Algal mats

MATALONI, GABRIELA

Encyclopedia of the Antarctic

Routledge

Lugar: New York; Año: 2007; p. 27 - 28

Algal mats are perhaps the most widespread microbial communities along Antarctica. They consist basically of a mesh of microscopic algal filaments embedded in their own mucillaginous sheaths, which in turn trap and bind sediment particles, providing shelter as well as nutrients to other algae, bacteria, heterotrophic microorganisms (flagellates, rhizopods, ciliates), and even metazoan (rotifers, tardigrades and nematodes). Because of this structural complexity they are also referred to as "microbial mats". Many scientists use the term "cyanobacterial mats" on account of the almost invariably dominance of filamentous cyanobacteria ("blue-green algae"), most of which  belong to the order Oscillatoriales (genera Phormidium, Oscillatoria and Leptolyngbya among others). Commonly nitrogen-fixing genera such as Nostoc, Anabaena  and Nodularia are also frequent. Diatoms, mainly represented by Navicula muticopsis and Pinnularia borealis  can surpass cyanobacteria in species number, yet not in biomass. Green algae do not contribute substantially to algal mats except for a few nitrophilous species and the green mats dominated by macroscopic Prasiola crispa. Growth and development of  algal mats also involves seasonal changes in species composition. Algal mats thrive in a wide range of environments, from wet soils or constantly flushed ones, to the bottom of temporarily drying-out pools and ephemeral runnels, to the channels of permanent rivers, to the edges and bottom of cryoconite holes and deep, permanently frozen lakes. In response to this wide spectrum of ecological conditions, they vary in thickness (from less than 4mm to more than 3 cm), species composition and colour. Differently coloured mats in one same area can represent distinct species compositions. Well-lit algal mats from soils and shallow water bodies have a layered structure, with reddish upper layers formed by dead filaments or by a dense mesh of actively moving Oscillatoriales. This layers are rich in mucillage, sheath pigments and carotenoids which provide the whole community with protection against dessication and high irradiances. Lower layers are typically dark green and host a higher number of species, which includes cyanobacteria as well as diatoms and few green algae. They have high chlorophyll-a concentrations and generally display a higher photosynthetic activity. I turn, algal mats from the dim-lit bottom of lakes have a photosynthetically active upper layer and an anoxic, heterotrophic low layer. Resistance to dessication and high irradiances allows algal mats succesful colonization from one environment to another. Different researchers have recorded the same species composition between the algal mats of lakes and the surrounding terrestrial ones, and between those mats covering the bottom of deep lakes and small cryoconite holes in the same area. Daily mat production can be as high as 1 to 38 mg C.m-2.day-1,  widely surpassing that of phytoplankton in the same lake. Also, algal mats can cover extensive areas of flushed soils at sites in Continental Antarctica. On account of this, they are the main source of carbon fixation in a vast region of Antarctica. Therefore, algal mats can be regarded as very succesful communities playing a key role in Antarctic trophic chains, and deserve close attention within the context of a changing environment.