Time-transgressive microbial diversity in a tropical bat guano accumulation, Deer Cave, Mulu, Borneo

Abstract

Bat guano accumulations in caves can constitute large deposits of organic material, particularly chitin and its degradation products, that host a diverse microbiome. Previous microbiological studies have not addressed the stratified nature of these deposits in the context of the timeframe of the decomposition process. We present such data (temperature, pH, DNA sampling of eight carbon-dated strata) for a 75 cm-deep tropical guano accumulation that undergoes complete decomposition over a timescale of ~120 years. The aerobic, surface layer of freshly-fallen guano has the greatest diversity of taxa, with Bacteroidota co-dominant with Pseudomonadota, and includes many taxa unique to this zone (including those from guanophagic invertebrates, bat guts, and insects). The ~20-40-year-old, more anaerobic, middle layer of increasing acidity and temperature has fewer taxa, with Pseudomonadota dominant and Bacteroidota almost disappearing, and many taxa tolerant of high temperatures and anaerobic conditions. The ~60-120-year-old, deepest, zone has the lowest diversity with a single genus of Actinomycetota (Mycobacterium) taking over as dominant. We also present data on the significantly different microbiomes of guano falling into aerobic versus water-logged, anaerobic conditions at the surface, the anaerobic having almost twice the diversity of the aerobic, including many taxa that are unique to this sample. Taxa of especial interest include methanogens, sulphur reducers and oxidizers, chitin decomposers, halophiles, acidophiles, and extremophiles.