Abstract:

Background & Aim: As a significant nitrogen source in many natural terrestrial ecosystems where nitrogen is limited, associations formed by bryophytes and nitrogen-fixing cyanobacteria play an essential role in the global nitrogen cycle. Species composition and habitat of bryophyte-cyanobacteria associations result in contrasting nitrogen fixation capacities in different ecosystems. However, the current studies on bryophyte-cyanobacteria associations are predominately conducted in high-latitude ecosystems of the Northern Hemisphere, with few and scattered reports in other ecosystems. Therefore, we summarized the distribution, species composition, cyanobacterial colonization rate, cyanobacterial abundance, and nitrogen fixation potential of reported bryophyte-cyanobacteria associations worldwide, aiming to provide a reference for further study on the diversity and nitrogen fixation capacity of bryophyte- cyanobacteria associations in various ecosystems globally.
Progress: According to our analysis and summarization, a total of 110 bryophyte species from 58 genera in 41 families and more than 26 cyanobacteria species from 17 genera in 9 families, were identified to be involved in bryophyte-cyanobacteria symbiotic associations. The associated bryophyte species varied among diverse ecosystems. For example, Pleurozium schreberi-cyanobacteria associations are dominate in the understory of boreal forests, while Sphagnum spp.-cyanobacteria associations dominate wetland ecosystems. Nostoc species are the dominant cyanobacteria in most ecosystems due to their unique physiological characteristics and strong ecological adaptability. Among different ecosystems the colonization rate, abundance, and nitrogen fixation capacity of cyanobacteria on bryophytes differed significantly. Nitrogen fixation was highest in Arctic tundra ecosystems (1.3-24.6 kg N·ha^-1·yr^-1), followed by boreal forests (0.04-11.53 kg N·ha^-1·yr^-1), and was the lowest in temperate grasslands (0.008-0.124 kg N·ha^-1·yr^-1).
Perspective: We emphasized that the research on species composition and the capacity for nitrogen fixation was quite inadequate. We also highlighted four research perspectives: (1) Employing techniques such as metagenomics and establishing observation and research stations to explore and study the colonization characteristics and nitrogen fixation ability of cyanobacteria associated with bryophytes. (2) Conducting further and systematic research on bryophyte-cyanobacteria associations in all types of ecosystems. (3) Unifying the measurement and estimation methods of nitrogen fixation rate in different ecosystems. (4) Determining the key influencing factors and regulatory pathways of the diversity, distribution, colonization rate, cyanobacteria abundance, and nitrogen fixation rate of bryophyte- cyanobacteria associations.

Key words: bryophyte-cyanobacteria associations, ecosystem, diversity, distribution, nitrogen fixation