Bryophyte-cyanobacteria associations increase bryophyte nitrogen content and photosynthetic performance but are vulnerable to environmental stress
Bryophytes play an important biogeochemical role in many ecosystems, where they regulate soil temperature and moisture, and influence carbon and nitrogen (N) cycling. Associations between bryophytes and N2-fixing cyanobacteria in northern latitude forests represent a substantial N input to these ecosystems, but among bryophytes that associate with cyanobacteria, it is unknown what proportion of bryophyte N comes from cyanobacteria, and how the association (and N supplementation) is influenced by environmental stress. Further, a rapid probe for cyanobacteria colonization presence in field-collected bryophytes has not yet been established. In this study, we first investigated the utility of bryophyte δ15N ratios to predict cyanobacteria colonization in common bryophytes from a northern temperate forest. Second, we conducted an experiment where bryophytes with or without cyanobacteria where subjected to precipitation containing NO3-/NH4+ of a known δ15N and differing levels of desiccation. After twelve weeks of treatment, plants were assessed for photosynthetic performance and the tissue δ15N of new growth was measured. We hypothesized that bryophytes that formed associations with cyanobacteria would exhibit δ15N signatures closer to the atmospheric signature for N2 compared to those that did not, and we predicted that the proportion of N received from cyanobacteria would be reduced by drought stress.
Among seven common northern temperate forest bryophytes, three formed associations with cyanobacteria. The taxa with cyanobacterial associations exhibited δ15N values that were 3.9‰ higher compared to those without, and δ15N signatures that were close to the atmospheric N2 signature (0‰), suggesting these taxa acquired a measurable fraction of their N from associated cyanobacteria. Bryophyte δ15N was also consistent with cyanobacteria presence assays performed using UV fluorescence microscopy, and the taxa that associated with cyanobacteria had 55% higher tissue N content compared to those that did not associate with cyanobacteria. Bryophytes that associated with cyanobacteria received 8-10% of their tissue N from cyanobacteria, but this amendment level was reduced by as much as 7% by exposure to drought conditions. Light-saturated CO2 fixation was up to 15% higher in bryophytes that associated with cyanobacteria, suggesting N amendments directly translate to increased photosynthetic performance, but CO2 fixation rates were reduced in all bryophytes exposed to the drought treatment. Collectively, these results suggest that cyanobacteria, by supplying N to bryophytes, increases bryophyte nutrient status as well as photosynthetic performance, but also reveals that the association (either one or both of the partners) is vulnerable to environmental stress in the form of drought.