A surprising diversity and frequency of cyano-bryo associations in a boreal forest

Background/Question/Methods The importance of nitrogen-fixing cyanobacteria associated with bryophytes in some arctic and sub-arctic systems is well-established.  Such associations contribute a sometimes large but highly-variable percentage of all nitrogen inputs to far-north ecosystems: from 2-58% of all nitrogen to the ecosystem, and from 42-84% of all nitrogen to the plants (Dodds et al., 1995).  But although bryophyte-cyanobacterial associations and their potential ecosystem impacts have been characterized in many parts of the circumpolar north, studies of these associations at temperate latitudes are relatively rare (Solheim & Zielke, 2003).  Also, many of these far-north studies have focused on a small number of moss taxa, particularly the dominant boreal moss Pleurozium schreberi (e.g., DeLuca et al. 2002; Gentili et al., 2005; DeLuca et al., 2006).  In our descriptive study, diverse moss and liverwort taxa (approximately 40 species) from a boreal forest at temperate latitudes (Door County, Wisconsin) were screened for the presence of cyanobacterial associates.  Approximately 500 bryophyte samples were collected using a completely-randomized design during four weeks in 2007-2008 (summer, fall, winter, and spring); these were screened for cyanobacteria using fluorescence microscopy.  In addition, the cyanobacteria were classified according to morphological type, examined for the presence of heterocytes, and characterized with respect to seasonal abundance.  

Results/Conclusions Because such associations are little-studied, previous estimates have suggested that bryophyte-cyanobacterial associations are relatively rare.  However, the percentage of bryophyte samples hosting cyanobacteria varied from approximately 20% to 50% for summer, fall, and winter samples; cyanobacteria were most abundant in fall, and least abundant in winter.  A wide array of cyanobacterial forms (unicells of various sizes and shapes, filaments, and mucilaginous colonies) were detected, many with heterocytes potentially capable of nitrogen fixation.  For all seasons, 50-60% of all cyanobacterial samples contained heterocytous forms; this means that approximately 25% of all bryophyte samples contained heterocytous cyanobacteria.  With respect to seasonality, the cold-tolerant cyanobacterium Nostoc appears to be most prevalent in the winter samples.  Although Pleurozium schreberi did not host cyanobacteria at our site, a number of potentially novel bryophyte-cyanobacterial associations are reported; these include common moss genera such as Fissidens, Brachythecium, and others.  Overall this evidence suggests that bryophyte-cyanobacterial associations are not necessarily rare; in this Wisconsin boreal forest, they are common, with seasonal variations that may have implications for nitrogen cycling.