Revisiting the precursors of the most abundant natural products on Earth: A look back at 30+years of bacteriohopanepolyol (BHP) research and ahead to new frontiers

In this review we look back on 30+ years of bacteriohopanepolyol (BHP) research within the field of organic geochemistry. BHPs are ubiquitous, intact polar lipids in modern environments. They have been found in lacustrine, marine, riverine and soil and peat environments, and they are noteworthy lipids in biological symbiont studies. BHPs are the precursors of hopanoids, which are the most abundant fossil lipids found in the geological record. BHPs are synthesized by members of various bacterial taxa, and their distributions are often used to help to identify bacterial communities, in studies of both modern and past environments. However, less than 10% of known bacterial species are genetically capable of synthesizing BHPs, and many BHPs are not specific to particular bacterial sources. Nonetheless, a range of BHPs with specific side chain configurations and/ or A-ring modifications have proven very useful for tracing bacterial metabolism and for identifying ecological niches in various environments (e.g., aerobic methanotrophy, possibly nitrite-dependent intra-aerobic methanotrophy, and anaerobic ammonium oxidation) or for tracing environmental processes (e.g., soil inputs into aquatic settings). Moreover, BHPs (with previously unknown terminal groups and side chain configurations) are continuously being discovered, thanks to recent methodological and instrumental advances. These highlight the advent of a new era of BHP lipidomics which awaits full exploitation in organic geochemistry. Here, we provide a summary of the state-of-the-art of BHP knowledge, analytical frontiers, and suggest directions for future research.