Archaeal abundance across a pH gradient in an arable soil and its relationship with bacterial and fungal growth rates.

Soil pH is one of the most influential factors for the composition of bacterial and fungal communities, but the influence of soil pH on the distribution and composition of soil archaeal communities has yet to be systematically addressed. The primary aim of this study was to determine how total archaeal abundance (qPCR based estimates of 16S rRNA gene copy numbers) is related to soil pH across a pH gradient (pH 4.0-8.3). Secondarily, we wanted to assess how archaeal abundance related to bacterial and fungal growth rates across the same pH gradient. We identified two distinct and opposite effects of pH on the archaeal abundance. In the lowest pH range (pH 4.0-4.7) the abundance of archaea did not seem to respond to pH. Above this pH range there was a sharp, almost 4-fold, decrease in archaeal abundance, reaching a minimum at pH 5.1-5.2. The low archaeal abundance of archaeal 16S rRNA gene copies at this pH then sharply increased almost 150-fold with pH, resulting in an increase in the ratio between archaeal and bacterial copy numbers from a minimum of 0.002 to more than 0.07 at pH 8. The non-uniform archaeal response to pH could reflect variation in the archaeal community composition along the gradient, with some archaea adapted to acidic conditions, and others to neutral to slightly alkaline conditions. This suggestion is reinforced by observations of contrasting outcomes of the (competitive) interactions between archaea, bacteria and fungi towards the lower and higher ends of the examined pH gradient.