Archaea Bacteria

 The word Archaea is derived from the Greek word Archaios which means ancient.

The Archaeabacteria, are quite diverse, both in morphology and physiology. They may be spherical, rod-shaped, spiral, lobed, cuboidal, triangular, plate-shaped, irregularly shaped, or pleomorphic. Some are single cells, whereas others form filaments or aggregates. They range in diameter from 0.1 to over 15 µm, and some filaments can grow up to 200 µm in length. Multiplication may be by binary fission, budding, fragmentation, or other mechanisms.

The Archaea are diverse physiologically- can be aerobic, facultatively anaerobic, or strictly anaerobic. They include psychrophiles, mesophiles, and hyperthermophiles that can grow above 100°C. Nutritionally they may be chemolithoautotrophs to organotrophs.

Ecology

Archaea are found in areas with either very high or low temperatures or pH, concentrated salts, or completely anoxic. These are generally referred to as “extreme environments.”

Extreme and hypersaline are situations where humans could not survive. Most of the Earth (the oceans) is an “extreme environment” where it is very cold (about 4°C), dark, and under high pressure. Many Archaea are well adapted to these environments, where they can grow to high numbers. Archaea constitute at least 34% of the prokaryotic biomass in some Antarctic coastal waters. In some hypersaline environments, the brine is red with archaeal pigments. Some archaea are symbionts in the digestive tracts of animals. Archaeal gene sequences have been found in soil and temperate and tropical ocean surface waters.

Thus, the Archaea are highly diverse with respect to morphology, reproduction, physiology, and ecology. Although best known for their growth in anoxic, hypersaline, and high-temperature habitats they also inhabit marine arctic, temperate, and tropical waters. Their RNA, ribosomes, elongation factors, RNA polymerases, and other components distinguish Archaea from Bacteria and eukaryotes. Much of archaeal metabolism appears similar to that of other organisms, but the Archaea differ with respect to glucose catabolism, pathways for CO₂ fixation, and the ability of some to synthesize methane.

Archaeal Cell Walls

The Archaeal cell wall, like the bacterial cell wall, is a semi-rigid structure which provide protection to the cell from the environment and from the internal cellular pressure. The cell walls of bacteria typically contain peptidoglycan, but it is absent in Archaea. 

The chemistry of Archaeal cell walls is different from that of Eubacteria. Archaea lack the muramic acid and D-amino acids that make up peptidoglycan. Thus they resist attack by lysozyme and β lactam antibiotics such as penicillin. 

Archaeal cell walls stain either Gram positive or Gram negative, depending on the thickness and mass of cell wall. 

Gram positive Archaea have a variety of complex polymers in their cell wall. Methanobacterium and some other methanogenic archaea have pseudomurein (a peptidoglycan-like polymer that is cross-linked with L-amino acids), N-acetyl talosaminuronic acid instead of N-acetyl muramic acid and β (1- 3) glycosidic bonds instead of β (1-4) glycosidic bonds. Methanosarcina and Halococcus lack pseudomurein and contain complex polysaccharides similar to the chondroitin sulfate of animal connective tissue. Other heteropolysaccharides are also found in Gram positive cellwalls. 

Gram negative Archaea have a layer of protein or glycoprotein (20-40 mm thick) outside their plasma membrane. Some methanogens (Methanolobus), Halobacterium, extreme thermophiles (Sulfolobus, Thermoproteus, Pyrodictium) have glycoproteins in their walls. Other methanogens (Methanococcus, Methanomicrobium, Methanogenium) and extreme thermophile Delsuphurococcus have protein walls.

Structure, function and chemical composition of archaeal cell membranes

One of the most distinctive archaeal features is their membrane lipids. 

Archaeal membrane lipids differ from those of other organisms in having glycerol connected to branched chain hydrocarbons by ether links. 

Bacterial and eukaryotic lipids have glycerol connected to fatty acids by ester bonds.

