In Bacteria, bacteriohopanepolyols (or BHPs) rigidify and regulate the cell membrane (Figure 1). When a cell dies, BHPs degrade, lose most of their functional groups and change their stereochemistry. The most simplistic hopanoid is the hopane, a pentacyclic (5-ringed) triterpenoid with 27-35 carbons. Unfortunately, hopanes are not particularly diagnostic biomarkers.
Methylation seen at the C-2 position is in green. Methylation is also possible at the C-3 position in certain methanotrophs.
In 1999, Roger Summons identified C2-methylated BHPs in cyanobacterial cultures and C2-methylated hopanoids in ancient sediments (Fig. 1). They argue that C2-methylhopanes are direct evidence for cyanobacteria and, as a result, oxygenic photosynthesis. This has been used “…as evidence for the antiquity of oxygenic photosynthesis and for the waxing and waning of cyanobacterial primary productivity and nitrogen fixation in the geological past”.
In 2010, Paula Welander, a post-doc working with Roger Summons, began to look at 2-methylhopanoids in more detail. As the squalene-hopene cyclase (shc) was the only known hopanoid biosynthetic step, Welander began to investigate the region surrounding shc. She discovered a gene that might encode for methylation at the C2 position (hpnP) but in order to confirm this, they had to delete the gene and re-run the lipid extracts.
When they did, they discovered that C2-methylated BHPs were no longer present. When this gene was re-introduced, the organism resumed synthesis of C2-methylated compounds, suggesting that hpnP was responsible for producing C2-methylated hopanoids.
The authors use this observation to show that:
1) not all cyanobacteria have the capability to synthesise C2-methylated hopanes
2) other organisms (e.g. acidobacteria, α-proteobacteria) have the capability to synthesise C2-methylated hopanes.
Blue shows which bacteria can make hopanes. Red shows which bacteria can make 2-methylhopanes (Welander et al 2010)
Jessica Ricci at CalTech argues that 2-methyhopanes are not reliable biomarkers for cyanobacteria (or indeed, for any other taxonomic group). Instead, they may beindicators of a specific environmental niche (low O2 and fixed N2, high osmolarity).Furthermore, the majority of shc and hpnP diversity is within terrestrial and freshwater environments.
So does that mean that every papers which uses 2-methylhopanes as a proxy for cyanobacteria is wrong? I am not sure and I probably don’t know enough to comment (If you want to read more, Kuypers thesis is a good place to go). But it seems that if you are going to use 2-Me hopanes as a proxy for cyanobacteria, it would make sense to locate a site which is unaffected by terrestrial and freshwater input (i.e. the open ocean).
Summons et al., 1999 http://azolla.fc.ul.pt/aulas/BiologiaCelular/docs/Cyanomarkers.pdf
Welander et al., 2010 http://www.pnas.org/content/107/19/8537.long
Riccie et al., 2013 http://www.readcube.com/articles/10.1038/ismej.2013.191
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