bMany environments on Earth experience nutrient limitation and as a result have nongrowing or very slowly growing bacterial populations. To better understand bacterial respiration under environmentally relevant conditions, the effect of nutrient limitation on respiration rates of heterotrophic bacteria was measured. The oxygen consumption and population density of batch cultures of Escherichia coli K-12, Shewanella oneidensis MR-1, and Marinobacter aquaeolei VT8 were tracked for up to 200 days. The oxygen consumption per CFU (Q O2 ) declined by more than 2 orders of magnitude for all three strains as they transitioned from nutrient-abundant log-phase growth to the nutrient-limited early stationary phase. The large reduction in Q O2 from growth to stationary phase suggests that nutrient availability is an important factor in considering environmental respiration rates. Following the death phase, during the long-term stationary phase (LTSP), Q O2 values of the surviving population increased with time and more cells were respiring than formed colonies. Within the respiring population, a subpopulation of highly respiring cells increased in abundance with time. Apparently, as cells enter LTSP, there is a viable but not culturable population whose bulk community and per cell respiration rates are dynamic. This result has a bearing on how minimal energy requirements are met, especially in nutrient-limited environments. The minimal Q O2 rates support the extension of Kleiber's law to the mass of a bacterium (100-fg range).
Heterotrophic bacterial respiration constitutes 50% to 90% of ocean community respiration (1) and plays a critical role in the recycling of organic carbon in all natural environments (2). Despite this importance, respiration remains poorly quantified in ocean models of metabolism, gas exchange, and carbon mass balances (3). This uncertainty in heterotrophy makes it difficult to predict when areas of the ocean will be sources or sinks of CO 2 (4). Microbial community ecosystem behavior is ultimately a function of auto-and heterotrophic relationships; hence, understanding the biogeochemical contribution of heterotrophic bacteria at the single-cell level is necessary for making predictions of how ecosystems will respond to global change (5). Extracting this information is difficult. Efforts are being made with single-cell observatories designed to trap cells in microwells and monitor their consumption of oxygen over time using optical techniques (6, 7). Field-based approaches involve modeling oxygen pore water profiles in marine sediments and dividing oxygen consumption rate by cell numbers (8). A more frequently employed method for analyzing cellular respiration rates is to determine bulk oxygen uptake (consumption) rates (OUR), using either batch or continuous cultures with large populations of microbes. The average cellular respiration rate is calculated by normalizing OUR to population size. Most studies of this nature determine respiration rates while cells are growing (9-14). In the rare cases wher...