Who demonstrated that O₂ comes from water instead from CO₂ during photosynthesis by using purple and green bacteria in their experiment? 

Van Niel

The correct answer is Option (1)- Van Niel

Cornelius van Niel's contributions to the understanding of photosynthesis were indeed significant and had a profound impact on our knowledge of this complex biological process. Let's break down his key findings and the implications for the source of oxygen (O₂) during photosynthesis:

1. Light-Dependent Reaction: Van Niel, through his studies of purple and green bacteria, demonstrated that photosynthesis is fundamentally a light-dependent reaction. This means that it relies on light energy to drive the process.
2. Hydrogen as a Reducing Agent: Van Niel's research revealed that during photosynthesis, hydrogen (H₂) from a suitable oxidizable compound serves as a reducing agent. In this reaction, hydrogen reduces carbon dioxide (CO₂) to form carbohydrates, such as glucose (C₆H₁₂O₆). The reduction of CO₂ to carbohydrates is a complex, multi-step process known as photosynthesis.
3. Source of Oxygen (O₂): Van Niel's work also clarified the source of the oxygen released during photosynthesis. In green plants, water (H₂O) serves as the hydrogen donor, and it is oxidized during photosynthesis. This oxidation of water results in the release of molecular oxygen (O₂). This insight is critical because it confirmed that the oxygen generated during photosynthesis comes from water, not from carbon dioxide (CO₂).
4. Different Hydrogen Donors: Van Niel pointed out that some organisms, like purple and green sulfur bacteria, do not release O₂ during photosynthesis. Instead, they use alternative hydrogen donors, such as hydrogen sulfide (H₂S), which leads to the production of sulfur or sulfate as oxidation products, rather than O₂. This observation emphasized the diversity of photosynthetic processes in various organisms.
5. Verification with Radioisotope Techniques: Van Niel's inferences about the source of oxygen were later confirmed using radioisotope techniques, which allowed scientists to trace the movement of atoms and molecules within photosynthetic reactions. These experiments provided further evidence that O₂ evolved during photosynthesis indeed originates from water. The correct equation, that would represent the overall process of photosynthesis is there :                                                                         

\[6CO_2 + 12H_2O \overset{light}{\longrightarrow}C_6H_{12}O_6 + 6H_2O + 6O_2\]

In summary, Cornelius van Niel's work highlighted the light-dependent nature of photosynthesis, the role of hydrogen as a reducing agent in carbohydrate formation, and the source of oxygen during this process. His research not only expanded our understanding of photosynthesis but also paved the way for future investigations into the intricate biochemical pathways involved in this vital process.