Bacterial biomass composition and its implications for implications for Earth's biogeochemical processes and feedbacks
Two contemporary effects of humans on aquatic systems are increasing temperatures due to climate change and increasing nutrient concentrations due to increased fertilizer use. The response of heterotrophic bacteria in aquatic systems to these perturbations has important feedback implications to ecosystem processes mediated through effects on nutrient availability and carbon metabolism and burial. We examined the effects of temperature and phosphorus availability on freshwater bacterial elemental composition and morphology by growing isolates from lakes in chemostats. We manipulated resource C:P and temperature in chemostats by altering the supply of inorganic P at temperatures varying from 10-30ºC. Dilution rates were manipulated to maintain all the strains at ca. 25% of their maximum growth rate to simulate relatively low growth in natural systems.
There were large effects of resource C:P but not temperature on cell dimensions and volume. Increasing P typically led to carbon limitation and decreased cell size in the strains but there was a great deal of variability in the magnitude of this response among strains. Decreased P led to decreased P content but also increased proportional C content. At uniform relative growth rates, the quantity of P in RNA and DNA did not change with changing temperatures, but increased P supply rates led to increases in both of these P-pools, despite no change in the relative growth rates. Increased P led to decreased biomass C:P and N:P but did not affect C:N content. Surprisingly, temperature did not affect C:N:P ratios except at the coldest temperature (10ºC). We conclude that the largest effects of temperature on microbial stoichiometry are mediated through effects on specific growth rates and the internal nucleic acid pools. The profound effects on resource C:P on internal pools and stoichiometry suggest that organic carbon and nutrient cycling in aquatic ecosystems are more sensitive to increased nutrient availability than climate warming. However, it is important to understand how temperature and nutrients affect the maximum and relative growth rates of microbes in natural systems.