    Sometimes, two glycerol groups are linked to form long tetraethers. Usually, the diether hydrocarbon chains are 20 carbons in length, and the tetraether chains are 40 carbons. Cells adjust the  length of the tetraethers by forming pentacyclic rings. 

    Such pentacyclic rings are used by thermophilic archaea to help maintain the delicate balance of the membrane at high temperatures. Biphytanyl chains contain 1 to 4 cyclopentyl rings. 

        Phosphate, sulfur- and sugar-containing groups can be attached to the third carbons of the diethers and tetraethers, making them polar lipids -phospholipids, sulfolipids, and glycolipids.  These predominate in the membrane, making up 70 to 93% of the membrane lipids.  The remaining lipids (7-30%) are nonpolar and are usually derivatives of squalene.

Archaeal membranes may contain a mix of diethers, tetraethers and other lipids. These lipids are combined in different ways to yield membranes of various rigidity and thickness. A regular bilayer membrane is formed when C₂₀ diethers are used. A much more rigid monolayer membrane is formed when the membrane is constructed of C₄₀ tetraethers. 

The membranes of extreme thermophiles such as Thermoplasma and Sulfolobus contain  tetraether monolayers which provide stability. Archaea that live in moderately hot environments have a mixed membrane containing some regions with monolayers and some with bilayers.

Differences between Archaebacteria and Eubacteria

 Eu Bacteria and Archaea – The Major Differences  

	ArchaeaBacteria	EuBacteria
	-Ancient bacteria-	-True bacteria-
Complexity	Simple in their organization	Complex than archaebacteria
Habitat	Can sustain in extremely harsh environment such as oceans, hot springs, marshlands, hot springs and gut of animals	Found everywhere - soil, organic matter, earth’s crust, water, bodies of animals and plants, radioactive wastes, hot springs
Size	0.1-15 μm in diameter	0.5-5 μm in diameter
Shape	spheres, rods, plates, spiral, flat or square-shaped	cocci, bacilli, vibrio, rods, filaments or spiral in shape
Cell wall	Pseudopeptidoglycan	Lipopolysaccharide/ Peptidoglycan with muramic acid
Membrane lipids	Ether-linked, branched, aliphatic chains, containing D-glycerol phosphate	Ester-linked, straight chains of fatty acids, containing L-glycerol phosphates
RNA	Consists of single RNA	Three types of RNA
RNA polymerase	Complex subunit pattern	Simple subunit pattern
Introns (a long stretch of noncoding DNA found between exons (or coding regions) in a gene)	Present in archaebacteria	Absent in eubacteria
Metabolism	Methanogenesis- exhibit neither glycolysis nor Kreb’s cycle	Autotrophy, Aerobic and Anaerobic Respiration, Fermentation and Photosynthesis-exhibit both glycolysis and Kreb’s cycle
Reproduction and Growth	Asexual Reproduction, by fragmentation, budding and binary fission	Other than binary fission, budding and fragmentation, eubacteria can produce spores in order to remain dormant during unfavorable conditions
Types	Methanogens, halophiles and thermophiles	Gram positive and Gram negative
Examples	Halobacterium, Thermoproteus, Pyrobaculum, Thermoplasma and Ferroplasma	Mycobacteria, Bacillus, E. coli, Pseudomonas, Clostridium etc

 

Thus, in short, Archaebacteria are called ancient bacteria whereas the eubacteria are called true bacteria. Eubacteria are usually found in soil, water, living in and on of large organisms. Eubacteria are divided into two groups known as gram positive and gram negative bacteria. Archaebacteria are found in salt brines, ocean depths and hot springs.  Three types of archaebacteria are found: methanogens, halophiles and thermoacidophiles.

The Archaea (Archaebacteria)

• Archaea are ancient bacteria - believed to have evolved just after the evolution of first life on earth.

• Archaea are prokaryotic cells. The cell walls of Archaea contain no peptidoglycan, hence Archaea are not sensitive to some antibiotics that affect the Bacteria.

