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Which of the following statements about the nitrogen cycle are false ?
1. Rhizobium and Frankia cause nitrogen fixation in their independent free - living stage .
2. Azotobacter and Anabaena are anaerobic, symbiotic , free - living , nitrogen fixers.
3. Rhodospirillum is an aerobic , nitrogen -fixing microbe found in the root nodules of legumes .
4. Nitrobacter oxidizes nitrites to nitrates .

A

1 and 2

B

2 and 4

C

1 , 2 and 3

D

3 and 4

Text Solution

AI Generated Solution

The correct Answer is:
To determine which statements about the nitrogen cycle are false, let's analyze each statement one by one. ### Step 1: Analyze Statement 1 **Statement 1:** Rhizobium and Frankia cause nitrogen fixation in their independent free-living stage. - **Analysis:** Rhizobium and Frankia are known for their symbiotic relationships with legumes and act as nitrogen fixers within root nodules. They do not primarily function as independent free-living organisms for nitrogen fixation. Therefore, this statement is **false**. ### Step 2: Analyze Statement 2 **Statement 2:** Azotobacter and Anabaena are anaerobic, symbiotic, free-living nitrogen fixers. - **Analysis:** Azotobacter is a free-living nitrogen fixer that is aerobic, meaning it requires oxygen for its metabolic processes. Anabaena, while it can be found in symbiotic relationships, is also primarily known for its ability to fix nitrogen in aquatic environments and is not strictly anaerobic. Thus, this statement is **false**. ### Step 3: Analyze Statement 3 **Statement 3:** Rhodospirillum is an aerobic, nitrogen-fixing microbe found in the root nodules of legumes. - **Analysis:** Rhodospirillum is indeed an aerobic nitrogen-fixing bacterium, but it is not found in the root nodules of legumes; that role is primarily filled by Rhizobium. Therefore, this statement is **false**. ### Step 4: Analyze Statement 4 **Statement 4:** Nitrobacter oxidizes nitrites to nitrates. - **Analysis:** This statement is accurate. Nitrobacter is a genus of bacteria that plays a crucial role in the nitrogen cycle by oxidizing nitrites (NO2-) to nitrates (NO3-). Therefore, this statement is **true**. ### Conclusion The false statements are: 1. Rhizobium and Frankia cause nitrogen fixation in their independent free-living stage. 2. Azotobacter and Anabaena are anaerobic, symbiotic, free-living nitrogen fixers. 3. Rhodospirillum is an aerobic, nitrogen-fixing microbe found in the root nodules of legumes. The only true statement is about Nitrobacter oxidizing nitrites to nitrates. ### Final Answer The false statements are 1, 2, and 3. ---
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