Final answer:
Algae, a group of plant-like protists, are responsible for consuming CO2 and producing O2, playing a pivotal role in aquatic ecosystems as primary producers through the process of photosynthesis.
Step-by-step explanation:
The group of protists that consumes CO2 and produces O2, serving as the base of food webs in lakes, streams, and the oceans, is algae. Algae are plant-like protists and are autotrophs, meaning they produce their own food through photosynthesis. They are crucial for the ecosystem as primary producers, particularly in aquatic areas where they feed a large proportion of species. In photosynthesis, algae use carbon dioxide, water, and sunlight to create sugars, releasing oxygen in the process, which is vital for the survival of many organisms. Unlike true plants, algae do not have true stems, roots, or leaves but can still perform photosynthesis efficiently.
The role of algae as oxygen-producers is paralleled by their significance in photosynthesis, contributing to approximately 25 percent of the world's photosynthesis. This process is not only crucial for oxygen production but also for the sustenance of marine life, as exemplified by the symbiotic relationship between coral polyps and photosynthetic dinoflagellates known as zooxanthellae.