Arctic Life/Bacteria/Blue-Green Algae (Cyanobacteria)
Blue-green algae contain chlorophyll, the photosynthetic pigment of higher plants, but they have the structural characteristics of bacteria. This has caused controversy among taxonomists, but it is now accepted that blue-green algae are eubacteria. Blue-green algae are also known as cyanobacteria or blue-green bacteria.
As in other Eubacteria, blue-green algae lack nuclear or mitochondrial membranes. Reproduction within Cyanobacteria, like Eubacteria, is strictly asexual. Despite these similarities, there are a few major distinctions which set the Cyanobacteria apart from the Eubacteria. Firstly, cyanobacteria have chlorophyll a, which is otherwise found only in higher plants. Secondly, cyanobacteria also possess blue and red pigments that are absent in other bacteria. Finally, they are the only organisms that can both fix nitrogen and produce oxygen. Blue-green algae typically occur free-floating in the water column and are ordinarily separated into groups based on their growth form: unicellular, colonial, and filamentous. Unicellular forms are difficult to see because they are very small; but colonial forms are visible as tiny blue-green particles and filamentous forms appear as long strands of green hair.
About 150 genera and 2000 species of blue-green algae have been described, many of which occur in the Arctic. Most arctic species live in freshwater, but some are marine, and a few grow on damp soil or glacial ice. Blue-green algae often colonize the underside of sea ice, causing it to change from white to a bluish-green colour. This under-ice community forms the basis of an important food web, providing food for small herbivorous invertebrates, such as copepods and amphipods, which then serve as a food source for larger invertebrates, fish, birds, and seals.
Although there are far fewer species of cyanobacteria in arctic marine systems versus arctic freshwater environments, some marine species of Anabaena are abundant and their filamentous, blue-green strands litter the ocean waters. They are more abundant near shorelines because light can penetrate to the bottom, warming the water more quickly in the spring. Anabaena species are known as enrichment indicators; they multiply rapidly when pollutants from fertilizers and industrial waste, such as nitrogen and phosphorous, are present in the water.
Most arctic lakes and ponds have a bluish-green colour. This hue results from the colour of the resident algal community, especially the Cyanobacteria. Due to their nutritional requirements and position as an important component at the base of food webs, blue-green algae provide information about ecosystem conditions. They respond rapidly and predictably to a wide variety of pollutants and to changing climatic conditions. When sufficient light – which is often a limiting factor for growth – is available, blue-green algae form some of the densest blooms in arctic lakes and serve as important food supplies for invertebrates like water fleas. Arctic species that thrive in freshwater environments include Gloeocapsa rupestris and Chroococcus spp.
Not only are blue-green algae key components of arctic ecosystems, they also play an important role in the fixation of nitrogen. Blue-green algae are able to break the nitrogen atoms of atmospheric nitrogen (N2) to produce nitrate (NO3) and ammonia (NH4), which can then be used by other algae and plants.
The blue-green algal genus Nostoc is one of the most important nitrogen fixers in the Arctic. It normally forms black, irregularly shaped mats on moist ground, but it also occurs as the algal partner in a circumpolar species of leafy lichen.