PS 40-111 - Micronutrient supply and high CO2 concentrations as possible drivers of biological nitrogen fixation in cyanolichens

Abstract: Biological nitrogen (N) fixation (BNF) is the main source of reactive N for unmanaged terrestrial ecosystems. Through BNF, N-fixing organisms make N available to the environment, affecting the N cycle and carbon (C) sequestration. Despite the importance of this ecosystem function, the factors that control BNF in terrestrial ecosystems remain poorly understood, especially in the tropics. Nitrogenase, the enzyme that catalyzes BNF, occurs in different isoforms; molybdenum (Mo)-nitrogenase, found in every N-fixing organism, and vanadium (V)-nitrogenase, found in only a few. Mo and V availability (onwards referred as micronutrients) can potentially limit BNF by limiting the synthesis of nitrogenases. Carbon can also affect BNF because the metabolisms of N and C are strongly coupled in many living organisms, with an increase in C requiring an increase in BNF to maintain the optimal C:N ratio.

To evaluate how micronutrient supply and C availability affect BNF under tropical conditions, we performed a 27-day experiment with the cyanolichen Leptogium cyanescens in a growth chamber. We transplanted 77 thalli from a forest to the growth chamber, where they were sprayed daily with a solution containing Mo and/or V and exposed to different CO2 concentrations, along with a control. Relative BNF rates were inferred from the number of heterocysts, specialized cells responsible for BNF in cyanobacteria. To accomplish this, we sectioned a fragment of the thalli at the beginning and end of the experiment using a cryostat, photographed the sections, and counted the number of heterocysts per total number of cells.

Our results showed that high CO2 concentrations increased heterocyst production, possibly due to an increase in the cyanobacterial demand for N. We also found that while V supply increased heterocyst production, Mo addition did not affect the production of these cells. One possible explanation is that while cyanobacteria produced more N-fixing cells under high V availability, cyanobacteria under high Mo availability increased the efficiency of the BNF without producing more heterocysts, in both cases increasing the availability of N. Our findings suggest that BNF in lichens is more complex than previously thought, with the possibility of tropical species having both Mo and V-nitrogenases and using them to better adapt to different environmental conditions. It also suggests that micronutrients and C drives BNF in lichens and therefore drive the amount of N lichens provide to the environment. Finally, our results indicate that BNF may respond to future increases in CO2 concentrations under climate change.