The role of bacteriohopanepolyols as biomarkers for soil bacterial communities and soil derived organic matter

Abstract

Bacteriohopanepolyols (BHPs) are a group of membrane lipids produced by a wide range of bacteria which have a high degree of structural specificity relative to bacterial source. This is the widest study to date into the BHP content of soils from around the world and investigates their use as biomarkers in modern soils and modern and ancient sediments using reversed phase HPLC-MSn. A total of 46 different BHP structures were observed during the study including the tentative identification of 11 novel BHP structures. Analysis of the BHP content in a series of soils from North East England, South West England, Canada, Sweden, Vietnam and Southern Spain has shown that soil BHP distributions are dominated by up to 5 different BHPs: bacteriohopane-32,33,34,35-tetrol (BHT), 35-aminobacteriohopane – 32,33,34-triol (aminotriol), 30-(5’-adenosyl)hopane (adenosylhopane), bacteriohopanetetrol carbopseudopentose ether (BHT cyclitol ether) and, less frequently, adenosylhopane type-1, although generally more than 20 different BHPs are identified in each soil sample. Soil bacterial population dynamics were investigated using existing knowledge of the relationship between BHPs and their specific bacterial sources and has demonstrated, for example, a decrease in the concentration of cyanobacteria related BHPs with soil depth in pasture (Palace Leas) and woodland (Hack Hall farm) environments confirming the validity of these BHP – bacteria relationships to infer BHP producing bacterial activity. Principle Component Analysis of the different bacterial types, identified by source BHPs, in the different soils can clearly identify variations between the soils with, for example, the influence of methanotroph and cyanobacteria derived BHPs having significant and opposite effects on the separation of the soils. This potentially enables BHPs to be used to identify source locations or environments. Investigation of seasonal variations at two sites in NE England (Palace Leas and Hack Hall Farm) has shown that soil BHP concentrations increase during the summer months and decline during winter with no net accumulation indicating that the aerobic conditions observed in near surface soils result in the rapid diagenesis or metabolism of BHPs. An investigation of estuary sediments from the Congo Fan, River Rhone and 7 arctic rivers has shown that this rapid degradation of these highly functionalized structures does not occur in anaerobic marine sediments with the aerobic methane oxidizing bacteria biomarker, 35- aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) and other complex BHPs being identified in Congo Fan sediments up to 1 million years old (100 metres below sea floor), thus enabling BHPs to be used as biomarkers in palaeo-environments. The estuary and fan sediments also contained a suite of BHPs that are ubiquitous in soil but absent in many lacustrine and open marine environments where their concentration decreases proportionally with distance from land. These BHPs; adenosylhopane, adenosylhopane type- 1 and their C-2 methylated homologues can therefore be used as markers for the transport of soil derived organic material (SOM) from terrestrial to marine environments. These results have opened up a wide range of opportunities for BHPs to be used as bacterial proxies in a variety of environments and ages, accessing a wide range of different proxies for different bacterial populations and processes from a single analysis.