Use of phospholipid fatty acids to detect previous self-heating events in stored peat

The use of the phospholipid fatty acid (PLFA) composition of microorganisms to detect previous self-heating events was studied in naturally self-heated peat and in peat incubated under temperature-controlled conditions. An increased content of total PLFAs was found in self-heated peat compared to that in unheated peat. Two PLFAs, denoted T1 and T2, were detected only in the self-heated peat. Incubation of peat samples at 25 to 55°C for 4 days indicated that T1 and T2 were produced from microorganisms with different optimum temperatures. This was confirmed by isolation of bacteria at 55°C, which produced T2 but not T1. These bacteria produced another PLFA (denoted T3) which coeluted with 18:1omega7. T2 and T3 were identified as omega-cyclohexyltridecanoic acid and omega-cyclohexylundecanoic acid, respectively, indicating that the bacteria belonged to the genus Alicyclobacillus. T1 was tentatively identified as omega-cycloheptylundecanoic acid. T2 was detected 8 h after the peat incubation temperature was increased to 55°C, and maximum levels were found within 5 days of incubation. The PLFA 18:1omega7-T3 increased in proportion to T2. T1 was detected after 96 h at 55°C, and its level increased throughout the incubation period, so that it eventually became one of the dominant PLFAs after 80 days. In peat samples incubated at 55°C and then at 25°C, T1 and T2 disappeared slowly. After 3 months, detectable levels were still found. Incubation at 25°C after heating for 3 days at 55°C decreased the amounts of T2 and 18:1omega7-T3 faster than did incubation at 5°C. Thus, not only the duration and temperature during the heating event but also the storage temperature following heating are important for the detection of PLFAs indicating previous self-heating.