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Distributions of Geohopanoids in Peat

A brief introduction (and some recent developments...)

This blog accompanies our recent paper entitled "Distributions of geohopanoids in peat: Implications for the use of hopanoid-based proxies in natural archive" ( Inglis et al., 2018; Geochimica et Cosmochimica Acta).

What are Hopanoids? (tl;dr)

Hopanoids are pentacyclic triterpenoids produced by a wide range of bacteria and appear to perform a regulating and rigidifying function similar to sterols in eukaryotes. They contain a C30 ring system with an additional polyfunctionalised side chain, both of which can be altered.

Structure of Bacteriohopanetetrol, one of the most common hopanoids found in living bacteria.

Hopanoids are ubiquitous and are relatively resistant to degradation. As such, they can be very important palaeoenvironmental proxies (more on this later...)

Useful Definitions:

Hopanoids: a family of pentacyclic triterpenoids with a particular fused ring structure composed of four six-membered and one five-membered ring.

Biohopanoids: hopanoids which are synthesised by living organisms (i.e. bacteriohopanepolyols (BHPs), diploptene and diplopterol).

Bacteriohopanepolyols: biohopanoids with an extended polyfunctionalised side chain.

Geohopanoids: degradation products derived from biohopanoids (e.g. hopanoic acids, hopanols, hopanones and hopanes)

Extended hopanoids: geohopanoids with more than 30 carbon atoms (i.e. C31 to C35).

Stereochemistry: the study of the spatial orientation of atoms in molecules and the reactions that change the orientation.

Stereoisomers: two or more forms of a molecular that differ only in the relative spatial orientation of atoms. If the atom in question is part of a ring structure, then the two configurations are designated as α or β depending on whether the attached hydrogen atoms points down from or up from the plane of the ring structure (see below).

The stereochemistry of hopanoids (source: http://summons.mit.edu)

Hopanoid Pre-History (1955 to 1984)

Our recent paper builds upon decades of previous research. However, I am fascinated by the evolution and discovery of different compounds and I have compiled a short summary of hopanoid pre-history1

To me, two things stand out: 1) Hopanoids began life as PLANT BIOMARKERS, and 2) Hopanoids were the first 'lipid family' to be discovered discovered through their MOLECULAR FOSSILS.

1955: Mills & Werner isolate hydroxyhopanone (2) from Hopea tree resin

(Fun Fact: the Hopea tree was named after John Hope (1725-1786), the first Regius Keeper of the Royal Botanic Garden, Edinburgh).

The first hopanoids: 1: Hopane (Henderson et al., 1969), 2: Hydroxyhopanone (Mills and Werner, 1955), 3: C29 norhopane ketone (Berti et al,. 1963) (source: Ourisson and Albrecht, 1992)

1963: Berti et al. isolate C29 norhopane ketone (3) from the fern Adiantum capillus-veneris.

1968: Albrecht, Ourrison et al. identify isoarborinol (a pentacyclic triterpenoid) in the Messel Shale (deposited ca. 50 million years ago).

1969: Henderson, Eglinton, Maxwell et al. identify the C30 hopane (1) in the Green River shale (also deposited ca. 50 million years ago). They also tentatively identify hopanes with more than 30 carbons. These will later be known as 'extended hopanoids'.

1971: Bird et al. identify hop-22 (29)-ene (also known as diploptene) in blue-green algae. This is the first evidence that hopanoids have a microbial (rather than plant) origin.  

1972: Ensminger et al. unambiguously identify pentacyclic triterpenoids with more than 30 carbons in the Messel Shale. These are known as the 'extended hopanoids'. At this stage, no suitable precursor is known.

1973: Forster, Biemann et al. isolate bacteriohopanetetrol from the cellulose-producing bacterium Acetobacter xylinum. This is a possible precursor for 'extended hopanoids'.

1976: Rohmer & Ourrison show that bacteriohopanetetrol (see above) can act as a precursor for the 'extended hopanoids'

1982: Rohmer et al. propose that bacteriohopanetetrol acts as sterol surrogate in bacteria

1984: Rohmer et al. identify bacteriohopanepolyols (BHPs), diploptene and/or diplopterol in diverse array of bacterial strains. Bacterial origin unambiguously confirmed.

Hopanoids as Palaeoenvironmental Proxies

Biohopanoids can be unique markers for specific bacteria or certain environmental conditions (e.g. nitrogen-fixing bacteria, methanotrophs etc). However, many of these are only preserved over relatively recent timescales (e.g. < 5 million years; Ma) and reconstructions of the ancient bacterial community are more commonly based upon the abundance, distribution and/or stable carbon isotopic composition of their degradation products (i.e. geohopanoids; see below).

From biohopanoids to geohopanoids: depending upon redox conditions, different geohopanoids will form (source: Ourisson and Albrecht, 1992)

In sediments, with increasing diagenesis, geohopanoids also undergo stereochemical transformations and the biologically-derived 17β,21β(H)-hopanoid is transformed into the more thermally stable 17β,21α(H) and 17α,21β(H)-stereoisomers. Such changes have been widely used to reconstruct the thermal history of sediments.

