Which type of algae can block out light and create large mats?

Mat-forming algae are specifically known for their ability to create large mats or blooms that can significantly block out light in aquatic environments. These algal mats can cover surfaces, including sediment and other vegetation, leading to decreased sunlight penetration. This can adversely affect aquatic ecosystems by diminishing the light available for photosynthetic organisms and disrupting the overall balance of the aquatic ecosystem.

In contrast, the other types of algae mentioned do not typically form large, dense mats that obstruct light. Microscopic algae, while abundant in many water bodies, exist as single cells or small colonies and do not form the large masses that characterize mat-forming species. Chara is a type of green algae that can grow in dense populations, but it's primarily a submerged plant and does not create thick mats in the same sense as mat-forming algae. Floating algae, such as some types of phytoplankton, stay suspended in the water column rather than forming extensive mats on the surface.

Thus, the choice of mat-forming algae is the most accurate answer when considering which type can effectively block light and create extensive areas that can significantly alter the aquatic environment.