I’ve been conflating hydrothermal vents and fields. I realized this after reading Chapter 3 of David Deamer’s Assembling Life. Most of us who study the origins of life are familiar with the hypothesis that life on Earth may have begun in submarine hydrothermal vents. The initial discovery in 1977, that life was teeming deep in the ocean bed where the water was locally hot around magma-driven minerals was a surprise! The heat and the minerals both act as energy sources (thermal and redox-chemical respectively). Since living organisms crave energy, they congregated to form a local ecosystem.
There are two types of hydrothermal vents. The first to be discovered were dubbed “black smokers” because that’s what the sulfide minerals look like under the very harsh conditions where temperatures could reach 400 Celcius. Water remains as a liquid because 2km deep in the ocean the pressure is approximately 300 atmospheres. The vent fluid is also quite acidic (pH 2-4) which can drive certain types of chemical reactions. Black smokers are transient, lasting up to a few hundred years before they collapse and reappear elsewhere along the mid-ocean ridge where the crust is thinner and underlying magma can break through.
Origin-of-life researchers have been more enthusiastic about “white smokers” because that’s what the carbonate minerals look like under the not-as-harsh conditions. Water temperatures might be 50-90 Celcius, and the vent fluid is alkaline (pH 9-11) which also drives chemical reactions. These vents can last thousands of years, perhaps longer, and are not associated with volcanic activity. Their existence was predicted before they were discovered in 2000, and the most famous of these, Lost City, is sometimes referred to as a hydrothermal field, which confuses things and likely contributed to my conflating vent and field. Abiotic chemical reactions at these minerals under these conditions can generate methane and molecular hydrogen, important precursors for prebiotic chemistry experiments. Scientists have set up experiments mimicking these alkaline vents and produced some key molecules that may be the building blocks of organic life.
Deamer distinguishes hydrothermal fields from vents in the following way. In the submarine vents, there is only one interface: mineral-seawater. Fields on the other hand have exposure to the atmosphere. Instead of being deep in the ocean, they are terrestrial in origin. In the aftermath of a volcanic eruption, the minerals slowly transform such that eventually rainwater collects to form pools. Hot springs and geysers at Yellowstone National Park are an example of such fields. There are three interfaces: mineral-water, mineral-atmosphere, and atmosphere-water. Crucially, the water initially derives from freshwater and although this will dissolve some of the minerals, the ionic strength of the solution is much lower than in seawater. This is critical if you want to form cellular structures from lipid molecules. The high Ca2+ and Mg2+ content in seawater inhibits the self-assembly of micelles and vesicles.
Another important feature of such pools is that the acidic water (pH 4-5) of hydrothermal fields also dissolves apatite (calcium phosphate), the same mineral that makes up your tooth enamel. Phosphate is a key constituent of living systems: it’s in your DNA backbone, it’s crucial in the energy transducing molecule ATP, and it’s also use as a biochemical tag in proteins. In neutral or alkaline pH, phosphate precipitates into a solid and is not available for chemistry in an aqueous solution; this is known as the “phosphate problem” in the origin of life. Sulfur compounds likely contribute to the acidity in hydrothermal field solutions, which is why I’m studying them.
Terrestrial pools of water have two other attractive features. Since they are not as deep, photosynthesis can play a role. By this I mean that an appropriate mix of molecules that can absorb solar photons that penetrate through the atmosphere provides an additional energy source of driving chemistry. There’s even a pigment-world hypothesis of the origin of life that makes this center-stage. Secondly, a shallow pool potentially allows for wet-dry cycling. This is important because as water evaporates, it concentrates the potential reactants in solution. In particular, evaporating conditions drive the assembly of polymers. If you’re deep in an ocean with lots of water, hydrolysis reigns and water chops up any short polymers back into smaller fragments. The wet-dry cycling of a shallower pool, on the other hand, allows polymers to form and re-form polymers, and a complex mixture could begin to “select” for the most robust ones.
Did life on Earth begin in hydrothermal fields (as opposed to vents)? I don’t know. Deamer makes an attractive case for the fields. But it’s a messy complex system and designing good experiments that allow you to extract good data is not easy. I’m thankful to Deamer for making the distinction between vents and fields explicit and I expect to use his definition in the future.
