Selenium may support deep microbial life in Earth's continental crust


Date:
July 27, 2021
Source:
Geological Society of America
Summary:
International drilling efforts over the last decades into the
seafloor have provided increasing evidence for the existence of an
extensive deep biosphere below the seafloor. There, circulating
fluids in the sub- seafloor deliver chemical compounds from
which energy is produced to fuel microbial life in such deep
ecosystems. Our understanding of the role of such chemolithotrophic
microbes in the continental deep biosphere, however, is much more
limited due to poor accessibility.



FULL STORY ========================================================================== International drilling efforts over the last decades into the seafloor
have provided increasing evidence for the existence of an extensive
deep biosphere below the seafloor. There, circulating fluids in the sub-seafloor deliver chemical compounds from which energy is produced
to fuel microbial life in such deep ecosystems. Our understanding of the
role of such chemolithotrophic microbes in the continental deep biosphere, however, is much more limited due to poor accessibility.


==========================================================================
Only a few places worldwide, mostly some of the oldest continental
crust fragments such as Archean cratons, enable direct sampling of
the continental deep biosphere. Such places indicate that deep-seated
fractures act as fluid pathways and deliver microbially important
nutrients to otherwise hostile habitats. The availability of selenium
(Se) might play a crucial role in such systems because the reduction
of oxidized Se species provides by far more energy than the reduction
of sulfate. Hence, even small amounts of Se could potentially sustain
microbial activity if the right physicochemical conditions are met.

An international team led by scientists from Brazil and Germany
investigated Se-rich platinum-palladium (Pt-Pd) nuggets from a placer
deposit in Minas Gerais (Brazil), a locality where Pd has originally
been identified. Although such Pt-Pd nuggets are covered by biofilms,
the formation of such nuggets remains elusive. High-precision Se-isotope
data were therefore combined with trace-metal data and Pt-Os ages to
assess the nugget formation and to identify potential microbial processes.

The combined data show that the Pt-Pd nuggets formed about 180 million
years ago, likely by replacement of precursor vein minerals in the host quartzite at 70 DEGC and approximately 800 meters below the surface. The
high levels of Se and other biophilic elements (iodine, organic carbon, nitrogen), together with an extremely negative Se isotopic composition,
the lowest yet measured in natural samples (d82/76Se = -17.4 to -15.4
0/00), are consistent with a microbial origin. Abiogenic processes cannot
be fully excluded yet, but the study suggests that the Pt-Pd nuggets
plausibly record Se-dependent microbial activity in the continental
deep biosphere.

Stephan Ko"nig and Benjamin Eickmann, who performed the Se-isotope
analysis, remark that the difficulty in sampling the continental deep
biosphere can be circumvented by applying novel isotope proxies to weathering-resistant minerals such as nuggets. This novel approach
may provide a wealth of information that is needed to enhance our
understanding of the continental deep biosphere.

========================================================================== Story Source: Materials provided by Geological_Society_of_America. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Alexandre Raphael Cabral, Stephan Ko"nig, Benjamin Eickmann, Michael
Brauns, Miguel Tupinamba', Bernd Lehmann, Mari'a Isabel Varas-Reus.

Extreme fractionation of selenium isotopes and possible
deepbiospheric origin of platinum nuggets from Minas Gerais,
Brazil. Geology, 2021; DOI: 10.1130/G49088.1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210727171533.htm

--- up 11 weeks, 4 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)