---
title: "Green Nonsulfur Bacteria | Microbiology"
description: "Green nonsulfur bacteria are phototrophic, anoxygenic bacteria that use bacteriochlorophylls and can also grow on organic compounds in Microbiology."
canonical: "https://fiveable.me/microbio/key-terms/green-nonsulfur-bacteria"
type: "key-term"
subject: "Microbiology"
unit: "Unit 4"
---

# Green Nonsulfur Bacteria | Microbiology

## Definition

Green nonsulfur bacteria are a group of phototrophic bacteria in Microbiology that do anoxygenic photosynthesis, so they use light without releasing oxygen. They can also switch to chemoorganotrophic growth when light is unavailable.

## What It Is

Green nonsulfur bacteria are phototrophic bacteria that use light for energy, but they do it without making oxygen. In microbiology, that puts them in the group of anoxygenic photosynthesizers, which means their photosystems do not split water the way plants and cyanobacteria do.

Their name is a little misleading if you read it too literally. “Green” points to the pigments they carry, especially bacteriochlorophylls c, d, or e, which absorb light differently from plant chlorophyll. “Nonsulfur” means they are not the classic sulfur-oxidizing green bacteria that use hydrogen sulfide and deposit sulfur granules. So the name helps separate them from green sulfur bacteria, even though both groups can look greenish and both do photosynthesis.

A big microbiology idea here is metabolic flexibility. Green nonsulfur bacteria can grow photoautotrophically when light is available, but many can also grow chemoorganotrophically by oxidizing organic compounds. That means they are not locked into one energy source. If conditions change, they can switch strategies instead of shutting down.

A well-known example is Chloroflexus, a genus often found in hot springs and other warm environments. Those habitats can be low in oxygen, high in temperature, and chemically unusual, so having flexible metabolism is a real advantage. In class diagrams, you may see these bacteria associated with mats or layered microbial communities where different microbes occupy different light and oxygen zones.

Mechanistically, their photosynthetic machinery still uses an electron transport chain, proton gradients, and ATP synthase to make ATP. The difference is the electron source and the end products. Because they do not use water as the electron donor, they do not release O2, which is why their photosynthesis is called anoxygenic.

## Why It Matters

Green nonsulfur bacteria show up whenever microbiology is comparing different ways microbes harvest energy. They help you see that photosynthesis is not one single pathway. Some bacteria use light but still avoid oxygen production, and that changes how you interpret their ecology, metabolism, and cell biology.

This term also connects directly to microbial diversity in extreme or unusual environments. If a lab or lecture mentions hot springs, microbial mats, or low-oxygen habitats, green nonsulfur bacteria are a good example of how microbes adapt to niches where plant-style photosynthesis would not be a good fit.

They also help with classification questions. If you are sorting bacteria by metabolism, pigment type, or oxygen production, green nonsulfur bacteria sit in a useful middle ground: phototrophic, but not oxygenic. That distinction comes up again when comparing them with green sulfur bacteria, cyanobacteria, and chemoorganotrophs.

In short, this term is less about memorizing a name and more about recognizing a metabolic pattern. Once you know what green nonsulfur bacteria do, you can predict where they live, what they need, and how they differ from other photosynthetic microbes.

## Connections

### Phototrophic Bacteria

Green nonsulfur bacteria are one kind of phototrophic bacterium, so they belong in the bigger group of microbes that use light as an energy source. The connection matters because not all phototrophs use the same electron donors or make the same products. When you see a phototroph question, this term helps you narrow the metabolism down to anoxygenic light use with flexible growth options.

### Chemoorganotrophs

Many green nonsulfur bacteria can grow chemoorganotrophically, which means they can oxidize organic compounds instead of relying on light. That switch is a big reason they survive in changing environments. In Microbiology, this is a useful example of metabolic versatility, where one organism can use more than one energy strategy depending on conditions.

### [Anoxygenic Photosynthesis](/microbio/key-terms/anoxygenic-photosynthesis)

This is the process green nonsulfur bacteria carry out when they use light without producing oxygen. The relationship is direct, but the term also helps you compare them with oxygenic photosynthesis in plants and cyanobacteria. If a question asks why no oxygen is released, the answer is that water is not the electron donor in this pathway.

### [green sulfur bacteria](/microbio/key-terms/green-sulfur-bacteria)

This is the closest comparison and a common source of confusion. Both groups are green, photosynthetic bacteria, but green sulfur bacteria are more closely tied to sulfur compounds and often live in very different environments. Green nonsulfur bacteria are defined more by flexible metabolism and by not being the classic sulfur-oxidizing green group.

## On the AP Exam

A quiz question might ask you to identify green nonsulfur bacteria from a metabolism description, a habitat, or a pigment clue. If you see bacteriochlorophylls c, d, or e, think of anoxygenic photosynthesis rather than plant-style oxygen production. If the prompt mentions hot springs, microbial mats, or growth with or without light, that is another clue.

In lab or short-answer work, you may need to explain why these bacteria do not release oxygen and how they can still make ATP from light. A good answer traces the path from light capture to electron transport to ATP synthesis, then notes the alternative chemoorganotrophic mode when light is absent.

## green nonsulfur bacteria vs green sulfur bacteria

These two groups sound similar because both are green, photosynthetic bacteria, but they are not the same. Green sulfur bacteria are more tightly associated with sulfur metabolism, while green nonsulfur bacteria are known for flexible metabolism and for thriving in places like hot springs. If a question asks about pigments plus habitat plus oxygen use, check which green group fits the clues.

## Key Takeaways

- Green nonsulfur bacteria are phototrophic bacteria that do anoxygenic photosynthesis, so they use light without producing oxygen.
- They contain bacteriochlorophylls c, d, or e, which are different from the chlorophyll used by plants.
- Many of them can also grow chemoorganotrophically, so they can use organic compounds when light is not available.
- Chloroflexus is a classic example and is often linked to hot springs and other warm, low-oxygen habitats.
- This term matters because it shows how bacterial metabolism can shift with the environment instead of following one fixed pathway.

## FAQs

### What is green nonsulfur bacteria in Microbiology?

Green nonsulfur bacteria are a group of phototrophic bacteria that carry out anoxygenic photosynthesis. They use light energy, but they do not release oxygen the way plants and cyanobacteria do. In Microbiology, they are a good example of how bacterial metabolism can be flexible.

### Why are green nonsulfur bacteria called nonsulfur?

They are called nonsulfur because they are not the classic sulfur-oxidizing green bacteria. The name helps separate them from green sulfur bacteria, which use sulfur compounds more directly in photosynthesis. Green nonsulfur bacteria are better known for flexible energy use and for living in places like hot springs.

### Do green nonsulfur bacteria make oxygen?

No. Their photosynthesis is anoxygenic, so oxygen is not a byproduct. That means they do not split water like plants do. Instead, they use a different photosynthetic setup that still makes ATP but avoids oxygen release.

### How do green nonsulfur bacteria survive without light?

Many can switch to chemoorganotrophic growth, which means they oxidize organic compounds for energy. That flexibility lets them survive when light levels drop or conditions change. It is one of the main reasons they show up in variable environments.

## Related Study Guides

- [4.3 Nonproteobacteria Gram-Negative Bacteria and Phototrophic Bacteria](/microbio/unit-4/3-nonproteobacteria-gram-negative-bacteria-phototrophic-bacteria/study-guide/PThwualrMvavw48o)

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