• Archaea have membranes composed of branched hydrocarbon chains (many also containing rings within the hydrocarbon chains) attached to glycerol by ether linkages. 

• The ether-containing linkages in the Archaea membranes is more stable than the ester-containing linkages in the Eubacteria  and are better able to withstand higher temperatures and stronger acid concentrations. 

• Archaea often live in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles.

• Archaea contain rRNA that is unique to the Archaea, distinctly different from the rRNA of Bacteria and Eukarya.

  Archaea are found in Volcanic hot springs- Grand Prismatic Spring of Yellowstone National Park

                            The very cold and ultra-salty Deep Lake in East Antarctica is home to haloarchaea.

Hydrothermal vents on the ocean floor, where the surrounding water can reach over 300° Celsius, are home  for some archaeal species.

The Bacteria (Eubacteria)

Bacteria (also known as eubacteria or "true bacteria") are prokaryotic cells that are common in human daily life. Eubacteria can be found almost everywhere and serve as antibiotic producers and food digesters, pathogens etc. 

Bacteria are prokaryotic cells. They have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages. 

The cell walls of Bacteria contain peptidoglycan. Bacteria are sensitive to  antibacterial antibiotics.

Bacteria contain rRNA that is unique.

Bacteria include mycoplasmas, cyanobacteria, Gram-positive bacteria, and Gram-negative bacteria.

Interesting Read

https://www.ck12.org/c/biology/archaea/lesson/Introduction-to-Archaea-Advanced-BIO-ADV/

https://www.science.org.au/curious/earth-environment/what-are-archaea

Oxygenic photosynthesis with reference to photosynthesis in cyanobacteria

 

Oxygenic means that oxygen is released during photosynthesis, distinguishing it from anoxygenic photosynthesis, which does not produce oxygen.

Anoxygenic and oxygenic photosynthesis differ in their electron donors, byproducts, and bacteriochlorophyll pigments. Oxygenic photosynthesis, performed by cyanobacteria and plants, uses water as an electron donor and releases oxygen as a byproduct. Anoxygenic types like green sulfur bacteria (GSB) using electron donors like hydrogen sulfide to produce elemental sulfur instead of oxygen. 

$$$$Oxygenic photosynthesis is summarized as, CO₂+6H₂O + light energy----->C₆H₁₂O₆+6O₂

Steps of oxygenic photosynthesis

1. Light-dependent reactions
• Occur in the thylakoid membranes of cell.
• Chlorophyll absorbs sunlight.
• Water molecules are split (photolysis), producing: Oxygen (O₂) + Electrons+ Protons (H⁺)
• ATP and NADPH are produced.

2. Light-independent reactions (Calvin cycle)
   • Occur in the cytoplasm.
   • ATP and NADPH are used to convert carbon dioxide (CO₂) into glucose (C₆H₁₂O₆).

Key features

• Electron donor: Water (H₂O)
• By-product: Oxygen (O₂)
• Pigment involved: Chlorophyll
• Organisms: Plants, algae, and cyanobacteria

Importance

• Produces the oxygen required for aerobic life.
• Forms the basis of most food chains by producing glucose.
• Removes carbon dioxide from the atmosphere, helping regulate Earth's climate.

The Light Reaction in Oxygenic Photosynthesis 

Phototrophic eucaryotes and the cyanobacteria carry out oxygenic photosynthesis, so named because oxygen is generated when light energy is converted to chemical energy. Central to this process, and to all other phototrophic processes, are light-absorbing pigments. In oxygenic phototrophs, the most important pigments are the chlorophylls.

 Several chlorophylls are found in eucaryotes, the two most important are chlorophyll a- absorption peak at 665 nm and chlorophyll b -absorption peak at 645 nm. 

Chlorophyll a is the primary photosynthetic pigment in cyanobacteria.  Chlorophyll a is embedded in the thylakoid membranes inside cyanobacterial cells. Chlorophylls absorb primarily in the red and blue ranges and green light is transmitted. Consequently many oxygenic phototrophs are green in color.

