As previously discussed in the Introduction, cyanobacteria may play a crucial role in the nutrient supply of tropical systems. Nitrogen fixation rates by cyanobacteria are influenced by several factors: moisture, energy source availability, mineral nutrient availability and sunlight availability (Bentley 1987). Epiphyllous liverworts may provide ideal habitats for cyanobacteria and subsequently nitrogen fixation. This figure (right) from Bentley (1987) illustrates the positive correlation between epiphyllous bryophyte biomass and nitrogen fixation rates. This relationship is probably due to increased moisture, mineral and exogenous sugar levels in the presence of the bryophyte. Studies have shown that bryophyte laden leaves dry more slowly than those without epiphylls (Berrie and Eze 1975). This is due to increased water-holding capacity affected by capillary action of the bryophyte (Berrie and Eze 1975) and may also be due to specialized anatomical featuresthat increase the bryophyte's ability to hold water. This water may be available to nitrogen fixers on the leaf surface. As discussed in the section The role of bryophytes in mineral and sugar cycling in the phyllosphere,epiphytic bryophytes undergo periodic pulse release of minerals and sugars after a period of desiccation followed by rehydration. These sugars and minerals may be available for uptake by the associated cyanobacteria. Free-living cyanobacteria may require up to 200 grams of glucose per gram of nitrogen fixed and are able to access external pools (Bentley 1987).
Nitrogen fixation rates increase when exogenous glucose is applied to the cyanobacteria as compared to deionized water independent of light levels (see figure left). Similarly, nitrogen fixation rates are positively correlated with the availability of mineral nutrients such as phosphate, ammonium, potassium, calcium, molybdenum and iron (Bentley 1987). Bryophytes can provide the moisture, nutrients and exogenous sugars that can support cyanobacteria growth and nitrogen fixation in the phyllosphere.
|What is the fate of this fixed nitrogen, and what are the costs or benefits to the host leaf supporting both bryophyte and cyanobacteria colonies?|