deficiency decreased the levels of those PUFA.
- Carbon dioxide and microalgae: its influence on growth and fatty acid composition of phaeodactylum tricornutumPublication . Carvalho, Ana P.; Meireles, Luís A.; Malcata, F. Xavier
- Supercritical fluid extraction of carotenoids and chlorophylls a, b and c, from a wild strain of Scenedesmus obliquus for use in food processingPublication . Guedes, A. Catarina; Gião, Maria S.; Matias, Ana A.; Nunes, Ana V. M.; Pintado, Manuela E.; Duarte, Catarina M. M.; Malcata, F. XavierFaster and more selective extraction methods are urged to recover food-grade pigments and antioxidants from microalgae - which do not resort to food-incompatible and environment-unfriendly solvents. Hence, this study ascertained the influence of pressure, temperature, CO2 flow rate and a polar co-solvent upon the yields of carotenoids and chlorophylls in supercritical fluid extraction of Scenedesmus obliquus biomass. The highest carotenoid yield was attained at 250 bar and 60 degrees C. The yields of chlorophylls, when using plain CO2, increased slightly with pressure, but decreased with temperature and CO2 flow rate; the highest yield of chlorophyll a was at 4.3 g(CO2), min(-1), whereas ethanol as co-solvent increased all yields except that of chlorophyll c. The highest ratio of total carotenoids to chlorophyll a was reached at 250 bar and 60 degrees C. A remarkable selectivity was observed under these operating conditions, which may enable easy separation and purification of the aforementioned pigments.
- Characterization of desmodesmus pleiomorphus isolated from a heavy metal-contaminated site: biosorption of zincPublication . Monteiro, Cristina M.; Marques, Ana P. G. C.; Castro, Paula M. L.; Malcata, F. XavierMicroalgae have been proven efficient biological vectors for heavy metal uptake. In order to further study their biosorption potential, a strain of Desmodesmus pleiomorphus (L) was isolated from a strongly contaminated industrial site in Portugal. Under different initial Zn2? concentrations, metal removal by that strain reached a maximum of 360 mg Zn/g biomass after 7 days, at 30 mg Zn/l, after an initial rapid phase of uptake. Comparative studies were carried out using a strain of the same microalgal species that is commercially available (ACOI 561): when exposed to 30 mg Zn/l, it could remove only 81.8 mg Zn/g biomass. Biosorption experiments using inactivated biomass of the isolated strain reached a maximum Zn2? uptake of 103.7 mg/g. Metal removal at various initial pH values was studied as well; higher removal was obtained at pH 5.0. The microalga strain L, isolated from the contaminated site, exhibited a much higher removal capacity than the commercial strain, and the living biomass yielded higher levels of metal removal than its inactivated form.
- Effect of culture media on production of polyunsaturated fatty acids by pavlova lutheriPublication . Carvalho, Ana P.; Malcata, F. XavierUse of microscopic algae as sources of polyunsaturated fatty acids for incorporation as feed additive in aquaculture has received increasing interest owing to beneficial effects upon growth rates of zooplankton. However, the biochemical composition of marine algae is a function of the composition of the culture medium. Pavlova lutheri is particularly interesting because it is able to modulate the fatty acid profile of its lipidic fraction when cultured with various nutrient media. Three enriched seawater media (i.e. GPM, f/2 and MN) and one synthetic medium (i.e. ASW), covering considerable quantitative and qualitative ranges of nutrients, were tested in order to determine the medium compositions that stimulate production of such polyunsaturated fatty acids as EPA and DHA, and the biochemical compositions of the algae grown in each were monitored accordingly. Relationships between the mineral components of the medium and the resulting biomass yield, as well as the biochemical profile and the polyunsaturated fatty acid productivity, are tentatively presented in attempts to shed further light on the effects of the processing environment upon microalgae. Statistical analyses of the experimental results have indicated that GPM provides the best compromise in terms of cell yield (12.1 × 106 cell mL−1) and polyunsaturated fatty acid productivities (0.502 mg l−1 d−1 and 0.228 mg l−1 d−1 in EPA and DHA, respectively). In addition, GPM is a simple medium, mainly composed of seawater, nitrogen and phosphorus, thus having a low production cost and being easy to prepare in industrial settings, so it is suitable for general use in aquaculture of marine species.
- Toxicity of cadmium and zinc on two microalgae, Scenedesmus obliquus and Desmodesmus pleiomorphus, from Northern PortugalPublication . Monteiro, Cristina M.; Fonseca, Susana C.; Castro, Paula M. L.; Malcata, F. XavierAquatic environments often contain toxic heavy metals that may enter the food web via uptake by microalgae and eventually cause severe poisoning problems at higher trophic levels. The effects of Cd and Zn cations upon growth of two native green microalgal species, Scenedesmus obliquus and Desmodesmus pleiomorphus (previously isolated from a polluted site in Northern Portugal), were accordingly evaluated. Growth inhibition of the microalgal cells was determined following exposure for 96 h to several initial concentrations of aqueous solutions of either of those two metals. At the higher end of Cd and Zn experimental concentration ranges, a significant reduction in cell density was observed in the cultures; EC50 values, calculated after fitting a Weibull model to the experimental data, were 0.058 and 1.92 mg L−1 for Cd and 16.99 and 4.87 mg L−1 for Zn in the case of S. obliquus and D. pleiomorphus, respectively. One observed that S. obliquus can tolerate higher Zn concentrations than D. pleiomorphus, but the reverse holds regarding exposure to Cd.
- Metabolic relationships between mineral composition of medium and polyunsaturated fatty acid profile of Pavlova lutheriPublication . Carvalho, Ana P.; Malcata, F. Xavier