Nostoc are a genus of cyanobacteria (often referred to as blue-green algae), which are photosynthetic bacteria that thrive in moist environments. They form gelatinous masses or mats, which can be found in soil, on gravel, or in aquatic habitats. Nostoc is capable of nitrogen fixation, converting atmospheric nitrogen into a usable form for plants. It plays a role in various ecosystems but can become a nuisance in areas with excess moisture.