Question

purple sulfur bacteria carry out photosynthesis in the presence of \(\mathrm{H}_{2} \mathrm{O}\) and \({ }^{14} \mathrm{CO}_{2}\), but only if \(\mathrm{H}_{2} \mathrm{~S}\) is added and \(\mathrm{O}_{2}\) is absent. During photosynthesis, measured by formation of \(\left[{ }^{14} \mathrm{C}\right]\) carbohydrate, the bacteria convert \(\mathrm{H}_{2} \mathrm{~S}\) to elemental sulfur but do not produce \(\mathrm{O}_{2}\). What is the role of the conversion of \(\mathrm{H}_{2} \mathrm{~S}\) to sulfur? Why doesn't photosynthesis produce \(\mathrm{O}_{2}\) in these bacteria?

Short answer

Purple sulfur bacteria use \(\mathrm{H}_{2} \mathrm{~S}\) as an electron donor, converting it to sulfur, to fix \(\mathrm{CO}_{2}\); no \(\mathrm{O}_{2}\) is produced as water isn't the donor.

Step-by-step solution

Understand Photosynthesis Mechanism

In typical photosynthesis (like in plants), water (\(\mathrm{H}_{2}\mathrm{O}\)) serves as an electron donor, which is split to release oxygen (\(\mathrm{O}_{2}\)). The electrons released are used to convert carbon dioxide (\({ }^{14} \mathrm{CO}_{2}\)) into carbohydrates.

Role of Hydrogen Sulfide (\(\mathrm{H}_{2} \mathrm{~S}\))

Purple sulfur bacteria use \(\mathrm{H}_{2} \mathrm{~S}\) instead of water as the electron donor in their photosynthesis process. When \(\mathrm{H}_{2} \mathrm{~S}\) is split, it releases electrons, which are then used to convert \({ }^{14} \mathrm{CO}_{2}\) into \([{ }^{14} \mathrm{C}]\) carbohydrates, while producing elemental sulfur as a byproduct.

Examining Sulfur Formation

The conversion of \(\mathrm{H}_{2} \mathrm{~S}\) to sulfur provides the necessary electrons for photosynthesis in these bacteria. This is similar to how water is split to provide electrons in plant photosynthesis, but it avoids oxygen production because sulfur is produced instead.

No Oxygen Formation In Purple Sulfur Bacteria

Since \(\mathrm{H}_{2} \mathrm{~S}\) is used instead of water, and the chemical reaction produces sulfur, elemental sulfur is formed instead of molecular \(\mathrm{O}_{2}\). Thus, no oxygen is released during the photosynthetic process in purple sulfur bacteria.

Key concepts

Electron Donor

In photosynthesis, an electron donor provides electrons for energy conversion. Typically, plants use water (\( \mathrm{H}_{2} \mathrm{O} \)) as the electron donor which splits to release oxygen (\( \mathrm{O}_{2} \)). This process converts carbon dioxide into carbohydrates. However, purple sulfur bacteria take a different route.
Instead of water, they use hydrogen sulfide (\( \mathrm{H}_{2} \mathrm{~S} \)) as their electron donor.

• Hydrogen sulfide's electrons play a key role in facilitating the conversion of carbon dioxide into carbohydrates.
• This process occurs without producing oxygen.

In these bacteria, hydrogen sulfide's transformation, rather than water splitting, drives photosynthesis. This unique mechanism is critical in environments lacking oxygen, enabling these bacteria to thrive and carry out essential processes.

Hydrogen Sulfide

Hydrogen sulfide (\( \mathrm{H}_{2} \mathrm{~S} \)) serves as a crucial electron donor in the photosynthetic processes of purple sulfur bacteria. It replaces water in typical oxygen-producing photosynthesis.
Here's how it works:

• When \( \mathrm{H}_{2} \mathrm{~S} \) is split, it releases electrons necessary for photosynthesis.
• These electrons enable the conversion of \( { }^{14} \mathrm{CO}_{2} \) into \([ { }^{14} \mathrm{C}]\) carbohydrates.
• This electron transfer doesn’t result in oxygen production.

Thus, in the absence of oxygen, \( \mathrm{H}_{2} \mathrm{~S} \) effectively substitutes water, ensuring that the bacteria can still perform photosynthesis. But instead of oxygen, it produces elemental sulfur as a byproduct.

Elemental Sulfur

When purple sulfur bacteria use hydrogen sulfide for photosynthesis, one of the byproducts is elemental sulfur. This process is different from the one in plants where oxygen gas is produced.
Key points about sulfur formation are:

• The splitting of \( \mathrm{H}_{2} \mathrm{~S} \) provides the electrons needed for carbohydrate production.
• The leftover matter from this reaction forms elemental sulfur.

Unlike plants that release oxygen, these bacteria's process culminates with sulfur rather than \( \mathrm{O}_{2} \) gas.
Elemental sulfur remains as solid yellow deposits, often seen in their habitats like sulfur springs. This distinction highlights the adaptive mechanisms of purple sulfur bacteria enabling them to sustain life in anaerobic conditions.