In the carbon cycle, how does carbon most directly enter the biotic (living) part of an ecosystem?

• A. Through nitrogen fixation, when bacteria convert atmospheric nitrogen into ammonium used to build organic carbon
• B. Through evaporation, when carbon-containing water vapor enters the atmosphere and is absorbed by plants
• C. Through sedimentation, when carbon in ocean sediments is directly incorporated into animal tissues
• D. Through photosynthesis, when autotrophs fix atmospheric $C_{O}2$ into organic molecules

Answer: (D)

Verified step by step guidance

1. Understand that the biotic part of an ecosystem refers to all living organisms, including plants, animals, and microorganisms.
2. Recognize that carbon enters living organisms primarily through the process of photosynthesis, where autotrophs (such as plants and algae) convert atmospheric carbon dioxide (CO\_2) into organic molecules.
3. Recall that nitrogen fixation involves converting atmospheric nitrogen (N\_2) into forms usable by organisms, but it does not directly involve carbon entering the biotic system.
4. Note that evaporation is a physical process involving water vapor and does not directly transfer carbon into living organisms.
5. Understand that sedimentation involves carbon being stored in ocean sediments and is not a direct pathway for carbon to enter living tissues.