Chlorophyll a, the primary photosynthetic pigment in cyanobacteria, has absorption peaks at approximately:

~430 nm (blue region)

~662 nm (red region)

Cyanobacteria also contain accessory pigments that help absorb additional wavelengths of light, including:

Phycocyanin (blue pigment)

Phycoerythrin (red pigment, in many species)

Carotenoids (yellow to orange pigments)

Accessory pigments also trap light energy. The most widespread of these are the carotenoids, usually yellowish in color. Carotene is present in cyanobacteria belonging to the genus Prochloron and most photosynthetic protists; fucoxanthin is found in protists such as diatoms and dinoflagellates.

 Red algae and cyanobacteria have photosynthetic pigments called phycobiliproteins-phycoerythrin is a red pigment and phycocyanin is blue (maximum absorption at 620 to 640 nm). 

Carotenoids and phycobiliproteins are often called accessory pigments because of their role in photosynthesis. Accessory pigments are important because they absorb light in the range not absorbed by chlorophylls (the blue-green through yellow range; about 470–630 nm). This light is very efficiently transferred to chlorophyll. In this way accessory pigments make photosynthesis more efficient over a broader range of wavelengths. In addition, this allows organisms to use light not used by other phototrophs in their habitat. For instance, the microbes below a canopy of plants can use light that passes through the canopy. Accessory pigments also protect microorganisms from intense sunlight, which could oxidize and damage the photosynthetic apparatus. 

Chlorophylls and accessory pigments are assembled in highly organized arrays called antennas, which creates a large surface area to trap as many photons as possible. An antenna has about 300 chlorophyll molecules. Light energy is captured in an antenna and transferred from chlorophyll to chlorophyll until it reaches a special reaction-center chlorophyll pair directly involved in photosynthetic electron transport.

 In oxygenic phototrophs, there are two kinds of antennas associated with two different photosystems. Photosystem I absorbs longer wavelength light (700 nm) and funnels the energy to a special chlorophyll a pair called P700. The term P700 signifies that this molecule most effectively absorbs light at a wavelength of 700 nm. Photosystem II traps light at shorter wavelengths (680 nm) and transfers its energy to the special chlorophyll pair P680.

 When the photosystem I antenna transfers light energy to the reaction-center P700 chlorophyll pair, P700 absorbs the energy and is excited; it donates its excited, high-energy electron to a specific acceptor, probably a special chlorophyll a molecule or an iron-sulfur protein. The electron is eventually transferred to ferredoxin and can then travel in either of two directions- cyclic or non-cyclic.

 In the cyclic pathway, the electron moves in a cyclic route through a series of electron carriers and back to the oxidized P700. The pathway is termed cyclic because the electron from P700 returns to P700 after traveling through the photosynthetic electron transport chain. PMF is formed during cyclic electron transport in the region of cytochrome b6 and used to synthesize ATP. 

This process is called cyclic photophosphorylation because electrons travel in a cyclic pathway and ATP is formed. Only photosystem I participates. Two ATP's are generated

Cyclic photophosphorylation

 

    Electrons also can travel in a noncyclic pathway involving both photosystems. P700 is excited and donates electrons to ferredoxin. In the noncyclic route,  reduced ferredoxin reduces NADP to NADPH. Because the electrons contributed to NADP cannot be used to reduce oxidized P700, photosystem II participation is required. It donates electrons to oxidized P700 and generates ATP in the process. 

The photosystem II antenna absorbs light energy and excites P680, which then reduces pheophytin a. Pheophytin a is chlorophyll a in which two hydrogen atoms have replaced the central magnesium. Electrons subsequently travel to the plastoquinone pool and down the electron transport chain to P700. 

Although P700 has been reduced, P680 must also be reduced if it is to accept more light energy. Thus, H₂O can be used to donate electrons to P680 resulting in the release of oxygen. 

ATP is synthesized by noncyclic photophosphorylation. One ATP and one NADPH are formed when two electrons travel through the noncyclic pathway. Both photosystem I & photosystem II participate.

 

 

In cyanobacteria, photosynthetic light reactions are located in thylakoid membranes within the cell.

The dark reactions require three ATPs and two NADPHs to reduce one CO₂ and use it to synthesize carbohydrate (CH₂O). 

CO₂ + 3ATP + 2NADPH +2H+ H2O ⎯⎯→ (CH₂O) + 3ADP + 3Pi +2NADP_ 

The noncyclic system generates one NADPH and one ATP per pair of electrons; therefore four electrons passing through the system will produce two NADPHs and two ATPs. A total of 8 quanta of light energy (4 quanta for each photosystem) is needed to propel the four electrons from water to NADP_. Cyclic photophosphorylation operates independently to generate the extra ATP. This requires absorption of another 2 to 4 quanta.

 Thus, around 10 to 12 quanta of light energy are needed to reduce and incorporate one molecule of CO₂ during photosynthesis.

Why are Cyanobacteria the most significant group of photosynthetic microorganisms?

Cyanobacteria are the largest group of photosynthetic prokaryotes, which harvest solar energy and perform photosynthesis through chlorophyll-a by fixing CO₂ and generating O₂. Cyanobacteria are important in global carbon fixation, and reduce atmospheric CO₂ levels. In addition to chlorophyll-a (green pigment), cyanobacteria produce accessory photosynthetic pigments carotenoids, which are protect against photooxidative damages & blue and red pigments known as phycobilin (phycocyanin (PC) and phycoerythrin (PE)), which enable them to grow under low-light conditions. Some cyanobacteria can also fix atmospheric nitrogen.

Cyanobacteria use chlorophyll a to perform oxygenic photosynthesis, releasing oxygen as a byproduct. They are among the earliest organisms to have produced oxygen on Earth, contributing significantly to the oxygenation of the atmosphere.

Phototrophy & Groups of Phototrophic Microorganisms

 Phototrophic microorganisms derive energy from light energy, which they capture and use to synthesize ATP and reduce power (e.g., NADPH) and the organic material required for growth. These organisms serve as the base of most food chains in the biosphere.  The process by which light energy is trapped and converted to chemical energy is called photosynthesis. Photosynthesis is one of the most significant metabolic processes on Earth because almost all our energy is ultimately derived from solar energy. 

We mostly associate photosynthesis with the larger, more obvious plants, but over half the photosynthesis on Earth is carried out by microorganisms. Photosynthesis is carried out by both eucaryotes and bacteria. 

  Photosynthesis as a whole is divided into two parts. In the light reactions light energy is trapped and converted to chemical energy. This energy is then used to reduce or fix CO₂ and synthesize cell constituents in the dark reactions (Calvin cycle).

 Phototrophic Microorganisms 

Phototrophic organisms are a diverse group of organisms which  carry out photosynthesis. They synthesize their own food and provide energy and nutrients to other organisms. They are important in a variety of ecological and biogeochemical processes and play a vital role in our ecosystem.

 Phototrophic microorganisms are broadly classified as either oxygenic or anoxygenic, based on their ability to produce oxygen during photosynthesis. Major groups include Cyanobacteria (oxygenic) and various anoxygenic bacteria, such as purple bacteria (Rhodospirillineae), green bacteria (Chlorobiineae), and aerobic anoxygenic phototrophs (AAPs). 

 Beyond their photosynthetic pathway, microorganisms are grouped by how they obtain carbon: 

·        Photoautotrophs: 

Use sunlight to convert carbon dioxide into organic compounds, making their own food. This includes most oxygenic phototrophs and many anoxygenic ones. eg; Cyanobacteria (Anabaena, Nostoc)

·        Photoheterotrophs: 

Convert light into energy but require organic compounds from their environment to make their own food. eg: Purple non-sulfur bacteria, green non-sulfur bacteria

 Oxygenic Phototrophs

These organisms produce oxygen as a byproduct of photosynthesis. eg, Cyanobacteria, eukaryotic microalgae. They harness light energy to convert carbon dioxide and water into carbohydrates, releasing molecular oxygen as a byproduct. 

