Perchlorate Reducing Bacteria: Evaluating the Potential for Growth Utilizing Nutrient Sources Identified on Mars

Introduction: Perchlorate reducing bacteria may serve as model organisms for potential life forms that could exist on Mars. Due to the high reduction potential of perchlorate it can be utilized in microbial metabolism as an electron acceptor [1]. Perchlorate was discovered to be present in martain surface materials (~0.5% wt.) using the Wet Chemistry Laboratory and the Thermal and Evolved Gas Analyzer on the Phoenix Lander [2]. An apparent widespread distribution of perchlorate on Mars was confirmed using the Sample Analysis at Mars (SAM) instrument suite on the Mars Curiosity Rover [3]. Nutrient sources that may support microbial metabolism on Mars potentially exist in the form of carbon as formate, nitrogen as nitrate, and phosphorus potentially as phosphate. Formic acid synthesis may occur on Mars due to UV irradiation of CO, CO2 and small quantities of water vapor [4]. The SAM investigation has detected oxidized nitrogen-bearing compounds on Mars and these results support nitrate in concentrations ranging from 110-1100 ppm [5]. The analysis of elemental composition of the “Paso Robles” evaporitic deposits on Husband Hill, using the Athena Science instrument package on Spirit, indicates the presence of Ca-phosphate [6]. To evaluate the potential for microbial growth through the utilization of these nutrients, perchlorate-reducing bacteria were cultured in media containing phosphate and different combinations of nitrogen and carbon sources, including formate and nitrate. Methods: Azospira suillum strain PS (formally Dechlorosoma suillum strain PS) was anaerobically grown in 4 different media treatments: 1) 5 mM ammonium chloride, 10 mM sodium acetate, 5 mM magnesium perchlorate, 5 mM monosodium phosphate, 7 mM disodium phosphate; 2) 8 mM sodium nitrate, 10 mM sodium acetate, 5 mM magnesium perchlorate, 5 mM monosodium phosphate, 7 mM disodium phosphate; 3) 10 mM sodium nitrate, 20 mM sodium formate, 5 mM monosodium phosphate, 7 mM disodium phosphate, 5 mM magnesium perchlorate; 4) 10 mM sodium nitrate, 10 mM sodium formate, 5 mM monosodium phosphate, 7 mM disodium phosphate, 5 mM magnesium perchlorate. Five replicates were done for all treatments. Growth curves of A. suillum strain PS for each treatment were obtained by measuring optical density at 600 nm using an Ocean Optics (HR4000CG-UVNIR) spectrometer and a halogen (HL-2000, Ocean Optics, Inc.) light source. To obtain growth curves, measurements were taken approximately every 24 hours over a 5-day period. Growth rates were determined using: