Difference Between Assimilatory and Dissimilatory Sulphate Reduction

The key difference between assimilatory and dissimilatory sulphate reduction is that assimilatory sulphate reduction produces cysteine as an end product while dissimilatory sulphate reduction produces sulfide as an end product.

Sulphate reduction is one of the main anaerobic respiratory pathways. Furthermore, some microbes that are dependent on anaerobic conditions are capable of reducing sulphates in order to obtain energy. Moreover, there are two main pathways in which sulphates reduce; they are the assimilatory pathway and the dissimilatory pathway. Wherein, in the assimilatory pathway, the reduction of sulphate yields Cysteine as its final product, which can be assimilated in living organisms such as plants and higher eukaryotes. On the contrary, the dissimilatory pathway of sulphate reduction yields sulfide as the end product. Hence, the most significant, distinguishable difference between assimilatory and dissimilatory sulphate reduction is the type of the end product each produce.

CONTENTS

1. Overview and Key Difference
2. What is Assimilatory Sulphate Reduction
3. What is Dissimilatory Sulphate Reduction
4. Similarities Between Assimilatory and Dissimilatory Sulphate Reduction
5. Side by Side Comparison – Assimilatory vs Dissimilatory Sulphate Reduction in Tabular Form
6. Summary

What is Assimilatory Sulphate Reduction?

As said before, assimilatory sulphate reduction is one of the two main pathways in sulphate reduction, which is one of the main anaerobic respiratory pathways. Specifically, it takes place in microbes including prokaryotic bacteria, fungi and photosynthetic organisms. These organisms are capable of undergoing anaerobic reactions for energy generation. Here, the main form of energy source for sulphate reducing organisms is the sulfate. The sulfate reduces into cysteine, which is the prominent end product of this pathway. Furthermore, the enzymes mediate this process. Also, this pathway depends on ATP. The final product, cysteine, is important for the formation of a carbon skeleton, which is in the form of cysteine amino acid or homocysteine.

In the assimilatory sulphate reduction pathway, the starting compound sulphate first converts into adenosine – 5 – phosphosulfate (APS). Afterwards, the APS reduces to form sulfide via a series of enzyme-catalyzed reaction. Then, the final step in the assimilatory pathway of sulphate reduction is the synthesis of cysteine from sulfide. This whole process requires an enzyme called O–acetylserine sulfhydrylase.

What is Dissimilatory Sulphate Reduction?

Dissimilatory sulphate reduction is an anaerobic process. It is the second pathway in pathways in sulphate reduction. Here too, some prokaryotes, eukaryotic fungi and photosynthetic organisms are capable of reducing sulphate in the dissimilatory pathway. However, the dissimilatory sulphate reduction produces sulfide as its final product. Like assimilatory sulphate reduction, it is also an enzyme-mediated process and depends on ATP.

Thus, similar to the assimilatory pathway, the first reaction here is the activation of sulphate to form Adenosine – 5 – phosphosulphate (APS). Subsequently, the APS becomes sulphite and then sulfide via a series of enzymes-facilitated chemical reactions. Therefore, in the dissimilatory pathway of sulphate reduction, the final product is sulfide, an inorganic compound.

What are the Similarities Between Assimilatory and Dissimilatory Sulphate Reduction?

• Both processes take place under anaerobic conditions.
• Also, the starting compound of both processes is sulphate.
• Furthermore, sulphate acts as the final electron acceptor in both processes.
• Moreover, both reduction processes are ATP dependent.
• In addition, the activation of sulphate to adenosine – 5 – phosphosulphate is common to both processes.
• Besides, they are enzyme-catalyzed reactions.
• Both reduction processes are carried out by prokaryotes, fungi and photosynthetic organisms.

What is the Difference Between Assimilatory and Dissimilatory Sulphate Reduction?

Assimilatory and Dissimilatory Sulphate Reduction are two anaerobic processes in which sulphate act as the starting material. In particular, microbes employ these processes to obtain energy for their activities. However, the key difference between assimilatory and dissimilatory sulphate reduction is that the assimilatory sulphate reduction finally produces cysteine while the dissimilatory sulphate reduction finally produces sulfides. Another difference between assimilatory and dissimilatory sulphate reduction is the enzymes that involve in the catalysis of the reactions. The enzyme O – acetylserine sulfhydrylase catalyzes the assimilatory sulphate reduction while dissimilatory sulfite reductase catalyzes the dissimilatory sulphate reduction.

The below infographic on the difference between assimilatory and dissimilatory sulphate reduction shows more details.

Summary – Assimilatory vs Dissimilatory Sulphate Reduction

Assimilatory and Dissimilatory Sulphate reduction processes are anaerobic processes. In both processes, sulphate acts as the final electron acceptor. Also, both processes are carried out by microbes and photosynthetic organisms. Moreover, they both depend on ATP as well. However, assimilatory reduction of sulphate is the process that produces cysteine and homocysteine as the final products. In contrast, dissimilatory sulphate reduction produces sulfide as the end product. Therefore, this is the difference between assimilatory and dissimilatory sulphate reduction.

Reference:

1.Koprivova, Anna, et al. “Assimilatory Sulfate Reduction in C3, C3-C4, and C4 Species OfFlaveria.” Plant Physiology, American Society of Plant Biologists, 1 Oct. 2001. Available here  
2.“Unifying Concepts in Anaerobic Respiration: Insights from Dissimilatory Sulfur Metabolism.” NeuroImage, Academic Press, 10 Sept. 2012. Available here  

Image Courtesy:

1.”Dvulgaris micrograph”By No machine-readable author provided. Graham Bradley assumed (Public Domain) via Commons Wikimedia  
2.”Dissimilatory Sulfate Reduction overall reactions”By CtSkennerton – Own work, (CC BY-SA 4.0) via Commons Wikimedia