Algae aften float on surface of water during day but sink down during night due to

Options

A. evolution and trapping of oxygen bubbles during day in their photosynthesis
B. becoming light as they consume most of their food in the night
C. warming action of sun during the day
D. release of absorbed air by warming of water

Answer

### Step-by-Step Solution:

1. **Understanding Algae and Photosynthesis**:
- Algae are photosynthetic organisms that perform photosynthesis during the day. This process converts carbon dioxide and water into glucose and oxygen using sunlight.

2. **Oxygen Production During Day**:
- During the day, algae produce oxygen as a byproduct of photosynthesis. This oxygen is released into the water and can form bubbles.

3. **Bubbles Trapping**:
- As the oxygen is produced, it gets trapped in tiny bubbles. These bubbles accumulate around the algae, providing buoyancy.

4. **Floating on Water Surface**:
- The trapped oxygen bubbles cause the algae to float on the surface of the water during the day. This buoyancy allows them to access more sunlight for photosynthesis.

5. **Nighttime Changes**:
- At night, photosynthesis ceases because there is no sunlight. Therefore, the production of oxygen stops, and the oxygen that was previously trapped in bubbles is no longer being produced.

6. **Respiration Process**:
- During the night, algae utilize the oxygen for respiration, which produces carbon dioxide instead. This process does not create new oxygen bubbles.

7. **Sinking of Algae**:
- Without the buoyant oxygen bubbles, the algae become denser than the water and sink to the bottom during the night.

8. **Conclusion**:
- Thus, the reason algae float on the surface during the day and sink at night is due to the evolution and trapping of oxygen bubbles during the day, which is a result of photosynthesis.

### Final Answer:
Algae often float on the surface of water during the day due to the evolution and trapping of oxygen bubbles produced during photosynthesis. At night, they sink because photosynthesis stops, and the oxygen is used for respiration, leading to the absence of buoyant bubbles.

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