Phosphorus and Molybdenum Limitations and Environmental Constraints on Cyanobacteria in a Tropical Rainforest Canopy

Abstract

Cyanobacteria in the rainforest canopy produce a significant amount of nitrogen (N), which is one of the dominant controlling factors on carbon storage (Luo, 2004). In order to determine the controls on carbon storage, we must determine the controls on cyanobacteria and other N-fixers. I studied the limitations of phosphorus (P), molybdenum (Mo), moisture, temperature, and light on cyanobacteria through an observational study and a canopy fertilization experiment. In my observational study, I measured the percent cover of cyanobacteria and other epiphytes on 5 species of phorophyte with different leaf characteristics such as size, longevity, and phosphorus and nitrogen concentrations, and at high, middle and low zones of the canopy. I found that epiphytes and cyanobacteria preferentially colonized certain phorophyte species over others, and were most abundant on the lower leaves of the canopy. In my canopy fertilization experiment, I suspended artificial substrates soaked in different treatments (P, Mo, Mo+P, control) from branches in the canopy. I found that heterocyst frequency (amount of nitrogen fixing cells to regular cells) decreased over time, indicating that individual cyanobacteria can down-regulate their N-fixation once enough N builds up within their colony. I also found a higher percent cover of cyanobacteria on substrates at the top of the canopy, which had more light, rainfall events, temperature and vapor pressure deficit (VPD) than substrates at the bottom of the canopy. This implies that cyanobacteria may be able to withstand periods of water stress and high temperatures and may be more controlled by light availability. I did not find a difference in percent cover or heterocyst frequency between nutrient treatments. I estimated an average N-input of 0.01 kg N ha-1 yr-1 for cyanobacteria. My results have important implications for predicting the response of cyanobacteria to climate change and other human disturbance, and the repercussions for terrestrial carbon storage.