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Something old, something blue - how cyanobacteria are helping forests grow

By Martin Leggett - 25 Feb 2011 14:55:0 GMT
Something old, something blue - how cyanobacteria are helping forests grow

Image credit: storm-crow.co.uk

Sometimes it is the little details which make the biggest impacts – and scientists from Montreal's McGill University have bought their microscopes to bear on tiny organisms which could influence the capacity of forests to offset greenhouse gases. Cyanobacteria, which appear blue under the microscope, are microorganisms that are both wildly abundant, and firmly ensconced in just about every habitat on the planet.

Dr. Zoë Lindo of McGill's Department of Biology, and a doctoral student, have been looking at the mosses of spruce trees in Canada's coastal rain forests. These tree-growing mosses have developed a close connection with cyanobacteria that grow on them. The bacteria provide the mosses with essential nitrogen, by 'fixing' it from the atmosphere – which aids the mosses growth.

It has already been shown that mosses at ground level, in the Arctic Boreal forests, aid nutrient flows by this process. Lindo decided to investigate whether this also applied to the mosses in temperate rain forests. They took samples of the mosses from the forest floor, and up to 30m (100 feet) high in the canopy, for trees of different ages. What they found was that the larger, older trees grow more mosses, and interestingly, that the cyanobacteria in the higher mosses fix twice as much nitrogen as those at ground level.

Given that the growth and development of forests depends on the amount of nitrogen available to them, this has major implications for how we manage forests. The age of the trees seems to be the fundamental factor in determining the nitrogen fixing capability of the forest as a whole.

As Lindo says ''You need trees that are large enough and old enough to start accumulating mosses before you can have the cyanobacteria that are associated with the mosses. Many trees don't start to accumulate mosses until they're more than 100 years old. So it's really the density of very large old trees that are draped in moss that is important at a forest stand level''.

Why does this matter at the bigger scale? Well, first of all increased nitrogen fixing can ultimately lead increased nitrogen dioxide release from the soil – and NO2 is a potent greenhouse gas. But equally, a more productive forest may absorb more CO2, so countering that effect. It is the interplay of these two factors makes the microscopic contribution of cyanobacteria to the global issue of global warming difficult to call.