Paula Welander is an Assistant Professor of Earth System Science at Stanford University. Her lab is focused on lipid biomarker studies in modern bacteria, specifically hopanoids and sterols. On her page she discusses the interest in lipids preserved in the rock record and how they might be signatures of early life, however we know little of evolutionary history and function of these molecules in modern bacteria. Without a fuller understanding of the function and history of these molecules in the 'here and now' it becomes difficult to interpret what we are seeing in the rock record. Her lab aims to fill this gap.
So let's learn about some 'fat' microbes.
Highlights from the talk:
- Fossil lipids cannot be degrade by diagenesis
- Diagenesis (because I drew a blank when she said it) - The physical and chemical changes occurring during the conversion of sediment to sedimentary rock. Basically, in oversimplified terms, we are going from 'loose' to 'compact'. This process is controlled by composition of the sediment, pressure, temperature, grain size, porosity, permeability and amount of fluid flow. An example below:
- Based on the age of the rock and types of organisms that produce these molecules today, we can infer 'who' was there and what type of environment was present when the molecules were deposited.
- So we'd like to:
- Learn how these molecules are biosynthesized, identify the genes responsible and look at transcriptional profiles.
- Link synthesis to physiological perturbations in the lab
- Link lipid production to a biological factor
- After diagenesis, cholesterol becomes cholestane which is detectable in the rock record and has been used as a biomarker of early eukaryotic life. Derivatives of cholestane include sterols.
- Sterols are essential to eukaryotes and require oxygen for synthesis.
- There are bacteria that produce sterols including Methylococcus capsulatus, Stigamatella aurantiaca and Gemmata obscurioglobus.
- To date very few bacteria have been shown to produce sterols and so often when it is found in bacteria it is assume the genes required for the process were acquired via horizontal gene transfer.
- So lets look at the pathway to Ianosterol - Squalene > Oxidosqualene > Ianosterol and lets look for the protein oxidosqualene cyclase (OSC) in bacteria.
- Present in 62 bacteria (by interrogating genomes) in a variety of families!
We've underestimated the fat microbes!...I mean...
We've underestimated bacteria's ability to produce these sterols
- Paula's lab inserted the genes into E. coli the provincial microbial lab rat of science and showed that E. coli can produce sterols! She then turned this finding into a class so students could be actively involved in sterol detection and production in bacteria.
- But we still needed to break it down - not all the genes necessary for sterol synthesis in eukaryotes were present in bacteria so it looked like an incomplete process. Specifically there was a gene missing for sterol modification in bacteria.
- Zooming in on Methylococcus capsulatus (where sterols are present) we find sdmA and sdmB, but are they enough? Just two proteins? Eukaryotes require 3.
This was a really interesting talk...a look at Paula's webpage, I think it has not be updated in a while (last news and pubs put up in 2015) so I went to her google scholar profile. To learn more about her work in lipids check out these publications -
- Lee, Alysha K., et al. "C-4 sterol demethylation enzymes distinguish bacterial and eukaryotic sterol synthesis." Proceedings of the National Academy of Sciences 115.23 (2018): 5884-5889.
- Hamilton, T. L., et al. "Microbial communities and organic biomarkers in a Proterozoic‐analog sinkhole." Geobiology 15.6 (2017): 784-797.
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