Chromatiaceae (Purple Sulfur Bacteria)

Introduction

The family Chromatiaceae (phototrophic purple sulfur bacteria) is a branch of gammaproteobacteria capable of performing anoxygenic photosynthesis. Located in mostly aquatic but sometimes terrestrial regions; populations of these bacteria are observed to grow near deposits of hydrogen sulfide because the gas is utilized as an electron donor for them to produce the chemical energy that sustains life. The nature of their habitat favors the synthesis of their distinct purple pigment as opposed to their other phototroph counterparts that usually present a green pigment from the chlorophyll. In the abstract, the photolithoautotrophs perform anoxygenic

photosynthesis by oxidizing hydrogen sulfide into elemental sulfur or carbohydrates, providing the necessary hydrogen molecules needed to charge the electron transport chain. This leads to photoassimilation of monosaccharides and synthesis of ATP, which is how these bacteria sustain themselves.

Fig. 1: Microscopic image (600x magnification) of the species Chromatium okenii, belonging to the family Chromatiaceae.[1].

Anoxygenic Photosynthesis

By using sulfur for metabolism, the bacteria is substituting the process of photolysis. This is when light is used to split a water molecule (H2O), which leads to the isolation of the hydrogen ion releasing oxygen as a byproduct. Instead of using a water molecule as an electron donor for the electron transport chain, the photolithoautotrophs break down reduced sulfur compounds or hydrogen sulfide gas (H2S) with light energy. In their case, instead of using chlorophyll to harness the sun’s energy, they use bacteriochlorophyll, which absorbs light at a larger wavelength than chlorophyll. As well, in oxygenic photosynthesis there are two photosystems, I and II; however, since photosystem II is the process in which water is broken down, the purple bacteria have no use for this photochemical reaction as no water molecules are present.
Despite the process of isolating the hydrogen ion being different between the two types of photosynthesis, the method of breaking down carbon dioxide (CO2) molecules is quite similar. Two carbon dioxide molecules which are contained in micro-compartments called carboxysomes are necessary for this equation. These compartments are located near the enzyme RuBisCo (ribulose, bisphosphate, carboxylase/oxygenase) which contributes to the first step in the process of photosynthesis. As well as being one of the most abundant enzymes in the world, RuBisCo is the primary enzyme that breaks down the carbon dioxide that is being stored within these carboxysomes. Rubisco fixes the broken down carbon dioxide molecules into bioavailable saccharide molecules. Once the hydrogen sulfide is broken down from the sun’s energy and the RuBisCo separates the carbon dioxide, the products of this equation come in the form of an organic molecule (represented by the chemical compound (CH2O), water (H2O), and elemental sulfur (2S). Upon separating the hydrogen molecules, an electrochemical proton gradient is created within the membrane, which allows for ATP synthesis to occur when ions flow through the enzyme ATP synthase.

Ecology

Populations of purple sulfur bacteria are found in an array of different environments, however, two conditions must be met for there to be a thriving population. There must be a presence of hydrogen sulfide, and there must be infrared light. As opposed to green bacteria that absorb red light with a wavelength of 600nm, the purple sulfur bacteria are optimized to absorb wavelengths of 800nm. Infrared light has a wavelength of 800nm and travels farther than visible light because of its longer length, which allows for colonies of these bacteria to grow in otherwise dark areas.
One of the main deposits of hydrogen sulfide gas comes from naturally occurring sulfur springs. These springs are an aquatic habitat with a consistent sulfide presence, which is fundamental to sustaining the bacteria. These springs emit geothermal heat, leaving the area around them hot all year round. Because of this heat, the colonies surrounding these geothermal vents have evolved to withstand the heat. Research has found that the colonies dependent on the springs have an optimal temperature range of between 48° - 50°C even though researchers believe these heat-tolerant purple sulfur bacteria can form populations at temperatures as low as 43°C. Some prominent observations of large heat-tolerant purple sulfur bacteria colonies can be found in Yellowstone’s Stygian Spring, as well as other hot springs in Japan and Poland.
Apart from heat-tolerant purple sulfur bacteria, by far the largest accumulations of these phototrophs can be found in lakes. This is because these bodies of water have remained for the most part constant. blooms of purple sulfur bacteria are dependent on location and depth level and can be determined by a couple of lake classifications. Firstly, at a depth no deeper than 15m, blooms of the bacteria form in summer and early fall.

Conclusion

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