Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific
Résumé
Aragonitic microbialites, characterized by a reticulate fabric,
were discovered beneath lacustrine microbial mats on the atoll of
Kiritimati, Republic of Kiribati, Central Pacific. The microbial
mats, with cyanobacteria as major primary producers, grow in
evaporated seawater modified by calcium carbonate and gypsum
precipitation and calcium influx via surface and/or groundwaters.
Despite the high aragonite supersaturation and a high photosynthetic
activity, onlyminor aragonite precipitates are observed in the
top parts of the microbial mats. Instead, major aragonite precipitation
takes place in lower mat parts at the transition to the anoxic
zone. The prokaryotic community shows a high number of phylotypes
closely related to halotolerant taxa and/or taxa with preference
to oligotrophic habitats. Soil- and plant- inhabiting bacteria
underline a potential surface or subsurface influx from terrestrial
areas, while chitinase-producing representatives coincide with the
occurrence of insect remains in the mats. Strikingly, many of the
clones have their closest relatives inmicroorganisms either involved
in methane production or consumption ofmethane or methyl compounds.
Methanogens, represented by the methylotrophic genus
Methanohalophilus, appear to be one of the dominant organisms in
anaerobic mat parts. All this points to a significant role of methane
and methyl components in the carbon cycle of the mats. Nonetheless,
thin sections and physicochemical gradients through themats,
as well as the 12C-depleted carbon isotope signatures of carbonates
indicate that spherulitic components of the microbialites initiate
in the photosynthesis-dominated orange mat top layer, and further
grow in the green and purple layer below. Therefore, these
spherulites are considered as product of an extraordinary high
photosynthesis effect simultaneous to a high inhibition by pristine
exopolymers. Then, successive heterotrophic bacterial activity
leads to a condensation of the exopolymer framework, and finally
to the formation of crevice-like zones of partly degraded exopolymers.
Here initiation of horizontal aragonite layers and vertical
aragonite sheets of the microbialite occurs, which are considered
as a product of high photosynthesis at decreasing degree of inhibition.
Finally, at low supersaturation and almost lack of inhibition,
syntaxial growth of aragonite crystals at lamellae surfaces leads
to thin fibrous aragonite veneers. While sulfate reduction, methylotrophy,
methanogenesis and ammonification play an important
role in element cycling of the mat, there is currently no evidence
for a crucial role of them in CaCO3 precipitation. Instead, photosynthesis
and exopolymer degradation sufficiently explain the
observed pattern and fabric of microbialite formation.