Are hydrocarbons viable foods for larger organisms?

Question

There are some oil consuming bacteria which can break down hydrocarbons in the ocean. Since these tend to store a lot of energy, I was wondering whether it's possible for larger organisms to take advantage of hydrocarbons (specifically, petroleum) as a food source.

I'm imagining something like an alien planet with vast petroleum deposits close to the surface, and tree-like plants with roots which could reach down and take from these deposits. If this is possible, I could imagine it evolving due to something like thick cloud cover preventing sunlight from reaching the ground.

I could also maybe imagine this working for animals that live underground, if the oil deposits were close enough to the surface that it would leak through cracks. It looks like petroleum has something like three times more energy density than glucose, so if it were possible for animals to take advantage of it seems that might be something they'd evolve to do.

Is this possible? If so, could it ever evolve in real life (assuming it's on a planet where all of the conditions happen to be right)?

Answer 1

Yes, But...:

Biologically, it's certainly possible for multicellular organisms (either directly or through the actions of symbiotic bacteria) to utilize petroleum as an energy source. Life makes similar compounds, like fats, to utilize. But life usually makes these with more enzymatically reactive parts to ease the process, so they generally don't have the enzymes to break down the petroleum as easily. Yet a shocking amount of the food you eat today is actually made from plastic (which is derived from hydrocarbons) so you already eat petroleum.

HOWEVER, petroleum is tricky to digest directly, tends to be highly localized as a food source, and IS NOT RENEWABLE. This seems like a trivial thing, but when you start talking evolutionary times, it is critical. To evolve an enzymatic system, a feeding system around traveling or being scattered from one tar pit to the next, and a way to directly eat tar/crude oil, you are talking millions of years and LOTS of great opportunities. All to evolve the ability to consume a non-renewable resource that that is typically sequestered deep underground.

Further, these multicellular organisms (most likely animals or fungi) would be directly competing with bacteria for these resources. The evolutionary time frame for bacteria is a lot shorter than for eukaryotic organisms, and bacteria are frighteningly efficient about consuming any resources laying around.

What I suspect would happen on such a world is that the local equivalent of bacteria would consume the tar/crude, and then tiny animals would consume the bacteria, and then other organisms would consume those, so you would have a food chain with petroleum at the base all the way up to apex predators at the top - assuming an insanely huge supply of abundant petroleum leaking out of the ground at numerous locations. Existing life forms could rapidly evolve to consuming bacteria (since they all ready do) and bacteria are fairly well established at eating petroleum.

Given enough time, and enough resources, you would have the petroleum equivalent of herbivores (petrovores? Napthavores?) consuming tar, then providing perfect internal growing conditions for bacteria that digest the tar, which the napthavore would then derive sustenance from. That would be the logical progression. All of this would inevitably lead to the consumption of all the petroleum faster and faster until your whole system collapsed for lack of food.

Answer 2

Yes, but it's unlikely to happen in an oxygen atmosphere.

DWKraus's answer does a good job of explaining the major impediment: in an Earthlike environment, petrochemicals are limited in distribution and non-renewable--so if the ability to eat them evolves, it won't last long. If you want to create a geologically-stable ecosystem using this food source, you need to solve the renewability problem. That means designing a planet that can naturally produce hydrocarbons at the surface in large quantities on a continuing basis... and that won't happen in an Earthlike, oxidizing environment. (Living organisms could do it if engineered to, but there's no reason for them to evolve the ability, given that fats are much more convenient.)

Rather than an oxygen-and-CO2-based metabolic ecosystem, you want hydrogen breathers in an atmosphere with a lot of methane. Entirely abiotic processes like upper-atmosphere photochemistry and lightning-induced electrochemistry will continuously produce complex hydrocarbons that rain down on the surface, which multicellular organisms would be incentivized to feed on to gain energy by hydrolyzing them to regenerate methane. And as you can see from the example of Titan, it is entirely plausible for such a world to be shrouded in haze such that very little light gets down to the surface, making direct consumption of hydrocarbons competitive with hydrogenic photosynthesis.

Answer 3

Yes, with the appropriate enzymes, there's no reason they couldn't.

But you need to solve a larger problem before that - the continuous (and renewable) availability of hydrocarbons. You can't do that with fossil petroleum. So, you need to start with organisms that produce hydrocarbons as energy storage (it's not too different from using fat, but the reactions aren't as efficient in the organic life temperature range; however, with the appropriate enzymes, who knows).

From there, you can have organisms evolve to predate on other hydrocarbon-rich organisms, gradually abandoning all other food sources. You could end with vampire-like predators and plant analogues storing hydrocarbon reservoirs like tubers deep underground. Once the vampire moles develop enough, they might also be able to detect, reach and exploit petroleum reservoirs, at least those not too far from the surface.

Answer 4

Symbiosis between a large organism and petroleum metabolizing bacteria

Living things operate in water and oil and water don't mix, so it is very difficult for living things to use pure hydrocarbons. Fats used by your body are mostly not pure hydrocarbons. For instance, fatty acids have carboxylic acid group that becomes negatively charged in water at neutral pH, making them much more water soluble than their purely hydrocarbon analogs (however, they still need help from lipoprotein particles to be carried around the body). The carboxylic acid group provides a handle for doing further chemistry as well.

This paper provides a lot of the details, but here's a good quote:

Due to the hydrophobicities and low water solubilities of most petroleum hydrocarbons, the biodegradation rate is generally limited in the environment.

For this reason, with the right selection pressures, I think that large organisms might actually be able to do much better than bacteria at metabolizing petroleum, since they can create internal environments (temperature, pH, oxygen concentration) more suitable for the reactions necessary to break down hydrocarbons than microorganisms.

While it's possible that the large organism could evolve the enzymes for degradation itself, I'm imagining that they might evolve like cows. The cows themselves do not produce the enzymes necessary to digest the tough plant matter that they eat (mostly cellulose and lignin) but create an environment that in which can do it for them. Chewing their cud provides mechanical mixing and the bacteria, protozoa, fungi, and archaea in their rumen convert the cellulose and lignin to fatty acids and other fairly water soluble molecules that can be used directly by the cow.

Petroleum-eating cows So, I imagine that a large organism could evolve a kind of stomach populated by petroleum-degrading microbes that provides an optimal environment for the microbes, uses mechanical mixing to expose more oil to these microbes, and helps to efficiently solubulize the oil by secretion of some kind of surfactant.

Why haven't such creatures evolved on Earth? As mentioned in the other answers, maybe there just isn't enough petroleum on Earth for this to be sustainable. However, you could imagine a planet where petroleum is more abundant and produced at a greater rate. Such petroleum cows might evolve there.

Answer 5

Fats from our real existing diets are basically long-chain hydrocarbons (more specifically, carboxylic acids with long hydrocarbon chain) connected to a glycerol backbone via ester bonds; so yes, it is possible.

Source: Wikipedia.

While fatty acids and fats are not hydrocarbons per se, they both have moieties made of long-chain aliphatic hydrocarbons. These hydrocarbon chain moieties are where their high energy density is coming from -- it does not come from the carboxylate group. The question seems to be about biology and physiology and they both "treat" similar molecules in similar ways.