Question

Why are most iron-oxidizing chemolithotrophs obligate aerobes and why are the better-studied iron oxidizers acidophilic? (Section 20.5)

Short answer

Iron-oxidizing chemolithotrophs are obligate aerobes because they use oxygen as the terminal electron acceptor, and they are acidophilic because low pH conditions keep Fe²⁺ soluble and accessible.

Step-by-step solution

Understanding Iron-Oxidizing Chemolithotrophs

Iron-oxidizing chemolithotrophs obtain energy by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺). This process releases electrons which are used in the electron transport chain to generate ATP.

Explaining Obligate Aerobes

Obligate aerobes require oxygen to survive. For iron-oxidizing chemolithotrophs, oxygen acts as the final electron acceptor in their respiratory chain. The conversion of Fe²⁺ to Fe³⁺ is typically coupled with the reduction of O₂ to H₂O.

Role of Acidic Environments

Acidophilic iron oxidizers thrive in acidic environments (low pH). In acidic conditions, ferrous iron (Fe²⁺) remains soluble and readily available for oxidation. This is crucial because iron tends to precipitate as ferric iron (Fe³⁺) in neutral and alkaline conditions, making it less accessible for oxidation.

Summarizing the Main Points

Most iron-oxidizing chemolithotrophs are obligate aerobes because they need oxygen as the terminal electron acceptor to generate energy from iron oxidation. They are acidophilic because a low pH keeps ferrous iron (Fe²⁺) soluble and easily accessible for their metabolic processes.

Key concepts

Obligate Aerobes

Iron-oxidizing chemolithotrophs are mostly obligate aerobes. This means they must have oxygen to live. Oxygen is important for their survival because it is used at the end of their electron transport chain.
Oxygen serves as the terminal electron acceptor in these bacteria.
When ferrous iron (Fe²⁺) is oxidized to ferric iron (Fe³⁺), electrons are released. These electrons move through the electron transport chain and finally combine with oxygen to form water (H₂O).
Without oxygen, this process cannot happen, and the bacteria cannot generate the energy they need to grow and survive.

Acidophilic Environments

Many iron-oxidizing bacteria prefer acidic environments. 'Acidophilic' means 'acid-loving', so these bacteria do well in low pH conditions.
In acidic environments, ferrous iron (Fe²⁺) stays dissolved and available for the bacteria to use.
This doesn’t happen in neutral or alkaline environments, where ferrous iron changes to ferric iron (Fe³⁺) and becomes solid. This solid form is hard for bacteria to use.
As a result, iron oxidizers are found more in places like volcanic soils, acidic mine drainage, and hot springs, where the pH is low and Fe²⁺ is plentiful.

Iron Oxidation

Iron oxidation is a process where ferrous iron (Fe²⁺) is changed to ferric iron (Fe³⁺). This chemical reaction releases electrons.
The bacteria use these electrons in their electron transport chain to create energy.
The oxidation of Fe²⁺ to Fe³⁺ produces energy for the bacteria, allowing them to live and grow.
This process is significant in natural ecosystems because it affects the iron cycle and helps in breaking down and cycling minerals.

Electron Transport Chain

The electron transport chain (ETC) is crucial in energy production for cells. In iron-oxidizing bacteria, it plays a major role.
When Fe²⁺ is oxidized to Fe³⁺, electrons are produced. These electrons pass through the electron transport chain, a series of protein complexes in the cell membrane.
As electrons move through the chain, they release energy that the cells use to make ATP, their main energy currency.
The final step in the ETC involves electrons combining with oxygen (O₂) to produce water (H₂O). Therefore, without oxygen, the ETC cannot complete, and energy production halts.

Ferrous Iron

Ferrous iron (Fe²⁺) is an iron ion with a +2 charge. It is important for iron-oxidizing bacteria because it is the form they can easily use and oxidize for energy.
In acidic environments, Fe²⁺ stays dissolved in water, making it accessible for bacteria. However, in neutral or basic environments, Fe²⁺ converts to ferric iron (Fe³⁺) and precipitates, becoming difficult for bacteria to use.
The availability of Fe²⁺ in acidic environments is one reason why acidophilic bacteria thrive there.
By oxidizing Fe²⁺ to Fe³⁺, these bacteria can generate the energy needed for their survival.