This process utilizes two photosystems (Photosystem I and II) to split water and drive an electron transport chain. 

 1. Cyanobacteria: Often referred to as blue-green algae, these prokaryotes were crucial in forming Earth's oxygen-rich atmosphere. 

·        Cyanobacteria can perform oxygenic photosynthesis – producing oxygen from CO₂ and water. 

cChlorophyll a, the primary photosynthetic pigment in cyanobacteria, has absorption peaks at approximately:

• ~430 nm (blue region)
• ~662 nm (red region)

Due to their chlorophyll pigments, they are typically greenish blue in color and therefore also known as blue-green algae. They are found in a variety of aquatic and terrestrial habitats, including even extreme locations like hot springs and deserts.

·        Cyanobacteria play a crucial role in the global carbon cycle and have had a significant impact on the evolution of our planet's atmosphere. As one of the oldest organisms on Earth, they were responsible for releasing oxygen into the atmosphere, which initiated the transformation of the atmosphere and created the environment in which we live today.

·        Cyanobacteria are commonly used in research, both as model organisms for studying photosynthesis and as potential sources of biofuels and other useful compounds.

 2. Eukaryotic Microalgae: These are microscopic, single-celled eukaryotic organisms that perform oxygenic photosynthesis. eg. diatoms, dinoflagellates. green algae etc.

 Eukaryotic microalgae perform photosynthesis to convert sunlight, carbon dioxide, and water into carbohydrates (like sugars) and oxygen. 

Photosynthesis takes place inside specialized organelles called chloroplasts, which are the site of both light and dark reactions. Chlorophyll, a green pigment, is crucial for absorbing sunlight. Other pigments may also be involved depending on the specific microalgae species.  Oxygen is a significant byproduct of the light-dependent reactions, released into the environment. 

The carbohydrates produced are the building blocks for vital organic molecules, including lipids and proteins, forming the microalgae's biomass. 

Microalgae are highly efficient photo-synthesizers and play a critical role in the global carbon cycle and oxygen production. 

Beyond generating a significant portion of Earth's atmospheric oxygen, these microorganisms form the foundational base of aquatic food webs and serve as key biological systems for understanding fundamental metabolic and genetic pathways.  eg; algae such as Chlamydomonas reinhardtii 

Algal Blooms

Under conditions of high nutrient availability from sources like agricultural runoff or wastewater,  algae such as cyanobacteria and diatoms, dinoflagellates undergo rapid overgrowth, known as an algal bloom. These blooms can discolor water and, when the algae die, their decomposition consumes oxygen, creating "dead zones" harmful to other aquatic life, though some blooms are harmless

Excessive levels of nutrients, particularly nitrogen and phosphorus from human activities like farming and wastewater discharge, lead to proliferation of microalgae, resulting in an algal bloom. Besides nutrient enrichment, climate change can also cause blooms.

Blooms can be recognized by the visible change in the water's color due to the pigments (red tide by dinoflagellates). 

Some algal blooms, known as harmful algal blooms (HABs), produce potent toxins that can harm or kill other organisms and negatively impact human health and economies. 

Anoxygenic Phototrophs

These organisms perform photosynthesis without producing oxygen, as water is not their electron donor. eg, purple bacteria, green bacteria  There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls; these are similar to chlorophyll found in eukaryotes. Bacteriochlorophyll a and b have wavelengths of maximum absorption at 775 nm and 790 nm, respectively. Only one photosystem is involved here unlike oxygenic photosynthesis. 

1. Purple Bacteria:  Also known as Rhodospirillineae, purple bacteria uses bacteriochlorophyll and can be further divided into purple sulfur and non-sulfur bacteria. 

