Paper Reviewed
Lindo, Z. and Griffith, D.A. 2017. Elevated atmospheric CO2 and warming stimulates growth and nitrogen fixation in a common forest floor cyanobacterium under axenic conditions. Forests 8: 73, doi:10.3390/f8030073.

Writing as background for their work, Lindo and Griffith (2017) state that "Nitrogen (N) fixation is a critical ecosystem-level process that brings unavailable atmospheric N into the biosphere." This process is particularly important in boreal forest ecosystems (which are generally N limited), where moss-associated cyanobacteria on the forest floor contribute "up to 3 kgN/ha/year to forest-level N budgets (Zielke et al., 2005; DeLuca et al., 2007; 2008; Sorensen and Michelsen, 2011), and may provide a mechanism for sustaining long-term forest productivity (Lindo and Whiteley, 2011; Rousk et al., 2013)." Unfortunately, little is known about how these ecosystems might respond to the global changes predicted to occur over the course of the next century (i.e., rising atmospheric CO2 and increasing temperatures). Therefore, it was the aim of these two Canadian researchers to examine the response of forest-inhabiting cyanobacteria N fixation to increases in both atmospheric CO2 and temperature.

To accomplish their objective, Lindo and Griffith conducted two separate experiments. In the first, they grew Nostoc punctiforme, a multicellular moss-associated forest floor cyanobacterium, for 30 days under ambient (430 ppm) or elevated (750 ppm) CO2 concentrations. In the second experiment, they also grew N. punctiforme, but in a full-factorial design of three temperature treatments (11.5, 15.5 and 19.5 °C) and two atmospheric CO2 concentrations (430 and 750 ppm), for a period of 90 days. Data pertaining to the growth and N fixation of N. punctiforme were acquired at regular intervals throughout the duration of both experiments. And what did those data reveal?

In describing their findings, Lindo and Griffith say that "elevated CO2 conditions stimulated N fixation and heterocyst production, and that cultures under elevated CO2 grew more quickly than under ambient CO2 conditions." With respect to the impacts of temperature, they report that "warming is likely to have a positive effect on forest floor cyanobacteria growth and N fixation as long as temperatures remain within or below the enzymatic optimum temperature for N fixation." And while the above findings indicate that "the cumulative overall amounts of N fixed [by N. punctiforme] will be greater under warmer and elevated CO2 conditions even when controlling for faster cycling of growth phases," Lindo and Griffith say it is "still unknown" whether the results of their laboratory experiments will translate "into greater N availability for N limited forest ecosystems," as that answer depends on the relationship between the cyanobacteria and the moss-associated host. But as we await those latter findings, these interim results appear quite encouraging.

References
DeLuca, T.H., Zackrisson, O., Gentili, F., Sellstedt, A. and Nilsson, M.-C. 2007. Ecosystem controls of nitrogen fixation in boreal feather moss communities. Oecologia 152: 121-130.

DeLuca, T.H., Zackrisson, O., Gundale, M.J. and Nilsson, M.-C. 2008. Ecosystem feedbacks and nitrogen fixation in boreal forests. Science 320: 1181.

Lindo, Z. and Whiteley, J.A. 2011. Old trees contribute to bio-available nitrogen through canopy bryophytes. Plant and Soil 342: 141-148.

Rousk, K., Jones, D.L. and DeLuca, T.H. 2013. Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems. Frontiers in Microbiology 4: 1-10.

Sorensen, P.L. and Michelsen, A. 2011. Long-term warming and litter addition affects nitrogen fixation in a subarctic heath. Global Change Biology 17: 528-537.

Zielke, M., Solheim, B., Spjelkavik, S. and Olsen, R.A. 2005. Nitrogen fixation in the high arctic: Role of vegetation and environmental conditions. Arctic, Antarctic, and Alpine Research 37: 372-378.