Villum Fonden - Teknisk og naturvidenskabelig forskning - The bacterium Gemmata obscuriglobus disguised as an early eukaryote

Project: Research

Project Details


Cells maintain fluidity of membranes through hopenes and sterols, including molecules like cholesterol. Compared to many other biological molecules, the structure of hopenes and sterols makes them extremely stable over long time scales. Through geological processes, these two types of compounds become chemically modified, being converted into reduced forms called hopanes and steranes, respectively. These molecules (often referred to as “biomarkers”) may persist for millions of years, and are detectable in some ancient rocks, giving a window into what organisms were present at the time the rock was formed.

The traditional view (Figure A) supposes that hopenes are produced by bacteria, while sterols are produced by eukaryotes. Therefore, finding hopanes or steranes in the rock record tells us whether the ecosystem was dominated by either bacteria or eukaryotes. Hopanes are the sole biomarker in rocks for the majority of the history of the planet, leading to the conclusion that these ancient marine ecosystems were dominated by bacteria. A stark contrast is then found in rocks from 600700Ma, where steranes suddenly are found at appreciable quantites, suggesting that eukaryotic organisms then became globally abundant until modern day.

Hopenes and sterols are produced by a series of shared enzymatic steps, but diverging at one critical step where oxygen is required to produce sterols, but not hopenes. Following the oxygen-dependent step, one of two enzymes then forms the multi-ring structure, either squalene-hopene cyclase (SHC) for hopenes or oxidosqualene cyclase (OSC) for sterols. The two enzymes are very similar at the sequence level, as well as the protein structures.

Some bacteria actually produce sterols and some eukaryotes produce hopenes, suggesting that the traditional interpretation is an oversimplification (Figure B). Previous work by Takashita et al. (2012, Biology Direct) had highlighted two horizontal gene transfer events from bacteria to eukaryotes. However, they had missed the fact that the eukaryotic group of SHC includes representatives from most eukaryotic lineages yet still shows a single origin; the same goes for eukaryotic OSC. Indeed, when considering the species tree of eukaryotes, rather than the phylogeny of SHC and OSC genes themselves, the implication is that ancestor of all eukaryotes must have had both SHC and OSC, and either one or the other gene was later lost in all species (Figure C). This suggests that early eukaryotes could have made hopenes and sterols, and also could have survived in both aerobic and anaerobic environments.

Layman's description

Traces of certain biological molecules, called biomarkers, can be found in ancient rocks and, in principle, can give a glimpse of what life was present at that time. However, without knowing details of who makes which biomarkers, or what conditions were needed to make them, the biomarker record remains controversial. Here, we want to study the bacterium Gemmata obscuriglobus for two reasons. This bacterium is capable of making a diverse set of biomarkers typical for bacteria, but also can make ones common in eukaryotes (group including all plants and animals). If environmental factors like oxygen level affect the biomarkers produced, then we can relate biomarker distribution to ancient environments. Secondly, the bacterium shares many features with eukaryotes and might give insight into how the first eukaryotes originated.
Effective start/end date01/10/201930/09/2020