Purple bacteria, are a diverse group of phototrophic bacteria that perform anoxygenic photosynthesis, which means they do not produce oxygen. They are called purple bacteria because their main pigments (bacteriochlorophyll pigment a or b located on chromatophores and plasma membranes) give them a purple or red color. They are further divided into: the purple sulfur bacteria and the purple non-sulfur bacteria.

·       The main difference between sulfur and non-sulfur purple bacteria is the electron donor they use during photosynthesis. Purple sulfur bacteria use reduced sulfur compounds, such as hydrogen sulfide or thiosulfate, as electron donors for photosynthesis. In contrast,  purple non-sulfur bacteria use organic compounds, such as lactate or succinate, as electron donors.

·         Purple sulfur bacteria use sulfide or thiosulfate as their electron donor during photosynthetic pathways. They oxidize sulfide to elemental sulfur, which accumulates as internal globules or granules within the cell. The sulfur deposition occurs inside the bacterial cell.  The purple sulfur bacteria can be used to reduce the concentration of harmful compounds like methane and hydrogen sulfide.

·       Sulfur purple bacteria are usually found in environments where sulfur compounds are abundant, such as hot sulfur springs, swamps, and sediments, while non-sulfur purple bacteria are found in a wider range of environments, including freshwater ponds and lakes, soils, and microbial mats.

Eg., Allochromatium and Thiocapsa

 2.  Green Bacteria: This group, including the Chlorobiineae, uses bacteriochlorophyll but has different pigments than purple bacteria.

·       Green sulfur bacteria are anoxygenic photosynthetic bacteria with a unique photosynthetic apparatus adapted to low light and anaerobic conditions. Most of them are nonmotile and obligate anaerobes. 

       Their name derives from their characteristic green color, which is due to the presence of chlorosomes – organelles that contain bacteriochlorophyll pigments. They have bacteriochlorophyll pigments c, d, a or e.        

·       They use sulphide as their ultimate electron donor for photosynthesis. Thus, they can thrive well in sulfur-rich environments with low light intensities. Most of these bacteria can reduce nitrogen to ammonia. This ammonia is later used to synthesise amino acids.

·       These bacteria can synthesize large amounts of sulfur granules, which protect them from oxidative stress and give them a distinctive appearance, visible under a microscope. The sulfur granules are stored outside the cell as a byproduct of their anaerobic photosynthesis, distinguishing them from purple sulfur bacteria that store them intracellularly. 

·       They are also known for forming complex microbial communities in sulfide-rich environments, called mats or biofilms, playing crucial roles in biogeochemical cycling and ecosystem function.

Eg., Chlorobium tepidum and Chlorobium vibrioforme 

Purple Sulfur bacteria store sulfur inside their cells (intracellular storage)  allowing immediate access and faster regulation, when external availability becomes limited. Also, intracellular location protect the granules from being washed away in aquatic environments

Green Sulfur Bacteria store sulfur outside their cells (extracellular storage) thus avoiding cluttering and providing more cytoplasmic space for other metabolic activities. Extracellular storage may facilitate sharing or recycling of sulfur among nearby microbes in dense microbial mats or sediments.

    3.    Aerobic Anoxygenic Phototrophic Bacteria (AAPs): 

        These bacteria perform anoxygenic photosynthesis but require oxygen to grow. They are mostly marine or freshwater genera. AAPB contain bacteriochlorophyll a as its main light harvesting pigment,. 

    Aerobic anoxygenic phototrophic bacteria are photoheterotrophic, meaning they obtain their carbon from organic compounds. They exist in a variety of aquatic environments and may constitute over 10% of the open ocean microbial community. AAPB are thought to play an important role in carbon cycling. 

1.  Acidobacteria and Heliobacteria 

   Heliobacteria perform anoxygenic photosynthesis and form endospore, while Acidobacteria is a vast phylum with diverse metabolic capabilities, primarily found in soil.  

