Composition and diversity of nifH genes of nitrogen-fixing cyanobacteria associated with boreal forest feather mosses

Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N-2 fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.