Photoheterotroph

Photoheterotrophs are organism in the food chain that uses sunlight as their energy source and organic compounds from the environment such as carbohydrates, fatty acids, and alcohols as the source of carbon. Unlike photolithotrophs, photoheterotrophic organisms cannot use carbon dioxide as their sole carbon source.

They are microorganisms that cannot make their food like any autotrophs. Some common examples are purple non-sulfur bacteria and green non-sulfur bacteria. Humans also exhibit this type of nutrition.

Photoheterotrophic organisms generate ATP using two methods:

1. Using Bacteriorhodopsin

It is a chlorophyll-based mechanism similar to photosynthesis. The light activates the molecules of the pigment bacteriorhodopsin resulting in the movement of electrons through the electron transport chain. This flow of electrons through the proteins pumps hydrogen ions across the membrane. The energy stored in the proton gradient is used for ATP synthesis.

The electron flow in photoheterotrophs is cyclic because the electrons released from the reaction center flow through the transport chain and return to the reaction center.

Purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria are examples of such photoheterotrophs.

2. Using Purple-Rhodopsin-Based Proton Pumps

These pumps are integral membrane proteins. Such pump consists of a single protein bound to retinal, a Vitamin A derivative. The pump may have accessory pigments like the carotenoids associated with the protein.

On absorbing light, the retinal molecule isomerizes, changing the molecule’s shape and pumping a proton across the membrane. The proton gradient generates ATP, transports solutes across the membrane, or drives the flagellar motion.

One flavobacterium is insufficient to reduce carbon dioxide using sunlight. Instead, it uses energy from its rhodopsin system to convert it through the process of anaplerotic fixation, as shown below:

CO₂ + pyruvate + ATP (from photons) → Malate + ADP +Pi, where malate is obtained by breaking down other carbohydrates:

Carbohydrate + O₂ → Malate + CO₂ + Energy

This method of carbon fixation is used by Halobacteria, Flavobacteria, and Vibrios when reduced carbon compounds are scarce. However, energy is plentiful in the form of sunlight.

Photoheterotroph and photoautotroph are two types of phototrophic organisms. Two ways in which both are similar are:

Similarities

• Use sunlight as the source of energy (ATP)
• Use electron transport chain to generate ATP

However, there are some critical differences between the two, based on which we place them under two groups. 

Differences

Basis	Photoheterotroph	Photoautotroph
1. Making Food	Depends on other organisms for food	Can make their food by photosynthesis
2. Carbon Source	Use organic carbon as its sole carbon source	Use carbon dioxide as the sole carbon source
3. Example	Purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria	Green plants, algae, green sulfur bacteria, and purple sulfur bacteria
3. Habitat	Live in deep-sea hydrothermal vents, active volcanoes, and ocean floors where sunlight is unavailable.	Grow on land or in the water wherever they can get sunlight.