Acidobacteria are primarily chemoheterotrophs, though some such as Chloracidobacterium thermophilum are capable of photosynthesis. They are ubiquitous and abundant, especially in soil ecosystems, peatlands and mineral-rich environments. They are motile eg., Acidobacterium capsulatum  

Heliobacteria use a unique form of chlorophyll, chlorophyll g as a light-harvesting pigment unlike other photosynthetic bacteria that use bacteriochlorophyll. They are obligate anaerobes and are typically found in anoxic environments such as freshwater sediments or soil. Heliobacteria are important members of the microbial community in these environments, where they play important roles in the cycling of nutrients and carbon. 

They are obligate anaerobes that are able to capture energy from light by photophosphorylation to produce ATP. Water is not used as an electron donor and, therefore, the production of oxygen is non-existent.

 Both phyla are important to soil ecology : 

·        Both are significant components of soil microbial communities. Acidobacteria are one of the most abundant phyla in soil, and heliobacteria are widespread in anaerobic soils.

·        Members of both groups are involved in biogeochemical cycles, including carbon and nitrogen cycling. Heliobacteria are known for their nitrogen-fixing capabilities.

·        The two phyla, show vast metabolic diversity found within bacteria. Acidobacteria possess a wide array of genes for degrading complex compounds, while heliobacteria use a unique photosynthetic process.  

Ecological Importance of Phototrophic Organisms 

Phototrophic organisms are essential for maintaining the balance of ecosystems and play a crucial role in the global carbon cycle. Phototrophic organisms produce organic matter that is then consumed by other organisms. This process forms the base of most food chains and supports the growth of all other living things in the ecosystem. 

Photosynthetic organisms are also responsible for producing the oxygen that we breathe. Oxygenic photosynthesis has played a critical role in the evolution of life on Earth.

 In addition to producing oxygen and organic matter, phototrophic organisms also play a significant role in regulating the Earth's climate. By removing carbon dioxide from the atmosphere through photosynthesis, these organisms help to mitigate the effects of climate change.

 Phototrophic organisms have significant economic importance and are used in a wide range of applications in industry, agriculture, and medicine.

·       Plants, algae, and other phototrophs are the primary source of food for humans and many other animals. In addition, photosynthetic bacteria are used in the production of certain types of food, such as fermented dairy products and pickled vegetables. 

·       Photosynthetic organisms are also used in the production of biofuels. Biofuels, such as ethanol and biodiesel, are produced from organic matter, such as crops and algae, that have been grown using photosynthesis. These biofuels are considered to be more sustainable than traditional fossil fuels, as they produce fewer greenhouse gas emissions and are renewable.

·       In medicine, photosynthetic organisms are used in a variety of applications, such as the production of antibiotics and other pharmaceuticals. The ability of photosynthetic organisms to carry out complex biochemical reactions makes them valuable tools in biotechnology and bioengineering. They are an important area of research for the development of new and innovative applications.

 Modern photobioreactors will help to conduct meaningful research with these interesting organisms. Photobioreactors are closed systems designed to grow photosynthetic microorganisms, such as algae and cyanobacteria, under controlled conditions of light, temperature, and nutrient supply.  These systems are used for a variety of purposes, such as the production of biomass for food, feed, or biofuels, the removal of pollutants from wastewater, and the cultivation of microorganisms for research or biotechnology applications.

Without light, the phototroph cannot perform photosynthesis. 

Obligate (Strict) Phototrophs have no alternative metabolism for energy production and cannot produce ATP or reducing power without light. They survive on stored energy (e.g., glycogen, polyhydroxybutyrate) for a limited time. If darkness persists, metabolism stops and cell death occurs due to energy depletion.

 Facultative Phototrophs/Mixotrophs such as cyanobacteria, algae, and some purple non-sulfur bacteria switch to heterotrophic metabolism when light is unavailable. Use organic compounds (e.g., sugars, acetate) from the environment as energy and carbon sources. and can survive depending on nutrient availability.