The following pathways describe the movement of carbon within the Biopod ecosystem:

• Pathway A: Dissolved inorganic carbon dioxide (${CO}_2$) is incorporated into organic glucose molecules by the photosynthetic algae.
• Pathway B: Organic carbon compounds in the algae are consumed and digested by the aquatic snails.
• Pathway C: Snail and algae organic compounds are broken down by cellular processes to release energy, releasing carbon dioxide back into the water.
• Pathway D: Accumulations of dead organic matter in the sediment are incinerated in an emergency waste burner to rapidly release carbon dioxide.

Match each Pathway A–D to its correct process from the list below:

• Combustion
• Feeding (Eating)
• Photosynthesis
• Respiration

The astrobiologist initially fills the sediment layer with synthetic sand containing essential mineral ions but no active bacteria or fungi.

Over a long duration, algal growth stalls because mineral ions (such as nitrates and phosphates) in the water and sediment are completely exhausted. Explain why these mineral nutrients are not recycled in this initial setup.

The astrobiologist then inoculates the sediment with active decomposer microbes. Despite the Biopod remaining completely hermetically sealed, both the algae and the microbes are able to survive long-term because they supply each other with vital gases. Explain how this gas exchange is maintained between the algae and the microbes.