Certain types of algae, such as green algae, are commonly found in freshwater environments. They play a crucial role in ecosystems by producing oxygen through photosynthesis and serving as a food source for various aquatic organisms. Green algae can vary in size, from microscopic phytoplankton to larger forms like sea lettuce. Another type, red algae, is primarily found in marine environments. They are known for their reddish pigment, which allows them to absorb light at greater depths. Red algae are important for coral reef ecosystems and are also harvested for food products like nori, commonly used in sushi.