BIOENERGETICS OF AEROBIC GROWTH BY SHEWANELLA PUTREFACIENS STRAIN CN32

Microbial metabolic activities are key variables in determining the geochemistry of groundwater systems. The facultative anaerobic bacterium Shewanella putrefaciens is widely considered a model dissimilatory metal-reducing bacterium (DMRB), as it can obtain energy for growth on a variety of metals. S. putrefaciens is unique among most DMRB’s because it is also able to grow aerobically, often flourishing at the oxic-anoxic boundary in groundwater settings. To date, however, little research has examined its aerobic growth. This is an important gap in the literature, as the metabolic plasticity exhibited by S. putrefaciens is relevant to the impact it and other taxa may have on metals in the subsurface. Specifically, it allows the organism to maintain its growth even if metal concentrations fluctuate, and, by reducing dissolved oxygen, S. putrefaciens affects the redox state of its environment, resulting in cascading effects on other microbial metabolisms. Clearly, a complete understanding of S. putrefaciens biology and its impacts on subsurface geochemistry needs to include a quantitative description of its aerobic growth and in particular the thermodynamics involved. In this study, we examined the bioenergetics of aerobic growth coupled to lactate oxidation in S. putrefaciens strain CN32. The bioenergetics were measured directly via isothermal calorimetry. From this, we quantified enthalpy, which we used to measure the total heat involved in growth. In addition, by repeating these measurements over a range of temperatures (25-35°C), we were able to infer how S. putrefaciens allocates its energy in terms of maintenance and growth. Results show a minimum enthalpic yield (total heat normalized to the change in biomass) at 30°C, with a statistically significant increase at 25 and 35°C. This suggests that the amount of energy allocated to maintenance increases as temperature shifts below and above 30°C, over the range of temperatures examined. In all, by quantifying the aerobic growth requirements of S. putrefaciens, this study advances our understanding of an important regulator of groundwater geochemistry.