The effect of growth rate, phosphorus concentration, and temperature on N₂ fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS101

Abstract

With the use of continuous culture systems, rates of dinitrogen (N 2 ) and carbon (C) fixation and nitrogen (N)-and C-based doubling times were assessed in Trichodesmium IMS101 growing exponentially at steady state dilution rates of 0.10, 0.20, and 0.33 d Ϫ1 (doubling times of 10, 5, and 3 d-within the range reported for natural populations). Rates of C fixation, N 2 fixation, and N release were examined in replicate culture systems with several techniques. Biomass-specific C uptake varied little with population doubling time, but N 2 fixation and N release varied markedly among treatments. Total daily gross N 2 fixation rates and estimated N release rates were higher in cultures with higher dilution rates. Cultures grown at lower dilution rates had higher daily C : N 2 fixation ratios and lower N release rates. Consistent with other studies, it was estimated that Trichodesmium released about 80-90% of their recently fixed N 2 during growth. Turnover of cellular C estimated from carbon fixation was a good estimator of population growth rates in steady state cultures, whereas turnover of cellular N estimated from gross or net N 2 fixation was not. Small changes in temperature (24ЊC vs. 28ЊC) did not appear to affect gross N 2 fixation, whereas inorganic phosphorus (1 vs. 5 mol L Ϫ1 ) supply had a large effect on N 2 fixation. These results suggest that continuous culture systems are excellent for elucidating physiological responses of Trichodesmium under ecologically relevant growth conditions and provide a framework for assessing highly variable field estimates of N 2 and C fixation.Trichodesmium spp. are nonheterocystous cyanobacteria commonly found in tropical and subtropical oligotrophic oceans where they can contribute substantially to new production through dinitrogen (N 2 ) fixation. Because these waters are generally nitrogen (N) deplete, Trichodesmium could be an important source of fixed N to ecosystems in which they occur. A number of studies have tried to quantify N 2 fixation by Trichodesmium in subtropical and tropical seas (e.g., see Capone et al. 1997;Karl et al. 1997;Orcutt et al. 2001); however, reported rates of N 2 fixation, carbon (C) fixation, and growth vary by orders of magnitude within and among studies (Mulholland and Capone 2000;Orcutt et al. 2001). For example, doubling times ranging from Ͻ1 to 243 d have been reported (e.g., Mulholland and Capone 2000). In general, C-based doubling times are faster than doubling times estimated from N 2 fixation. Similarly, the ratio of C : N 2 fixation ranges from 1.2 to 703 in natural populations, with the bulk of the estimates far in excess of canonical Redfield numbers (ϳ6.6; Mague et al. 1977;McCarthy and Carpenter 1979;Orcutt et al. 2001), suggesting an imbalance in C and N metabolism or the use of unquantified N sources in nature.The availability of combined N and other essential nonnitrogenous nutrients (e.g., phosphorus [P] and iron [Fe]), temperature, and the quality and quantity of light can all affect Trichodesmium growth rates a...

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

“…P limitation has been shown to play an important role in biomass growth and functioning of peatlands (Larmola et al, 2013;Hill et al, 2014;Fritz et al, 2012) and appeared to control N 2 fixation rates (Toberman et al, 2015;Vitousek et al, 2002;Chapin et al, 1991). Besides, isolated cyanobacteria were shown to be directly stimulated by P (Mulholland and Bernhardt, 2005) and, in Azolla spp., a fern species with symbiotic cyanobacteria within its leaves, P was shown to drastically increase plant growth and N content (Cheng et al, 2010). In peat mosses from N-rich sites, increased P availability can be expected to complement the high N supply and lead to an increase in photosynthesis (by 14 %) (Fritz et al, 2012) and moss growth (by 42 %) (Carfrae et al, 2007).…”

The fate of nitrogen fixed by diazotrophs in the ocean

“…In numerous paired comparisons between acetylene reduction and 15 N 2 uptake, ratios of acetylene (C 2 H 2 ) reduced to N 2 taken up have varied by at least an order of magnitude (Table 2). Because C 2 H 2 reduction measures just the reduction step, it is a measure of gross N 2 fixation while movement of 15 N 2 from the dissolved to the particulate pool measures net N assimilation (see Gallon et al, 2002;Mulholland et al, 2004aMulholland et al, , 2006Mulholland and Bernhardt, 2005). Release of recently fixed N 2 and the difficulty in chemically recovering all possible dissolved pools into which products of N 2 fixation might be released, may make intercalibration between the two methods impossible.…”

“…It is thought that rates of N 2 fixation and growth by Trichodesmium are limited by phosphorus (P), iron (Fe), or light (Sañudo-Wilhelmy et al, 2001;Mills et al, 2004;Fu et al, 2005;Mulholland and Bernhardt, 2005). However, the range of responses to these variables and their interactions is unknown (Mulholland and Bernhardt, 2005).…”

“…Release of recently fixed N 2 and the difficulty in chemically recovering all possible dissolved pools into which products of N 2 fixation might be released, may make intercalibration between the two methods impossible. However, the difference between N 2 reduction (gross N 2 fixation) and net N 2 assimilation may prove to be an excellent index of the release of recently fixed N 2 (Gallon et al, 2002;Mulholland et al, 2004aMulholland et al, , 2006Mulholland and Bernhardt, 2005). If this is the case, and the theoretical ratio, three, is assumed to be correct (although this itself is a subject of debate; see Mulholland et al, 2006, for a discussion of this assumption) in estimating N 2 fixation from C 2 H 2 reduction, paired comparisons in which C 2 H 2 :N 2 reduction ratios of approximately 3:1 are observed, would indicate no N release while C 2 H 2 :N 2 reduction ratios of six would translate into a release rate of about 50%, and so on.…”

“…Based on laboratory studies, rates of N 2 fixation vary with environmental factors and according to physiological state (e.g., Mulholland and Bernhardt, 2005;Breitbarth et al, 2006;Hutchins et al, 2007) and yet the physiological state of natural populations is impossible to assess at the time of sampling. It is thought that rates of N 2 fixation and growth by Trichodesmium are limited by phosphorus (P), iron (Fe), or light (Sañudo-Wilhelmy et al, 2001;Mills et al, 2004;Fu et al, 2005;Mulholland and Bernhardt, 2005). However, the range of responses to these variables and their interactions is unknown (Mulholland and Bernhardt, 2005).…”