Effects of Abiotic Factors on Acetylene Reduction by Cyanobacteria Epiphytic on Moss at a Subantarctic Island

Abstract

Acetylene reduction (AR) rates by cyanobacteria epiphytic on a moss at Marion Island (46°54' S, 37°45' E) increased from-5°C to a maximum at 25 to 27°C. QlO values between 0 and 25°C were between 2.3 and 2.9, depending on photosynthetic photon flux density. AR rates declined sharply at temperatures above the optimum and were lower at 35°C than at 0°C. Photosynthetic photon flux density at low levels markedly influenced AR, and half of the maximum rate occurred at 84 ,umol m2 s-1, saturation occurring at ca. 1,000 ,imol m-2 s-I. Higher photosynthetic photon flux density levels decreased AR rates. AR increased up to the highest sample moisture content investigated (3,405%), and the pH optimum was between 5.9 and 6.2. The addition of P, Co, and Mo, individually or together, depressed AR.

What drives biological nitrogen fixation in high arctic tundra: Moisture or temperature?

“…In our study, the highest N 2 fixation activity was found at temperatures below 14°C. This stands in sharp contrast to previous findings on the temperature dependence of N 2 fixation, with a temperature optimum around 25°C for symbiotic as well as moss-associated N 2 fixation (Smith 1984, Houlton et al 2008. Then again, most previous findings are from ecosystems outside the Arctic.…”

“…This was ascribed to the drying-out effect of the diazotroph associations by increased temperature. The large range in temperature optimum reported previously (e.g., Smith 1984, Hobara et al 2006 might have been recorded while the N 2 fixer associations have been optimally moist. Further, mosses differ in their capacity to hold water (Elumeeva et al 2011), which might also explain differences in N 2 fixation activity between studies investigating different moss species.…”

“…These temperature optima are much lower than previously reported. For instance, Hobara et al (2006) report increasing N 2 fixation rates by free-living and lichenassociated diazotrophs in high arctic tundra up to 30°C, and maximum temperature for N 2 fixation in mosses from the Antarctic have been found between 25°and 27°C (Smith 1984). On the other hand, N 2 fixation rates in different legume species in the high Arctic differ in their temperature optimum ranging between 10°and 25°C (Schulman et al 1988).…”

Nitrogen Fixation Increases With Successional Age in Boreal Forests

“…The apparent drop in activity in July may correspond to the reported period of dormancy in mosses that is likely a function of both dry conditions and photoinhibition (Sveinbjö rnsson and Oechel 1992, Whitehead and Gower 2001). Although epiphytic cyanobacteria in both the high arctic (Lennihan et al 1993, Zielke et al 2002 and in the Antarctic (Smith 1984) have been found to reach light saturation at relatively low light intensity, cyanobacteria have not been shown to experience decreased N fixation with increasing light intensity. Nitrogen fixation rates, as estimated by use of the acetylene reduction assay, in P. schreberi were found to follow a linear increase with time since last fire for the window of time (320 years) provided by the chronosequence.…”

“…In contrast to N additions that would likely only decrease N fixation rates, P additions to the moss bottom layer could either increase (Chapin et al 1991) or decrease (Smith 1984) N fixation rates. Therefore, we chose to conduct the experiment at both a late-succession site (Ruttjeheden) that had high rates of N fixation and at one early-succession (Granliden) site that had low rates of N fixation.…”