蓝菌(cyanobacteria)，亦称蓝绿藻（blue-green algae）。

大群原核生物，是光合作用的主要有机体。虽被分类归为细菌，但蓝菌在许多方面似真核的藻类，包括一些形态特徵和小生境，有一段时间被视为藻类。蓝菌包含某些色素，与它们的叶绿素一起，往往呈现出蓝绿色，虽然许多品种实际是绿的、棕的、黄的、黑的或红的。蓝菌分布广泛，常见於土壤、海水和淡水中，在很宽的温度范围内都能生长，从冰下几公尺深的南极湖泊到黄石公园里的温泉中都有蓝菌。蓝菌是在裸露的岩石及土壤上最先生长繁殖的生物物种之一。有些蓝菌有固氮的能力；其他的则包含能够产生游离态氧的色素，氧是光合作用的一种副产品。在合适的条件下（包括被氮的废弃物污染的环境下），它们繁殖极快，短期内即形成稠密的大群，称为起霜作用，通常为一层不透明的绿色。蓝菌在地球大气形成之初提升自由氧的含量方面扮演了重要的角色。

English version：

cyanobacteria

Any of a large group of prokaryotic, mostly photosynthetic organisms. Though classified as bacteria, they resemble the eukaryotic algae in many ways, including some physical characteristics and ecological niches, and were at one time treated as algae. They contain certain pigments, which, with their chlorophyll, often give them a blue-green color, though many species are actually green, brown, yellow, black, or red. They are common in soil and in both salt and fresh water, and they can grow over a wide range of temperatures, from Antarctic lakes under several meters of ice to Yellowstone's hot springs. They are often among the first species to colonize bare rock and soil. Some are capable of nitrogen fixation; others contain pigments that enable them to produce free oxygen as a by-product of photosynthesis. Under proper conditions (including pollution by nitrogen wastes) they can reproduce explosively, forming dense concentrations called blooms, usually colored an opaque green. Cyanobacteria played a large role in raising the level of free oxygen in the atmosphere of early earth.