How do protozoans obtain carbon and energy from their surroundings?

Protozoans obtain carbon and energy through diverse nutritional strategies, primarily categorized as heterotrophy and autotrophy, with many species employing a combination of both, known as mixotrophy.12

Heterotrophy

As nonfilamentous heterotrophs, protozoans acquire organic carbon from external sources. They utilize two main methods:3
* Phagotrophy: The engulfment of particulate food, such as bacteria, algae, other protozoans, or small animals. This process may involve specialized structures like pseudopodial extensions or buccal organelles to trap prey.13
* Osmotrophy: The absorption of dissolved nutrients directly from the surrounding medium, often through pinocytosis or simple diffusion across the cell membrane.1

Autotrophy and Mixotrophy

Some protozoans supplement their energy balance through photosynthesis. This is achieved by:2
* Endosymbiosis: Harboring symbiotic algae that provide photosynthetic products to the host.4
* Kleptoplasty: Consuming plantlike flagellates and sequestering their plastids to perform photosynthesis, though these organelles must be replaced periodically.14

Once nutrients are acquired, mitochondria break down molecules to release chemical energy. In anaerobic environments, protozoans may rely on hydrogenosomes or symbiotic bacteria for energy production.1