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 (H2S) with light energy. In their case, instead of using green chlorophyll to harness the sun’s energy by absorbing red light, which has a shorter wavelength, they use purple-pigmented bacteriochlorophyll. These absorb infrared light, which has a longer wavelength. This allows blooms to form in visibly darker regions because infrared light can reach these deeper places. 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 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, the largest accumulations of these phototrophs can be found in lakes. The most favorable type of lake for blooms to grow would be a confined meromictic lake. This is because these bodies of water form layers depending on salinity and depth. These layers never mix, which is why sulfide accumulations can form in lower anoxic regions of the lake. The sulfide seeps into the lower regions of the lake through sediments at the bottom that contain sulfides. Blooms of purple sulfur bacteria are prone to grow in these lower anoxic regions of these meromictic lakes in part because of the abundance of resources within the area and not a lot of competition but also because these lakes are stagnant, meaning the collection of these resources are incredibly confined and will never mix with the upper levels of the lake.

Fig. 2: Phylogenetic tree of the Chromatiaceae family. The construction is based on 16S rRNA and created using the PHYML algorithm (Guindon and Gascuel, 2005). The sequence dataset and alignment were used according to the All-Species Living Tree Project (LTP) database (Yarza et al., 2010; http://www.arb-silva.de/projects/living-tree) and strain info project (Dawyndt et al., 2006; http://www.straininfo.net). Representative sequences from closely related taxa were used as outgroups. The scale bar indicates the estimated sequence divergence.

Conclusion

Edited by Silas Richland: Student of Joan Slonczewski BIO 116, 2024, Kenyon College.