Ascomycete fungi are a diverse group of fungi characterized by their unique reproductive structure called the ascus, which is a sac-like structure that contains spores. They play essential roles in ecosystems as decomposers, breaking down organic matter, and some are important in food production, such as yeasts used in baking and brewing. These fungi can be found in various habitats, including soil, water, and on plants. Many Ascomycetes are also known for their symbiotic relationships, forming partnerships with algae to create lichens, which can thrive in harsh environments.