Effects of CO2 and micronutrient supply on heterocyst production and nitrogen fixation in cyanolichens

Abstract

Nitrogen (N) fixation is an important ecosystem function as it is one of the main sources of N to the environment. Despite that, the factors that control this process in terrestrial environments are still poorly understood. Micronutrient availability and excess carbon have been considered key regulators of N fixation, but there is still great uncertainty about how they act in the regulation. Future projections predict an increase in carbon dioxide (CO2) concentrations, which can pressure organisms to fix N. However, a micronutrient limitation may restrict N fixation, affecting the supply of reactive N to the ecosystem. Thus, this work aimed to verify how the supply of molybdenum (Mo) and vanadium (V) and high concentrations of CO2 affect N fixation and heterocyst production, fixing cells, in the cyanolichen Leptogium cyanescens (Rabenh.) Körb. To do it, we performed a 27-day experiment, with 64 thalli transplanted from a riparian forest located within Reserva Biológica Boqueirão to a growth chamber with a controlled environment. Heterocysts were counted after we sectioned the thalli using a freezing microtome, and N fixation rates were obtained through an acetylene reduction assay. We found that the addition of micronutrients and exposure to high concentrations of CO2 increased the thalli investment in heterocysts. However, specimens exposed to Mo, V, and high concentration of CO2 simultaneously did not produce more heterocysts, and they did not fix more nitrogen than thalli exposed to these conditions separately, as expected. In addition, we also found a negative relationship between the number of heterocysts and N fixation. These results lead us to the conclusion that investment in heterocysts is affected by micronutrients because they participate in the synthesis of nitrogenase and by a high concentration of CO2 because it induces the demand for nitrogen.