Stereochemical transformations: diagenesis promotes the formation of βα & αβ hopanes (source: Ourisson & Albrecht, 1992)

Whilst geohopanoids in modern sediments typically occur in the biological 17β,21β configuration, in some modern peatlands the 'thermally-mature' 17α,21β configuration dominates. The occurrence of these compounds in recent peat deposits which have not undergone thermal maturation has puzzled scientists for years and has typically been attributed to one of two mechanisms:

1) Direct input of αβ hopanoids by indigenous bacteria

2) Oxidation and decarboxylation reactions of bacteriohopanepolyols followed by isomerization at the C-17 position catalysed by the acidic environment

It has also been have argued that temperature and hydrology exert a control upon the formation of the αβ hopanoids. As such, it remains unclear why the αβ configuration is so abundant in some peatlands.

To answer this question, we have analysed the distribution and isomerisation of geohopanoids in 395 samples from > 25 peatlands. Crucially, these peats span a wide temperature (-1 to 27°C) and pH (3 to 8) range and allow us to investigate the impact of environmental change upon geohopanoid isomerisation ratios.

Our results indicate that peatlands are dominated by a range of geohopanoids including hopanoic acids, hopanols, hopanes and hopenes (consistent with decades of previous research). We show that functionalised geohopanoids (e.g. hopanoic acids and hopanols) occur as αβ- and ββ-stereoisomers in peat, with the ββ-isomer typically dominating. In contrast, hopanes occur predominantly as the αβ-stereoisomer. The fact that hopanes exhibit a greater degree of isomerisation than functionalised bio- and geohopanoids, their putative precursors, suggests that isomerisation is not inherited from original biological sources. As such, we argue that biosynthesis of αβ-hopanoids is unlikely to directly account for the majority of αβ geohopanoids in peat.

When we compare hopanoid isomerisation ratios with different environmental parameters, we find that there is no evidence for a strong temperature control (c.f. Huang et al., 2015). However, both hopanoic acid and hopane ββ/(αβ+ββ) ratios exhibit a linear positive correlation with pH. The correlation between the C31 hopane ββ/(αβ+ββ) ratio and pH is statistically significant (r2 = 0.64, p < 0.001; n = 94; see below) and indicates that pH exerts a first-order control upon hopane isomerisation in peats.

Impact of pH upon geohopanoid isomerisation (Inglis et al,. 2018)

Crucially, isomerisation appears to be fixed during early diagenesis, suggesting that geohopanoid ββ/(αβ+ββ) indices could be a useful proxy for understanding pH over a range of timescales. However, the error associated with this proxy remains large and we only recommend using ββ/(αβ+ββ) indices to interrogate large amplitude and more long-term pH variation.

For example, the C31 ββ/(αβ+ββ) index could be applied to immature coal deposits (i.e. lignites) to understand environmental change during past greenhouse periods and across hyperthermal events. To explore this, we assessed the geohopanoid distribution within a thermally immature, early Paleogene (~56 Ma) lignite deposit (Schöningen, Germany). Within this setting, the C31 αβ isomer dominates the hopane assemblage, suggesting an acidic (pH <6), ombrotrophic peatland. This is consistent with the occurrence of Sphagnum-type spores and biomarkers within this lignite seam.

pH and vegetation change within Seam 1, Schöningen during the latest Paleocene and/or earliest Eocene. a) the C31 hopane ββ/αβ+ββ index, b) C31 hopane ββ/αβ+ββ-derived pH estimates, c) CBTpeat-derived pH estimates , d) C23/C31 n-alkane ratio (i.e. proxy for input of Sphagnum moss), e) the relative abundance (total palynomorphs) of Sphagnum-type spores.

We have also previously suggested that low C31 ββ/(αβ+ββ) indices could also be a useful proxy to trace the input of acidic peat (or eroded lignite) to marine sediments. While our results generally support this hypothesis, some acidic peats exhibit relatively high C31 ββ/(αβ+ββ) indices (e.g. Brazil), dictating caution in this approach – in particular, an absence of substantive αβ-hopane inputs should not be interpreted as evidence for an absence of peat inputs

In summary, our findings suggest that geohopanoid ββ/(αβ+ββ) indices could be used to reconstruct pH within modern and ancient peat-forming environments. Furthermore, we envisage that geohopanoids can provide important new insights into understanding depositional environments and interpreting terrestrial organic matter sources in the geological record.

Link to paper: https://www.sciencedirect.com/science/article/pii/S0016703718300036

1 If you are interested, I also highly recommend reading Ourisson & Albrecht (1992; ACS), Ourisson & Rohmer (1992: ACS) and Echoes of Life by Susan Gaines, Geoff Eglinton and Jurgen Rullkotter.

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