Microbialites are organosedimentary rocks that have occurred throughout the Earth’s
history. The relationships between diverse microbial metabolic activities and isotopic
signatures in biominerals forming within these microbialites are key to understanding
modern biogeochemical cycles, but also for accurate interpretation of the geologic
record. Here, we performed detailed mineralogical investigations coupled with
NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry) analyses of pyrite S
isotopes in mineralising microbial mats from two different environments, a hypersaline
lagoon (Cayo Coco, Cuba) and a volcanic alkaline crater lake (Atexcac, Mexico).
Both microbialite samples contain two distinct pyrite morphologies: framboids and
euhedral micropyrites, which display distinct ranges of δ34S values1. Considering
the sulfate-sulfur isotopic compositions associated with both environments, micropyrites display a remarkably narrow range
of Δpyr (i.e. Δpyr ≡ δ34SSO4 − δ34Spyr) between 56 and 62‰. These measured Δpyr values agree with sulfate-sulfide equilibrium
fractionation, as observed in natural settings characterised by low microbial sulfate reduction respiration rates. Moreover, the
distribution of S isotope compositions recorded in the studied micropyrites suggests that sulfide oxidation also occurred at
the microbialite scale. These results highlight the potential of micropyrites to capture signatures of microbial sulfur cycling
and show that S isotope composition in pyrites record primarily the local micro-environments induced by the microbialite.