1- The main problem with metal eating microbes is that most metals are already found in oxidized, low energy states. This means that you would have to transform low-energy compounds to even lower energy compounds in order to use them for food.

This means that if you eat iron oxide (rust) and convert it to pure iron, you would actually have to provide it energy instead of hoping to obtain any energy from it. However if you convert oxides into chlorides and chlorides into sulphates/flourides, you can obtain energy from the process.

However remember that you cannot obtain energy from converting oxidized metal compounds back into pure metal. Suggesting that would send waves of angry chemistry professors at your door.

2- The lethal miasma issue. While some metal compounds are extremely deadly, there is no compound which is equally deadly to all creatures.

Plus, even if you do have a highly lethal miasma, it automatically suggests it would be a reactive compound. Which means that if you leave it be, it will react with water vapor, oxygen, carbon dioxide or something else in the environment (specially in the soil) and become non-toxic. There is NO toxic compound which you cannot turn non-toxic by treating it with other chemicals.

3- A self sustainable cycle always involves recycling. If you have a process that only goes in one direction, there would be only so much time before the reactants finish and the process stops.

For example, in your scenario, you have plants processing metallic compounds into lethal, unusable compounds.

plants find metallic compounds and utilize them => plants proliferate and render more and more metal reserves unusable as their (plants) number grows => all metal reserves are soon rendered unusable and all plants die.

You see the problem with this scenario? There is no recycling involved. Incorporate it and your ecosystem would be more practical and believable.

#Edit To Add

4- A very serious problem with your world would be that all complex life forms on Earth require some seriously large amounts of metals in their bodies. For example, take the case of chordates (backboned animals). All chordates require several grams of iron in their bodies for blood (haemoglobin). All chordates also require metals (calcium) for bones and teeth formation.

You cannot have complex life on your planet where metals are out of reach.

1- The main problem with metal eating microbes is that most metals are already found in oxidized, low energy states. This means that you would have to transform low-energy compounds to even lower energy compounds in order to use them for food.

This means that if you eat iron oxide (rust) and convert it to pure iron, you would actually have to provide it energy instead of hoping to obtain any energy from it. However if you convert oxides into chlorides and chlorides into sulphates/flourides, you can obtain energy from the process.

However remember that you cannot obtain energy from converting oxidized metal compounds back into pure metal. Suggesting that would send waves of angry chemistry professors at your door.

2- The lethal miasma issue. While some metal compounds are extremely deadly, there is no compound which is equally deadly to all creatures.

Plus, even if you do have a highly lethal miasma, it automatically suggests it would be a reactive compound. Which means that if you leave it be, it will react with water vapor, oxygen, carbon dioxide or something else in the environment (specially in the soil) and become non-toxic. There is NO toxic compound which you cannot turn non-toxic by treating it with other chemicals.

3- A self sustainable cycle always involves recycling. If you have a process that only goes in one direction, there would be only so much time before the reactants finish and the process stops.

For example, in your scenario, you have plants processing metallic compounds into lethal, unusable compounds.

plants find metallic compounds and utilize them => plants proliferate and render more and more metal reserves unusable as their (plants) number grows => all metal reserves are soon rendered unusable and all plants die.

You see the problem with this scenario? There is no recycling involved. Incorporate it and your ecosystem would be more practical and believable.

Edit To Add

4- A very serious problem with your world would be that all complex life forms on Earth require some seriously large amounts of metals in their bodies. For example, take the case of chordates (backboned animals). All chordates require several grams of iron in their bodies for blood (haemoglobin). All chordates also require metals (calcium) for bones and teeth formation.

You cannot have complex life on your planet where metals are out of reach.

1- The main problem with metal eating microbes is that most metals are already found in oxidized, low energy states. This means that you would have to transform low-energy compounds to even lower energy compounds in order to use them for food.

This means that if you eat iron oxide (rust) and convert it to pure iron, you would actually have to provide it energy instead of hoping to obtain any energy from it. However if you convert oxides into chlorides and chlorides into sulphates/flourides, you can obtain energy from the process.

However remember that you cannot obtain energy from converting oxidized metal compounds back into pure metal. Suggesting that would send waves of angry chemistry professors at your door.

2- The lethal miasma issue. While some metal compounds are extremely deadly, there is no compound which is equally deadly to all creatures.

Plus, even if you do have a highly lethal miasma, it automatically suggests it would be a reactive compound. Which means that if you leave it be, it will react with water vapor, oxygen, carbon dioxide or something else in the environment (specially in the soil) and become non-toxic. There is NO toxic compound which you cannot turn non-toxic by treating it with other chemicals.

3- A self sustainable cycle always involves recycling. If you have a process that only goes in one direction, there would be only so much time before the reactants finish and the process stops.

For example, in your scenario, you have plants processing metallic compounds into lethal, unusable compounds.

plants find metallic compounds and utilize them => plants proliferate and render more and more metal reserves unusable as their (plants) number grows => all metal reserves are soon rendered unusable and all plants die.

You see the problem with this scenario? There is no recycling involved. Incorporate it and your ecosystem would be more practical and believable.

1- The main problem with metal eating microbes is that most metals are already found in oxidized, low energy states. This means that you would have to transform low-energy compounds to even lower energy compounds in order to use them for food.

This means that if you eat iron oxide (rust) and convert it to pure iron, you would actually have to provide it energy instead of hoping to obtain any energy from it. However if you convert oxides into chlorides and chlorides into sulphates/flourides, you can obtain energy from the process.

However remember that you cannot obtain energy from converting oxidized metal compounds back into pure metal. Suggesting that would send waves of angry chemistry professors at your door.

2- The lethal miasma issue. While some metal compounds are extremely deadly, there is no compound which is equally deadly to all creatures.

Plus, even if you do have a highly lethal miasma, it automatically suggests it would be a reactive compound. Which means that if you leave it be, it will react with water vapor, oxygen, carbon dioxide or something else in the environment (specially in the soil) and become non-toxic. There is NO toxic compound which you cannot turn non-toxic by treating it with other chemicals.

3- A self sustainable cycle always involves recycling. If you have a process that only goes in one direction, there would be only so much time before the reactants finish and the process stops.

For example, in your scenario, you have plants processing metallic compounds into lethal, unusable compounds.

plants find metallic compounds and utilize them => plants proliferate and render more and more metal reserves unusable as their (plants) number grows => all metal reserves are soon rendered unusable and all plants die.

You see the problem with this scenario? There is no recycling involved. Incorporate it and your ecosystem would be more practical and believable.

#Edit To Add

4- A very serious problem with your world would be that all complex life forms on Earth require some seriously large amounts of metals in their bodies. For example, take the case of chordates (backboned animals). All chordates require several grams of iron in their bodies for blood (haemoglobin). All chordates also require metals (calcium) for bones and teeth formation.

You cannot have complex life on your planet where metals are out of reach.