In our boreal forests (coniferous forests, dominated by pines and spruces), there is generally a great lack of the essential nutrient nitrogen. This means in practice that the availability of nitrogen determines how productive these forests are. Previous research has shown that feather mosses contribute with nitrogen to the coniferous forest ecosystem by forming symbioses with nitrogen-fixing cyanobacteria. Cyanobacteria can fix atmospheric nitrogen and make it accessible for plants. By this symbiosis, the forests are supplied with two to three kilo nitrogen per hectare and year.

In a recent study, scientists at DEEP (Department of Ecology, Environment and Plant Sciences, at Stockholm University in collaboration with SLU, Umeå) show that mainly the feather moss symbioses with a specific group of cyanobacteria is the largest contributor of nitrogen.

- The cyanobacterium that is most abundant on the moss is not, as earlier thought, the one that contributes the most with nitrogen. Instead it is the previously overlooked cyanobacterium (Stigonema) that is the major nitrogen fixer on the mosses, says Ulla Rasmussen. This study is now presented in an article in the prestigious international research journal ISME Journal.

Boreal forest from the sampling site in Arvidsjur in north of Sweden. Photo: Denis Warshan.
Boreal forest from a sampling site in Arvidsjur in northern Sweden. Photo: Denis Warshan.

 

Boreal forests covers 11-14% of the Earth's land surface and form the world's largest continuous land ecosystems. This means that about a third of the world's forests consist of conifers. In Sweden, 82% of all coniferous forests. Because of the boreal forests major global distribution they are important for the absorption of carbon dioxide from the atmosphere and can therefore act as carbon sink which could slow down the greenhouse effect. The soil surface in the boreal forest are mostly covered by the mosses.

The symbiosis between cyanobacteria and mosses was relatively recently discovered and has for some time been studied more closely by researchers at DEEP. An interesting finding in the study is that the cyanobacterial composition differs between different mosses, and also that the composition undergoes major changes during the growth season.

Väggmossa - Pleurozium schreberi.
Feather moss - Pleurozium schreberi - one of the most abundant moss species in the boreal forest. Photo: Ulla Rasmussen.

- These results are extremely important in our search to understand the input of nitrogen into this ecosystem. We have in previous studies shown that the mosses attracts cyanobacteria when they need nitrogen, this by a communication that we now are investigating in detail, says Denis Warshan.

 

 

 

The cyanobacterium
The cyanobacterium Stigonema on the surface of a moss leaf, visualized with fluorescent light in microscope. Photo: Ulla Rasmussen.

Cyanobacteria are one of Earth's most important microorganisms. They fix both nitrogen and carbon dioxide from the atmosphere and are important actors in the global cycles of nitrogen and carbon. Understanding the details about how nitrogen fixation in mosses is regulated is therefore important if we want to predict and manage many of the global environmental changes that humanity is causing in the nitrogen and carbon cycles.

 

What is interesting about this study is that we now know what the coniferous forest’s main source of nitrogen is, which in practice has great influence on the productivity of these forests. There is also an interesting opportunity if we could learn to use the ability of cyanobacteria to fix nitrogen from the atmosphere to improve the productivity of food plants and other useful plants.

About the research

”Seasonal variation in nifH abundance and expression of cyanobacterial communities associated with boreal feather mosses” är publicerad i ISME Journal 26 februari 2016. www.nature.com/ismej

Read the paper here: http://www.nature.com/ismej/journal/vaop/ncurrent/pdf/ismej201617a.pdf