How concentrated would a metal ore need to be to be extractable by a suitable plant?

Question

So there is a Minecraft mod that includes various oreberry bushes. One for gold, one for silver, etc. Now I know that various plants can extract specific minerals in higher quantities that what would normally be expected, so my question revolves around concentrations in the environment. If I wanted a plant that extracted, say gold, from the environment and produce pea-sized nodules of pure gold how concentrated would the gold have to be to have a bush actually do this? I think it would be interesting to have a bush that a harvest gave a couple dozen of these little balls of pure metal instead of normal fruits. I know that this might make propagation of the bush difficult, so maybe in addition to the ore, the plant also produces something like hollow seed pods and to keep refreshing the minerals maybe the plant lives in or near an area of frequent volcanic activity...

Answer 1

Ignoring all other mechanisms, a plant can only pull as much ore as its root system can reach. It can't create gold or silver from nothing, so it's a matter of how much actually exists. If you would mine and refine some ore and end up with say, 1 kg of gold, then your plant can't ever produce more than 1 kg of gold - whether it does that in one harvest or over several is up to how efficient you want it.

But there's an alternative - what if your plants extract ores from water?

I haven't found a great reference for this, but it seems like on earth reasonable levels for silver and gold in river water are going to be around .1 to 1 ppb - that's part per billion. Taking the upper bound means you'd need to process 1 billion liters to get 1 gram of the mineral of your choice. Now that sounds like a lot, but major rivers have flow rates on the order of 10 million liters per second. Now obviously a single bush can't sample the entire river, but let's say you want a harvest of 10g per year. Then your bush only needs to sieve ~16 liters a second, which is less than a cubic foot. A larger plant that sieved a cubic yard would be able to produce 300g-400g per year.

There's a couple of issues:

1. If you plant two bushes next to each other, the one directly downstream won't get as much because a lot of its water will already be sieved. So you need to spread them out enough that random fluctuations will replenish the gold for the downstream plant.
2. There's no major benefit for a plant to provide gold or silver in easily harvestable pellets. So instead, what if the plants are genetically engineered as part of an environmental cleanup strategy? You'd have plants for major minerals, plastics and other environmental wastes, and they'd collect and concentrate them for human harvest. That also lets you plant bushes closer together, because you can alternate types to avoid diminishing returns.

Answer 2

The necessary density of the mineral would be directly related to the extent and density of the root structure of the plant in question, and the amount that you hope to harvest.

Contrary to what people might expect, most of the mass of a plant comes from the air, specifically the carbon dioxide that it filters into pure oxygen. However, the remainder of a plant's mass is generally made of retained water and minerals extracted from the earth, so, on a very small scale, all plants do what you're asking about.

The real limit for what a plant could uptake is how much metal its roots come in contact with. If a plant has a root-ball that is about a cubic meter, and each cubic meter of dirt contains, say, three grams of gold, then the most gold you could expect to extract from the plant would be three grams or less, probably much less.

This leads us to the first obstacle to harvesting nice little berries made of gold. If the soil concentration of a useful mineral is high enough to be harvestable from the plant, it would probably be in such high concentrations in the soil that you would still be better off just digging it up and smelting it.

Additionally, If the mineral in question was a nutrient, it would most likely be distributed throughout the plant fairly evenly, with only slightly higher concentrations in new growth.

The only good reason I can see for a plant to produce metal berries is if there is a high groundwater concentration of the metal, and the metal is a mild toxin to plant life. In that case, I could envision a plant that purified its own soil by sequestering as much of the metal as it could into tiny nodules, so that the rest of the plant wouldn't be affected. The constant flow of new toxins carried by the ground water would overcome the soil density problem too, since the trace amounts of the mineral would build up over time, instead of already being in useful concentrations in the first place.

Answer 3

Let's think about what the plant needs to do.

The metal needs to be pulled from the ore and transported. Clearly the best way to do this is chemically, so we'll assume that the process centers around a reaction which converts the metal in the ground (most likely in an oxide form) into an aqueous ionic compound which can be transported up the roots and into the foliage where it can be deposited. The depositing is very reasonable; there are countless metal deposition examples in nature.

The process of getting the metal may be a bit interesting. Some metal processing methodologies require heat, and they would be inaccessible to a plant. On the other hand, you have access to the entire repertoire of enzymatic activities, so let's assume this process works.

