Anaerobic microbiology of an alkaline Icelandic hot spring

Samples of sediments and biofilm, defined as biomats or microbial mats, collected from a hot spring in Hveragerdi, Iceland at approximately 55°C and 75°C, were examined for anaerobic microbial activity and bacterial numbers. Generally more bacteria were present in the biomat at 75°C than at 55°C; 9.98 × 105 and 9.48 × 103 xylan degrading bacteria/gram volatile solids and 6.89 × 107 and 0 cellulose degrading bacteria/gram volatile solids were found at 75°C and 55°C, respectively.

No proteolytic or lipolytic bacteria were present in the biomat at 75°C, but 1.38 × 103 proteolytic and 4.48 × 102 lipolytic cells/gram volatile solids were found in the biomat at 55°C. Acetate, propionate and butyrate were not degraded by the biomat either at 55°C or 75°C. No methanogenic activity was found at 75°C.

However, most-probable number enumerations and indirect immunofluorescence using antibody probes against selected methanogenic archaea showed that methanogens were present at this temperature. H2 was the major electron donor for the methanogenic archaea at 55°C. No methane production was observed with acetate as substrate during short time (i.e. 24 h) incubation at 55°C.

However, a significant amount of methane appeared after long periods of incubation at this temperature. Indirect immunofluorescence using antibody probes revealed a diverse methanogenic population in the spring. Cells immunologically related to Methanobacterium thermoautotrophicum ΔH and Methanobacterium thermoformicicum CB 12 were the dominating methanogenic archaea at both temperatures.

In addition, cells immunologically related to Methanoculleus thermophilus strain UCLA were present in high numbers at 75°C. Furthermore, cells immunologically related to Methanosaeta thermophila CALS-1 were found in low numbers in the biomat at 55°C.