| Summary: | Nitrogen fixation is an enzymatic process which converts atmospheric nitrogen into biologically available molecules and also provides nitrogen to aquatic ecosystems where nitrogen otherwise limits growth and productivity. As a metabolic process, nitrogen fixation rates typically increase with temperature. However, it is unclear whether nitrogen-fixing species will respond to altered temperatures in a similar fashion, or whether the range of growth forms and physiological adaptations possessed by nitrogen-fixers that allow them to persist in a variety of physical conditions may constrain their response to changes in temperature. Mean temperatures are increasing around the globe, with the most extreme shifts occurring at high latitudes, including arctic regions. Our primary research goal was to determine how different species of freshwater nitrogen-fixing algae respond to temperature change. Nitrogen-fixing Nostoc species are found across a wide range of stream temperatures in Iceland and different species are acclimated to cold (5-8 °C), intermediate (10-15 °C) and warm (20+ °C) streams that are maintained by geothermal groundwater heating. We predicted that if nitrogen fixation rates are regulated by enzymatic activity alone, rates should change predictably when temperature is altered. If, however, nitrogen fixation rates are constrained by traits possessed by species that naturally occur at different temperatures, some species may be physiologically unable to adjust nitrogen fixation rates in response to changing temperature. To test these predictions, samples of two Nostoc species, both of which are found in streams at 14 °C, were transplanted to different temperature environments 24 hours prior to measuring nitrogen fixation rates. Nitrogen fixation rates were measured in the resident stream (at 14°C), as well as 7, 10 and 17 °C for Nostoc spongiaeforme and 18 °C for Nostoc amplissimum. There was a significant positive relationship between nitrogen fixation rate and temperature in Nostoc spongiaeforme (R2=0.9235, p<0.01), while there was no significant difference in nitrogen fixation rates for Nostoc amplissiumum at 14 and 18 °C (one-way ANOVA, p>0.05). These results indicate that the response to temperature varies between nitrogen-fixing species and that some more readily acclimate to temperature change. Which species persist as temperatures increase may therefore influence how much nitrogen is supplied to freshwater ecosystems, influencing productivity and food web dynamics.
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