Abstract
Cyanobacteria-plant symbioses play an important role in many ecosystems due to the fixation of atmospheric nitrogen (N) by the cyanobacterial symbiont. The ubiquitous feather moss Pleurozium schreberi (Brid.) Mitt. is colonized by cyanobacteria in boreal systems with low N deposition. Here, cyanobacteria fix substantial amounts of N2 and represent a potential N source. The feather moss appears to be resistant to decomposition, which could be partly a result of toxins produced by cyanobacteria. To assess how cyanobacteria modulated the toxicity of moss, we measured inhibition of bacterial growth. Moss with varying numbers of cyanobacteria was added to soil bacteria to test the inhibition of their growth using the thymidine incorporation technique. Moss could universally inhibit bacterial growth, but moss toxicity did not increase with N2 fixation rates (numbers of cyanobacteria). Instead, we see evidence for a negative relationship between moss toxicity to bacteria and N2 fixation, which could be related to the ecological mechanisms that govern the cyanobacteria – moss relationship. We conclude that cyanobacteria associated with moss do not contribute to the resistance to decomposition of moss, and from our results emerges the question as to what type of relationship the moss and cyanobacteria share.
Highlights
Symbiotic relationships between plants and microbes have been proposed to be important drivers of evolution [1]
Chemical analyses We investigated the N concentration in moss tissue of samples collected along gradients of cyanobacterial N2 fixation in Pleurozium schreberi induced by road pollution in boreal forests
The EC50-values were used as indices for moss toxicity to bacterial growth
Summary
Symbiotic relationships between plants and microbes have been proposed to be important drivers of evolution [1]. Cyanobacteria are facultative autotrophs, generally fixing atmospheric carbon (C) and nitrogen (N), but are known to use host-C through symbiotic relationships [4]. One example is the relationship between one of the dominant primary producers in mid- to late succession boreal forests [8], the feather moss Pleurozium schreberi (Brid.) Mitt., and associated cyanobacteria. Substantial amounts of atmospheric N2 are fixed by cyanobacteria colonizing P. schreberi [8] This process is sensitive to N inputs: higher N input through deposition results in lower numbers of cyanobacteria in the feather mosses and, in lower N2 fixation rates [9,11,12]
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