A sandwich-designed temperature-gradient incubator for studies of microbial temperature responses

Abstract

A temperature-gradient incubator (TGI) is described, which produces a thermal gradient over 34 aluminium modules (15×30×5 cm) intersected by 2-mm layers of partly insulating graphite foil (SigraFlex Universal). The new, sandwich-designed TGI has 30 rows of six replicate sample wells for incubation of 28-ml test tubes. An electric plate heats one end of the TGI, and the other end is cooled by thermoelectric Peltier elements in combination with a liquid cooling system. The TGI is equipped with 24 calibrated Pt-100 temperature sensors and insulated by polyurethane plates. A PC-operated SCADA (Supervisory Control And Data Acquisition) software (Genesis 4.20) is applied for temperature control using three advanced control loops. The precision of the TGI temperature measurements was better than ±0.12°C, and for a 0–40°C gradient, the temperature at the six replicate sample wells varied less than ±0.04°C. Temperatures measured in incubated water samples closely matched the TGI temperatures, which showed a linear relationship to the sample row number. During operation for 8 days with a gradient of 0–40°C, the temperature at the cold end was stable within ±0.02°C, while the temperatures at the middle and the warm end were stable within ±0.08°C ( n=2370). Using the new TGI, it was shown that the fine-scale (1°C) temperature dependence of S o oxidation rates in agricultural soil (0–29°C) could be described by the Arrhenius relationship. The apparent activation energy ( E a) for S o oxidation was 79 kJ mol −1, which corresponded to a temperature coefficient ( Q 10) of 3.1. These data demonstrated that oxidation of S o in soil is strongly temperature-dependent. In conclusion, the new TGI allowed a detailed study of microbial temperature responses as it produced a precise, stable, and certifiable temperature gradient by the new and combined use of sandwich-design, thermoelectric cooling, and advanced control loops. The sandwich-design alone reduced the disadvantageous thermal gradient over individual sample wells by 56%.