Q.36 Which combination of the following statements is CORRECT for cyanobacteria?
P. They can perform oxygenic photosynthesis
Q. Usually filamentous forms are involved in nitrogen fixation
R. Nitrogen fixation occurs in heterocysts
S. They cannot grow in a mineral medium exposed to light and air
(A) P, Q and R (B) P, S and R (C) Q, R and S (D) P, Q and S

Cyanobacteria, often called blue-green algae, play a vital role in biotechnology, microbiology, and environmental science due to their unique abilities in photosynthesis and nitrogen fixation. This SEO-optimized article breaks down a common multiple-choice question (MCQ) from exams like CSIR NET or GATE Biotechnology, explaining each statement (P, Q, R, S) with scientific evidence. Ideal for bioengineering students tackling microbial physiology and biochemical processes.

Correct Answer: (A) P, Q and R

This combination is accurate because cyanobacteria excel in oxygenic photosynthesis, rely on filamentous forms for nitrogen fixation, and use specialized heterocysts for the process. Statement S is false, as cyanobacteria thrive in such conditions. Let’s dive into each statement with evidence from microbiology and biochemistry.

Detailed Explanation of Each Statement

P. They can perform oxygenic photosynthesis – TRUE

Cyanobacteria are prokaryotes that carry out oxygenic photosynthesis, similar to plants. They use water as an electron donor, splitting it via Photosystem II to produce oxygen, ATP, and NADPH. This process follows the equation:

Key enzymes like Rubisco enable the Calvin cycle for carbon fixation. This makes cyanobacteria essential in algal blooms and biofuel production in biotechnology.

Q. Usually filamentous forms are involved in nitrogen fixation – TRUE

Nitrogen fixation in cyanobacteria typically occurs in filamentous species like Anabaena, Nostoc, and Oscillatoria. These non-heterocystous or heterocystous filaments house nitrogenase enzyme complexes that convert atmospheric N₂ to ammonia:

Unicellular forms like Crocosphaera fix nitrogen, but filamentous types dominate ecological and lab studies, especially in rice paddies for biofertilizers.

R. Nitrogen fixation occurs in heterocysts – TRUE

Heterocysts are specialized, thick-walled cells in filamentous cyanobacteria (e.g., Anabaena) dedicated to nitrogen fixation. They lack oxygen-evolving Photosystem II, maintaining an anaerobic environment via glycolipid layers and high respiration rates. This protects the oxygen-sensitive nitrogenase enzyme, allowing diazotrophy under aerobic conditions.

S. They cannot grow in a mineral medium exposed to light and air – FALSE

Cyanobacteria grow well in mineral media like BG-11 under light and air. They fix CO₂ via photosynthesis and N₂ (in diazotrophs), requiring only inorganic salts, trace metals, and light. Lab cultures of Synechococcus or Anabaena thrive this way, contradicting S and making it the incorrect statement.

Why Option (A) P, Q and R is Correct

• P + Q + R align with core cyanobacterial biology: Oxygenic photosynthesis powers growth, while filamentous heterocysts enable nitrogen fixation without oxygen interference.
• S disqualifies other options: (B), (C), and (D) include the false S, leaving (A) as the only valid choice.
• Biotech Relevance: Understanding this aids in microbial fermentation, enzyme kinetics modeling, and genetic engineering for sustainable agriculture.

Quick Reference Table

This MCQ tests molecular biology and microbial growth kinetics—crucial for your biotech research.