Biocatalytic Chromium(VI) reduction by bacterial culture grown in symbiotic autotrophy with fresh water microalgae

Abstract

The ability to reduce Cr(VI) to Cr(III) has been discovered in multiple species of bacteria. This ability has manifested as either a detoxification strategy to ensure survival in Cr(VI) rich environments, or as a metabolic necessity due to other properties of the bacteria. Bacterial species that can reduce Cr(VI) to Cr(III), and can survive in Cr(VI) rich environments, and are called chromium reducing bacteria (CRB). In this study, several pure cultures of bacteria were isolated from sludge, wastewater and soil samples from a Cr(VI) contaminated site in Brits (South Africa). Colonies were tested individually for Cr(VI) tolerance and reduction capability. The locally isolated cultures proved successful in reducing Cr(VI) and were identified using 16S rRNA sequencing as Escherichia coli, Citrobacter sedlakii, and Bacillus thermoamylovorans. Cr(VI) reduction in bacteria is facilitated either by passive reaction systems of reaction such as chemical oxidation organic compounds or by enzymatic reactions catalysed by specially expressed Cr(VI) reducing enzymes biochemically classified as Cr(VI) reductases. Cr(VI) reduction in the presence of oxygen, although fast, does not yield energy for cell growth and metabolism, and is therefore cometabolic in nature. Cr(VI) under anaerobic conditions, on the other hand, is known to be dissimilatory in nature whereby Cr(VI) is used as the solo terminal electron sink in a process that yields energy for cell metabolism and growth. In this study Cr(VI) reduction experiments were conducted under aerobic conditions to simulate possible application in an open surface water body with algal growth. The rationale for the investigation was to develop a self-sustained bioremediation process for Cr(VI) reduction where carbon sources are produced internally. Such a system could be energy efficient and carbon-negative in nature. Algae offer a solution to the problem since they produce organic carbon from CO2 in the presence of sunlight as an energy source. Engineered algal cultivation has the benefit of not requiring diversion of agricultural land, as cultivation can take place in freshwater, marine, and brackish water environments. Additionally, algae cultivation can be used as a carbon sink to consume CO2 emitted from specific industrial processes. The freshwater algae used in this study were obtained from the Hartbeespoort Dam, an artificial reservoir located in Hartbeespoort (North West Province, South Africa). The algae were identified as Chlorococcum Ellipsoideum by 18S rRNA and 28S rRNA genotype fingerprinting followed by Basic Local Alignment Search Tool (BLAST) Search of the gene sequence in the National Center for Biotechnology Information (NCBI) database. Control algal cultures, Chlamydomonas reinhardtii and Tetradesmus obliquus, were purchased from the Culture Collection of Algae and Protozoa (CCAP) and China, respectively. In the algae-CRB system, the Cr(VI) reduction process in the batch experiment was determined to be enzyme mediated with minimal adsorption taking place. In the batch experiments, complete reduction and removal of Cr(VI) from solution was achieved in less than 24 hours in batches loading with an initial concentration of up to 50 mg/L. At 100 mg/L initial Cr(VI) concentration, 92% of the Cr(VI) was reduced within 24 hours. The algal species tested in this study provided carbon sources for bacterial growth with a resultant Cr(VI) reduction even though the process was mostly sacrificial with respect to the survival of algae. A biokinetic model was developed for the bacterial reduction of Cr(VI) in the algae-CRB system based on Michaelis-Menten or Monod model. Two apparent Cr(VI) reduction rates prevailed in the algae-CRB system, i.e., a rapid reduction rate, followed by a slow reduction rate. The kinetic parameters in the Cr(VI) reduction model was determined using the software AQUASIM 2.3. The predicted model was able to fit the experimental data well.