🦠Microbiology Unit 4 Review

Nonproteobacteria gram-negative bacteria are a broad collection of microbes that share the classic gram-negative cell wall but fall outside the Proteobacteria phylum. They show up everywhere, from the human gut to volcanic mudpots, and their metabolic flexibility is a big reason why. Understanding these groups matters because they drive key nutrient cycles and directly affect human health.

Characteristics of nonproteobacteria gram-negatives

Like all gram-negative bacteria, these organisms have a thin peptidoglycan layer sandwiched between an inner membrane and an outer membrane. That outer membrane contains lipopolysaccharides (LPS), which serve as a protective barrier and can trigger strong immune responses in humans (endotoxin activity).

Between the two membranes sits the periplasmic space, a compartment where enzymes accumulate. Many of these enzymes handle nutrient breakdown and detoxification before molecules even enter the cytoplasm.

What really sets these bacteria apart is their metabolic diversity:

Aerobic respiration uses oxygen as the final electron acceptor (e.g., Cytophaga)
Anaerobic respiration substitutes alternative electron acceptors like nitrate or sulfate (e.g., Bacteroides in the gut)
Fermentation breaks down organic compounds without any external electron acceptor (also common in Bacteroides)

Beyond these core strategies, some groups have specialized metabolic modes like phototrophy (using light energy, as in Chlorobium) or chemolithotrophy (deriving energy from inorganic compounds, as in Thiobacillus). This range of energy strategies explains why nonproteobacteria gram-negatives colonize such different habitats.

Comparison of nonproteobacteria groups

Bacteroidetes are among the most abundant bacteria in the human gut microbiome. They specialize in breaking down complex polysaccharides like cellulose and pectin that human enzymes can't touch. Without them, you'd lose access to a significant source of short-chain fatty acids that your colon cells depend on for energy.

Planctomycetes stand out structurally because they have intracellular membrane-bound compartments, which is unusual for bacteria. Even more striking, some species lack peptidoglycan in their cell walls entirely, which means they won't respond to antibiotics like penicillin that target peptidoglycan synthesis. One notable species, Candidatus Brocadia anammoxidans, performs anaerobic ammonia oxidation (anammox), converting ammonium and nitrite directly into nitrogen gas ( $NH_4^+ + NO_2^- \rightarrow N_2 + 2H_2O$ ). This reaction is a major pathway for nitrogen removal in marine environments and is now used in wastewater treatment plants.

Verrucomicrobia are found across diverse habitats, from soil to ocean sediments. Two species worth knowing:

Akkermansia muciniphila lives in the human gut and helps maintain the protective mucus layer lining the intestine. Research links its abundance to healthier metabolic profiles.
Methylacidiphilum fumariolicum thrives in volcanic mudpots, where it oxidizes methane and fixes $CO_2$ . This makes it both a methanotroph and an autotroph, contributing to carbon cycling in extreme environments.

Phototrophic Bacteria: Mechanisms, Adaptations, and Ecological Roles

Phototrophic bacteria capture light energy and convert it into chemical energy. Some of these organisms produce oxygen, while others don't. This distinction has enormous consequences for the environments they inhabit and for Earth's history.

Photosynthetic mechanisms in bacteria

There are two fundamentally different approaches to bacterial photosynthesis:

Oxygenic phototrophs (primarily cyanobacteria) use water as an electron donor and release $O_2$ as a byproduct. Their pigment toolkit includes chlorophyll a and accessory pigments called phycobilins (phycocyanin and phycoerythrin), which expand the range of light wavelengths they can harvest. They run a two-stage process using both Photosystem I and Photosystem II, which is the same basic setup found in plant chloroplasts. That's not a coincidence: chloroplasts are thought to have evolved from an ancient cyanobacterial endosymbiont.

Anoxygenic phototrophs (purple bacteria and green bacteria) use electron donors other than water, such as hydrogen sulfide ( $H_2S$ ), hydrogen gas, or organic compounds. Because they lack the oxygen-evolving complex associated with Photosystem II, they never produce $O_2$ . Their pigments are bacteriochlorophylls (types a, b, c, d, or e) and carotenoids (like spirilloxanthin and okenone), which absorb wavelengths of light that chlorophyll a doesn't capture well. This lets them thrive in deeper or dimmer environments where oxygenic phototrophs can't compete. They use only a single reaction center, which resembles either Photosystem I or Photosystem II depending on the group.

Quick comparison: Oxygenic phototrophs split water, release $O_2$ , and use two photosystems. Anoxygenic phototrophs use reduced compounds like $H_2S$ , produce no $O_2$ , and use one photosystem.

Ecological roles of phototrophic bacteria

Phototrophic bacteria are primary producers, converting light energy into organic compounds that feed entire food webs in both aquatic and terrestrial ecosystems.

Their contributions extend across multiple biogeochemical cycles:

Nitrogen cycle: Certain cyanobacteria, such as Anabaena and Nostoc, fix atmospheric nitrogen ( $N_2$ ) into ammonia ( $NH_3$ ). They do this using specialized thick-walled cells called heterocysts, which maintain the low-oxygen environment that the enzyme nitrogenase requires. This process makes nitrogen biologically available in ecosystems where it would otherwise be limiting.
Carbon cycle: All phototrophic bacteria sequester $CO_2$ during carbon fixation, helping regulate atmospheric $CO_2$ levels. Cyanobacteria in the oceans are responsible for a substantial fraction of global primary production.
Sulfur cycle: Purple and green sulfur bacteria oxidize reduced sulfur compounds like $H_2S$ and thiosulfate ( $S_2O_3^{2-}$ ) in anoxic environments such as stratified lakes and deep sediments. This links the sulfur cycle to energy production in places where oxygen is absent.
Earth's oxygenation: Cyanobacteria drove the Great Oxidation Event roughly 2.4 billion years ago, fundamentally transforming Earth's atmosphere from anoxic to oxygen-rich. This was arguably the single most consequential metabolic innovation in the history of life.

Photosynthetic structures and processes

Phototrophic bacteria house their photosynthetic machinery in specialized structures:

Light-harvesting complexes are protein-pigment assemblies that capture photons and funnel that energy toward reaction centers. Think of them as antennae that increase the surface area for light absorption.
Thylakoid membranes (in cyanobacteria) or invaginations of the cytoplasmic membrane (in purple bacteria) are the internal membranes where the photosynthetic reactions actually take place. Reaction centers and electron transport chain components are embedded here.

The overall process of photosynthesis has two linked stages:

Light-dependent reactions: Light energy excites electrons in the reaction center. These electrons pass through an electron transport chain, a series of membrane-bound protein complexes that use the electron flow to pump protons across the membrane. The resulting proton gradient drives ATP synthase to produce ATP. In oxygenic phototrophs, water is split to replace the lost electrons, and $NADPH$ is also generated.
Carbon fixation (Calvin cycle): ATP and NADPH from the light reactions power the conversion of $CO_2$ into organic molecules like glyceraldehyde-3-phosphate (G3P). This is where inorganic carbon becomes the organic building blocks of life.

🦠Microbiology Unit 4 Review