As has been mentioned in other posts, transport of metal is the issue. You can only collect what you can reach with the chemical processes. If you're looking for gold, you're out of luck. The average volume of a plant root ball doesn't have more than a scant amount of gold. How small, consider this site's assay of soil, multiplied by a sample 1 cubic meter root ball:

• 0.002 ppm Gold = 38ug
• 0.01% Aluminum = 270g aluminum
• 0.01% Iron = 780g iron

This is, of course, assuming complete extraction from the root ball.

However, we're not playing by nature's rules here. Nature doesn't reward massive strip-mining of entire countrysides to harvest metals. Nature builds things up in ecosystems, where each participant is dependent on the actions of the other participants. What if we could convince these plants (by breeding, or something more extreme like genetic engineering) to work with some helper bacteria or insects to aggregate the metals for us.

Yosemite has been measured to have a tree density of roughly 130 trees/Ha (give or take). At 300,000 Ha, that's 39 million trees. If we could have them all transfer their gold to one master tree, it would be able to acquire roughly 1.5kg of gold.

At gold's density of 19.7 g/cm^3, that's 77 cm^3 of gold. 77 cm^3 is roughly a sphere 5cm in diameter. This means, deep in the heart of your forest, you can have one tree which produces a small apple of pure gold. But only the true of heart would find it.

Answer 4

There was one factor in the question that seems to have been overlooked in the other answers that may make a gold harvesting plant a little more plausible...

If the plant evolved in an area with significant volcanic activity you may have a reasonable explanation of both why the plant collected valuable minerals/metals and how it would have access to more significant quantities of them.

If your bush looked more like a mangrove and grew in water run off from active geysers/hot-springs, in a place like Yellowstone you may be be able to suspend disbelief enough to make it work.

The reason I'm pointing to the mangrove, and specifically the black mangrove, is the way that it has adapted to living in salt water.

It is a hardy species and expels absorbed salt mainly from its leathery leaves.

"Avicennia germinans-salt excretion" by Ulf Mehlig - Own work. Licensed under CC BY-SA 2.5 via Wikimedia Commons.

Basically you would have a mangrove like plant, that grows in heavily mineral/metal laden water, that excretes excess mineral/metal build up in nice little crystals on its leaves as a byproduct. The plant doesn't really want or use most of the minerals/metals it just developed a handy way to get rid of them so that it can live in water that is tainted with them. Perhaps the mangrove developed the adaptation in order to have access to a relatively warm microclimate that supports an active microbial ecology to feed off of.

I know this isn't exactly the "berries" that you were looking for, but this scenario may be a bit more believable.

Answer 5

Presuming that you had a plant that could extract and concentrate a particular material, the answer is that it could happen at very low soil concentrations indeed, however in such a circumstance, the yield would be very low.

A plant might concentrate material by absorbing water that contained dissolved material, or it might more actively release then reabsorb a solvent for that material. Once the material was in the plant, it would simply be a matter of transporting it to the site of expression and precipitating it out of solution.

A plant which does this might produce nodules of the material in question, separate from its seeds. As to evolutionary advantage, if a plant evolved to do this and humans discovered it, its success would be assured, as humans would cultivate and protect it. There is relatively little other advantage to the plant except in the presence of humans, so this would be a symbiotic relationship.

Answer 6

There are species of common flowering clover that are found to remove nickel from soil and absorb it into every cell in the planet. it concentrates the nickel at above 2% by dry weight of planet matter, easily recoverable by burning. it's been used for ground contamination cleanup. surely there's a plant out there that would consume gold, and for all the useful chemical processes it would benefit itself from the uptake of gold from the soil. perhaps even a salt tolerant plant that can pull that scant gold found in seawater that everyone says is rather pointless to try to recover. and then of course are the nanites. I mean. Why bother with plants at all when you got ant-brained nanotech swarms harvesting all the metals from the earth? nanites beat BOTH paper AND rock. but they just sort of stare at the scissors with a distrustful glare.

Answer 7

There's a modern method for extracting gold from pine trees grown on mine tailings but you're talking about grams from a whole pine tree and repeat plantings over many years to get a few percent of what's there. In the case of a custom organism the concentration in the soil isn't that important; what you really need to look at is the absolute amount of gold or silver in the soil because really you can concentrate that amount as much as you like by having a greater or lesser extent and retrieval efficiency to the bushes' root systems in grams (or ounces) per length of time. That will give you an idea of how many bushes you'll have in an area and how often you can make a harvest of them.