consequences, showing the profound link between life at the planetary scale and events at the atomic scale.
Figure 4.11 1,4 Glycosidic links between glucose molecules can occur between alpha glucose (α-D-glucose) molecules or beta glucose (β-D-glucose). The former chains result in the formation of starch, the latter form cellulose.
Sugars do not just have to bind to other sugars. One of the remarkable versatilities of the sugar rings is their ability to link to molecules through nitrogen (N-glycosidic bonds) and sulfur (S-glycosidic) bonds, thus generating a great variety of molecules with a complex structure. The N-glycosidic bond turns out to be essential for the binding of the sugar backbone of DNA to the bases that make up the genetic code, as we shall see in a later section.
Like amino acids, sugars are chiral, being found in L and D forms. All life on Earth uses primarily D-sugars.
Focus: Astrobiologists: Scott Perl
Affiliation: NASA Jet Propulsion Laboratory (JPL), Pasadena, CA, USA
What was your first degree subject? My first set of degrees were two bachelor's degrees, one in Geology and one in Materials Science. I learned in my master's degree how to build an astrobiology mission, and finally I went on to earn my PhD in Geobiology and Geological Sciences. I was fortunate to also work as a mission scientist on the 2004 Mars Exploration Rover mission while in college. My job on the mission was to examine microtextures from abraded rocks in outcrops where groundwater had infiltrated the sedimentary sections that we were exploring. This helped pave the way for my future career as an astrobiologist. While it was proven that water had existed on Mars, my questions started to form around how life could utilize such waters for survival and how evidence of biogenic activity could be preserved in salt and evaporite mineral veins.
What do you study? I am a geobiologist and astrobiologist who studies how life is preserved and maintained in hypersaline and evaporite salt mineral environments where ancient lakebed systems have dried or in modern lakes that are currently evaporating. I also study these ancient aqueous systems on Mars using in-situ and orbital data from rovers and orbiters. Moreover, I examine brine environments that are analogous to Europa's modern ocean–ice interfaces.
What science questions do you address? How can we validate true biological (biogenic) features from abiogenic ones? How can evidence of life and its processes be maintained deep underground in the Martian subsurface? How different would that life be from Earth's biomes and would we even detect it? What are the features of preserved microbiological processes in evaporites? How sustainable are these same features over geologic time? How can we use the single data point of life on Earth (including its diversity) and use that for life as we don't know it in our Solar System? Since 2017, I've co-led the Origins and Habitability Laboratory at JPL where we are looking to answer these questions and many others related to physical biosignatures and chemical biomarkers in extreme environments.
How did you get involved in astrobiology research? When I started my career, the focus was showing how widespread water was on Mars during the late Noachian/early Hesperian. Eventually that evolved to understanding how the mineralogy, formed from those same waters, could capture potential evidence of life and act as a record of biogenic processes. I'm also a geobiologist for extreme environments on Earth. Essentially, on our own planet astrobiology is geobiology. I wanted to contribute answers to astrobiology questions that dealt with validating records of ancient and modern preservation of life, how this evidence was recorded in halite and gypsum and other evaporite mineralogy, and using life “as we know it” to help understand “life as we don't know it” in our Universe. I got involved with astrobiology research essentially from seeing mineral formations on Mars precipitated and modified by ancient waters. This led me to closed-basin lake systems on Earth that recorded microbial communities that were once living in the nearby waters. The fieldwork led me to lab work where I created analog evaporite samples of what I observed in the field and on Mars through in-situ and orbital images and spectra. Incorporating all three aspects: field geobiology, laboratory work, and the context of ancient Mars and modern Europa has allowed for a clear and challenging pathway for understanding how cellular life can be preserved and maintained.
4.8 Lipids
Another class of compounds is the lipids, which encompass a wide diversity of chained and ring-containing molecules. They include long-chained carboxylic acids (also called fatty acids) which are chains of carbon compounds joined together, mainly through single bonds (e.g. saturated fatty acids) and some containing double bonds (unsaturated fatty acids; Figure 4.12). Fatty acids in life typically contain between 12 and 22 carbon atoms. One important group of lipids is the fats, or triglycerides (Figure 4.12), which are a combination of glycerol and three fatty acids and are found widely in animal fats and plant oils. The energy stored in the many bonds of fatty acids and triglycerides has made them useful as energy-storage molecules in life. Lipids have a variety of functional groups, which allow for important molecular interactions.
Figure 4.12 The molecular structure of some lipids. Free fatty acids are found as energy-storage molecules. Triglycerides are found in vegetable oils and animal fats. Phospholipids form cell membranes in microbes and other organisms. Also shown are two types of molecules found as structural components of membranes. Bacteriohopanetetrol is a hopanoid and is found in bacterial lipid membranes. Cholesterol is a steroid and a structural component of animal cell membranes.
Many lipids have the important characteristic that one end is charged. This end tends to be attracted to water, and the charge helps it to dissolve. We call it the hydrophilic end from the Greek hydro (water) and philos (love; it loves water). The other end, which is non-polar, does not dissolve readily, as it is uncharged. It is called the hydrophobic end because it dislikes water (from the Greek phobos, or fear). This property of having an uncharged and charged end means that these molecules are also referred to as amphiphilic. The phospholipids are one important example of lipids with a hydrophilic phosphate at one end (Figure 4.12). They are involved in cell membrane assembly. In the next chapter, we look at how their amphiphilic properties play a central role in cell membrane assembly, leading to compartmentalization, a fundamental characteristic of cellular life.
Like sugars and proteins, lipids also come in a vast variety, allowing for their complex and diverse functional roles in life. Some contain ring structures such as cholesterol (Figure 4.12), a steroid. Cholesterol is a component of animal cell membranes that is involved in maintaining membrane fluidity and integrity.
The lipids are immensely important to astrobiologists for a number of reasons. The tendency for life to link carbon atoms together to form well-defined chains makes them a strong signature of life. Non-biological processes can produce a wide range of chains, but they tend to have random variable numbers of carbon atoms, and in general, complex long chains are rarer than shorter chains. Life, however, tends to deliberately make long- chained lipids to construct membranes, so a preponderance of long-chained lipids in a sample with well-defined carbon numbers (e.g. 12, 14, 16) tends to suggest the presence of life. An example of such lipids is the hopanoids, lipids that have a five-ringed (pentacyclic) chemical structure (Figure 4.12). They have various roles, including membrane stabilization in bacteria. When bacteria are fossilized in the rock record, the bacterial shapes themselves may be destroyed, but the hopanoids can be preserved for potentially hundreds of millions of years.
