Chemosynthesis

Up here, near the surface of Earth, where the sun shines, life depends on photosynthesis. I hope that everyone has, at least, seen this equation that represents photosynthesis.

CO₂ + H₂O –> O₂ + C₆H₁₂O₆

I hate to tell you this, but this is really kind of wrong. Like much of what we learn in the high school and the first year of college, it is a gross over simplification. This is the end result of photosynthesis from the moment the first photon hits a chlorophyll a molecule to the point where the collected energy is stored in a carbohydrate (sugar).

There are the light dependent reactions that absorb photons to begin an electron transport chain that converts ADP into ATP and NADP⁺ into NADPH. ATP and NADPH are the energy carrying molecules that are used by cells to accomplish various tasks.

Cells cannot use sugar. In the mitochondria, sugar is reduced and the energy gained is used (by another electron transport change to change ADP into ATP, which the cells then use to do things (like assemble proteins or move molecules across the cell wall).

In the light-independent reactions in the plant cell, the high energy ATP and NADPH molecules are used to convert CO₂ into long term storage carbohydrates.

After the Calvin cycle produces 3-carbon sugars, which are further refined into glucose, and ADP, which waits for the next light dependent cycle to be transformed into ATP again.

This is all based on photons from the sun.

But there are places that don’t get photons from the sun. The deep holes of Earth, under tens of thousands of feet of water (and coincidentally, in places elsewhere in the solar system, like Europa).

The neat thing about chemosynthetic reactions is that there are a couple of different ways that organisms can extract energy from compounds that are readily available.  Here’s a few… for the present, we’ll use the over-simplified ones with the understanding that the chemical pathways are much more complex than this.

Hydrogen Sulfide is the stuff that makes rotten eggs smell bad and is the principle component of stink bombs. However, it’s a common component in volcanic events. The “black-smokers” which are extremely hot under-sea volcanic vents can emit tons of hydrogen sulfide (H₂S).

H₂S + 3O₂ –> 2SO₂ + 2H₂O + energy

A variant on this one is a little more interesting because of one of the products.

6CO₂ + 6H₂O  + 3H₂S –>  C₆H₁₂O₆  + 3H₂SO₄

Yeah, one of the products is sulfuric acid. Talk about Montazuma’s Revenge… ouch.*

Another common component of volcanic vents is methane gas (CH4).  All of the hydrocarbons (molecules with carbon and hydrogen) are high energy molecules. We run our cars on them. Even the smallest of them, methane, has enough energy to be extracted by organisms.

CH₄ + 2O₂ –> 2H₂O + CO₂ + energy

That reaction should look pretty familiar. It’s the same reaction that is used by mitochondria and our cars. A carbon molecule, burning in oxygen, produces energy, water, and carbon dioxide. We use it in our bodies… in a more controlled way than a marshmallow burning in a fire.

And that equation is the same, except that methane can form without life.

There are also several nitrogen based systems used by bacteria.

First bacteria that nitrify ammonium for their energy source (generally of Nitrosomonas or Nitrosococcus genus ). Do the following:

NH₃ + O₂ + 2H+ + 2e⁻ → NH₂OH + H₂O

NH₂OH + H₂O → NO⁻² + 5H⁺ + 4e⁻

What this means it that the bacteria use a small amount of energy (2 electrons) and two hydrogen ions to create ammonium hydroxide and water. Then they convert the ammonium hydroxide into nitrites, 5 hydrogen ions, and more energy (4 electrons). It’s not very efficient, but it’s enough for these bacteria to live on.

Now the nitrifying bacteria that work on nitrites go to work. These are generally Nitrobacter or similar genera. They do the following:

NO⁻² + H₂O → NO⁻³ + 2H⁺ + 2e⁻

Basically, the is taking the nitrites and hydrolysing it to form nitrates, 2 more hydrogen ions and 2 electrons for the energy.

The bacterial species Thiobacillus denitrificans, Micrococcus denitrificans and the genus Pseudomonas use oxygen as a terminal electron receptor which is why it must be in an anaerobic environment.  The chemical reaction in these bacteria is as follows:

2NO₃^– + 10e^– + 12H⁺ → N₂ + 6H₂O

So, those are some of the nitrogen reactions that will generate energy without use of the sun.**

Then there’s a whole cluster of bacteria that use metals as their electron source (instead of water or sulfur). Common metals used include iron, manganese, chromium, uranium (yeah!), nickle, copper, and cadmium. Most of those are highly toxic to us.

So what’s the point of all these chemical reactions? Believe me, it’s not so I can type these into WordPress… that’s a pain.

The point is that these are very simple chemical reactions that generate energy that is usable by cells. As I said, burning sugar or methane generates plenty of energy, but it’s kind of hard on the cell.

Cellular systems can “control” the rate of energy released (like in an electron transport chain) and collect small amounts of energy over time instead of one giant lump of energy all at once.

But this also shows that there are many ways life can exist without having access to the sun. Which is important considering deep sea trenches or other bodies in the solar system. Given some water, methane, and trace elements, there’s no inherent reason that life couldn’t exist in millions of comets, small asteroids, and moons in our solar system. Yes, bacteria-like life, but life never-the-less.

In another post, I hope to talk about the evolution of chemosynthesis and the implications for the development of life on Earth.

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* Yes, I am 9.

** I wrote this some time ago for another blog of mine and modified it for this one.

 

REFERENCES:
Geomicrobiology of Deep-Sea Hydrothermal Vents. Holger W. Jannasch and Michael J. Mottl. Science 23 August 1985: 229 (4715), 717-725. [DOI:10.1126/science.229.4715.717]

Christian Gaillard, Michel Rio, Yves Rolin and Michel Roux. PALAIOS Vol. 7, No. 4 (Aug., 1992), pp. 451-465