Microbial abundance across a salinity and mineralogical transect in the Ntwetwe Pan of Botswana: A terrestrial analogue for playa deposits on Mars

Abstract

The current conditions of the Martian surface are considered prohibitive for life as we know it, due to strong radiation, highly oxidizing conditions, concentrated evaporative salts (including highly toxic perchlorates), and relatively low water activity. Earth hosts a multitude of extreme environments whose physico-chemical properties partly match Martian conditions. Such environments are defined as “analogue sites” and may offer a critical test-bed for astrobiological studies in characterizing the physical and chemical boundaries within which terrestrial life may exist and in assessing the habitability of Mars, and understanding the biological mechanisms for survival in extreme environments.

For example, the Makgadikgadi Basin, located in central Botswana is considered one of the largest evaporitic basins on Earth, characterized by deposition of NaCl crusts from brines and surface water, high UV radiation and strong evaporitic conditions. These conditions may be compared with those detected on the Martian surface and/or hypothesized for early Mars.

Here, we provide, for the first time, a comparison between the abundance of microorganisms (both bacteria and fungi) and the chemical and physical properties of the surficial sediments from the Ntwetwe Pan, in the western Makgadikgadi Basin, providing a description of the morphological characteristics of halophilic communities and highlighting interactions with different mineral phases. These results show that fungi communities are more prone to variations due to changes in salinity and evaporite mineralogy. In general, the abundant filaments found in the surface sediments of the Makgadikgadi pans provide templating for the nucleation of carbonates and other evaporitic minerals such as trona and thenardite.

This study draws links between the effects of a hypersaline environment on the survival potential (abundance) of microorganisms and their preservation potential within mineral phases. Studies of the Makgadikgadi pans could help to understand if hypothetical life-forms may exist or have existed on Mars, and if they are likely to be preserved in the evaporitic playa deposits described across the planet.