Lanthanum Anomalies as Fingerprints of Methanotrophy - 2020

Geochemistry & Hydrochemistry

Wang, and 4 co-authors, reported in mid-2020 that methane is an important greenhouse gas whose emissions into the oceans and atmosphere are regulated by relatively unconstrained anaerobic and aerobic microbial processes. The aerobic pathway for methane oxidation is thought to be largely dependent upon the use of rare earth elements (REE), but to date the effects of this process on their abundances in bacteria or in organisms living in symbiosis with methanotrophs remain to be evaluated.

The authors demonstrated that deep sea chemosynthetic mussels prospering at methane seeps display distinctive lanthanum enrichments linked to the enzymatic activities of their symbionts. These results demonstrate that methanotrophy is able to fractionate efficiently REE distributions in organisms and possibly in the environment. Lanthanum anomalies recorded in ancient sediments are potential chemical fossils that could be used in the geological record for tracking early evidence of microbial life.

The authors concluded that their findings have implications for the understanding of other lanthanum excesses previously identified in seawater and various marine precipitates, but remaining unexplained so far. However, this element was assumed to be more stable than other light REEs during complexation in seawater (de Baar et al., 1985) or could be released from suspended barite particles (Grenier et al., 2018). In marine precipitates, lanthanum anomalies are common features that are usually inferred to be largely inherited from seawater (e.g., Bau and Dulski, 1996; Kamber and Webb, 2001), but for which no clear explanation has been proposed.

The authors' data demonstrated unambiguously that positive lanthanum anomalies can be generated by the enzymatic activity of methanotrophs, hence providing the possibility of promising applications in various fields of research. Importantly, in future studies, the presence of marked lanthanum anomalies or any other large light REE enrichments in fossil shells could be used as proxies for past methanotrophic activity. Methanotrophic organisms have been present since the Archean, but their detection in the geological record, and in the sedimentary archives of the early Earth oceans are still the subject of intense debate (Thomazo et al., 2009; Knoll et al., 2016). In particular, lanthanum anomalies are frequent in ancient microbial carbonates and banded iron formations (Bau and Dulski, 1996; Kamber and Webb, 2001; Planavsky et al., 2010), which may point towards a putative REE dependent microbiological origin. Revisiting the REE geochemistry of these ancient rocks could provide fresh insights into the emergence and diversification of life.

Original working URL: https://www.geochemicalperspectivesletters.org/article2019/?hilite=%27Rare%20Earth%20Elements%27