A comparisson of methods measuring nitrogen fixation across a stream temperature gradient

• Main Author: Goldschmidt, Jackelyn B
• Format: Text
• Language: unknown
• Published: SOPHIA 2014
• Subjects: Arctic
• Online Access: https://sophia.stkate.edu/undergraduate_research_symposium/2014/natural_sciences/6

The supply and availability of nitrogen often determines the growth of organisms. Cyanobacteria are able to fix nitrogen gas from the atmosphere and convert it into a useable form of nitrogen, supporting their own growth, and ecosystem production; therefore nitrogen fixation can be an important source of nitrogen in nitrogen poor ecosystems. Yet, measuring nitrogen fixation can be difficult. Our primary objective was to assess the effectiveness of current methods for quantifying nitrogen fixation, as well as how those methods might vary as a function of temperature. Nitrogenase, the enzyme responsible for nitrogen fixation, also reduces acetylene into ethylene. Thus, the conversion of acetylene into ethylene provides an indirect measure of nitrogen fixation. However, the efficiency or ratio of conversion between acetylene and nitrogen gas is highly debated and may also be sensitive to temperature. In contrast, direct measurements that quantify the fixation of nitrogen gas directly are preferable, yet are more expensive and sensitive. It can also be difficult to quantify the amount of nitrogen gas dissolved in water, as it does not readily dissolve. To compare the methods, we measured nitrogen fixation rates and algal biomass across a gradient of temperatures (n=3) in a sub-arctic grassland watershed. We chose this landscape for the natural geothermal properties that create the temperature gradient between streams within a single watershed. Three methods were used to measure nitrogen fixation rates, which are quantified per unit of algal biomass and per unit of area. We found that when using the acetylene method, there was a significantly positive relationship between nitrogen fixation rates and temperature per area after the experiment was run for 4, 6, and 8 weeks (p<0.05). However, when compared to rates per algal biomass, there was only a significant positive relationship between nitrogen fixation rates and temperature after 6 weeks (p<0.05). It appears that after 8 weeks, the relationship between temperature and nitrogen fixation rates breaks down.