Nutritional and ecotoxicological aspects of the acidotolerant alga Keratococcus rhaphidioides (Chlorophyta): a potential candidate for algal mediated bioremediation of extremely acidic waters

Abstract

The green alga Keratococcus rhaphidioides was isolated in axenic culture from water samples of the extremely acid Lake Caviahue (Neuquén, Argentina). The lake pH is 3.0 and K. rhaphidioides is tolerant to conditions such as the very low pH and the high concentrations of different elements. In this work, bioassays were done to study the tolerance of the alga to different pH values and high contents of Fe, Al, and Mn combined with two different chelators, fulvic acids and nitrilotriacetic acid; in addition to the ability to grow with different organic and inorganic carbon, nitrogen and phosphorus sources. In addition to the ability to grow with different organic and inorganic carbon, nitrogen and phosphorus sources. Keratococcus rhaphidioides optimum pH range was 3.0 to 4.0 with a sub-optimum range from pH 5.0 to 7.0. Growth was completely inhibited at pH 2.0. The alga can grow on inorganic CO2, glucose, and acetate, while urea and amino acids did not work as carbon sources in axenic culture. Inorganic nitrogen such as nitrate and ammonium and organic nitrogen sources like urea and leucine induced algal growth, whereas nitrite and aspartic acid had an inhibitory effect. Aluminum had toxic effects when combined with both organic chelators, nitrilotriacetic acid and fulvic acids. Iron induced inhibition only with the latter. Finally, the alga was grown in a photobioreactor under the optimum conditions determined during this work with continuous air bubbling. Algal biomass production was 10 times higher than in the nutritional assays although the time frame was also larger. In summary, the broad nutritional and pH spectrum and the tolerance to low light intensities and to metals in acid medium make K. rhaphidioides a good prospect for acid effluents bioremediation.