The cyanobacterial genus Nostoc includes many species that are highly diverse with respect to morphology, functional properties, biotic relations and habitat distribution. Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts and they periodically form resistant akinetes for survival and short motile filaments (hormogonia) for reproduction. Some species are free-living and many species engage in loose or obligate cooperation with land plants and fungi (e.g. lichens). A third, fascinating Nostoc type forms large gelatinous colonies of variable shape and structure in rice fields, freshwater lakes, ponds and streams and on alternating wet and dry soils or rock surfaces. The large gelatinous species require special adaptations to obtain sufficient light, nutrients and dissolved inorganic carbon in water and to survive the extreme variations in temperature, water supply and irradiance on naked soils and rock surfaces.
The gelatinous Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.
This review in Annals of Botany evaluates the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.
As free-living organisms and as symbionts in lichens, Nostoc species are both pioneers and permanent members of the vegetation of deserts, semideserts, dry grasslands and rock surfaces ranging in geographical distribution from polar to tropical regions. Their N input to these biomes can be of utmost importance. In a carefully mapped Low-Arctic tundra landscape, N2 fixation by cyanobacteria was twice the annual wet deposition of nitrogen. It remains unexplored how the high water-absorbing capacity and N2 fixation of Nostoc can facilitate the colonization of bare or newly exposed mineral surfaces by mosses and higher plants, thereby forming more stable vegetation and more organic soils. With the exposure of new mineral surfaces behind retreating glaciers on a warming Earth, this ecosystem service of Nostoc deserves future attention. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range. Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.
Sand-Jensen, K. (2014) Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments. Annals of Botany, 114(1): 17-33.