Carbon fixation rates

P vs I Relationships. In our investigation we em-ployed a technique widely used by plant physiologists, namely, the study of rate of photosynthesis (P) as a function of irradiance (I). The results of one experiment in which phytoplankton were kept under the four experimental treatment conditions for 24 hours arc shown in Fig. 4. It is clear from this figure that phytoplankton exposed to visible light alone (i.e., when UV was excluded) had the highest photosynthctic rates, and these rates were maintained over a higher range of irradiance levels. On the other hand, the lowest carbon fixation rates occurred when the phytoplankton were exposed to ambient and enhanced levels of UV. 

Vgp Carbon fixation rate per dry cell weight based on sucrose... 

Given all of this, the addition of iron to HNLC waters should result in higher levels of carbon fixation, increased growth of diatoms, local drawdown of CO2, and enhanced export of carbon to the deep sea. Iron-addition experiments in bottles and in situ on the mesoscale unequivocally support the first two points. Addition of iron to HNLC waters results in madly blooming phytoplankton (Martin and Fitzwater, 1989 Martin et al, 1994 Coale et al, 1996 Boyd et al, 2000 Strass, 2002). Chlorophyll concentrations may quadruple (e.g., Strass, 2002) and carbon fixation rates may triple (e.g., Coale et al, 1996) over the first few days after iron addition (Figure 12). 

It is not clear that the acquisition of carbon is the rate-limiting step for photosynthesis even in the larger phytoplankton cells. Phytoplankton are not limited to the passive diffusion of CO2 to the cell surface for carbon acquisition. HCO may be taken up directly by phytoplankton and used as a source of CO2 for photosynthesis (Korb et al, 1997 Nimer et al, 1997 Raven, 1997 Tortell et al, 1997). The considerably greater abundance of HCO than dissolved CO2 in seawater would imply that carbon-fixation rates of marine phytoplankton are not CO2 limited, and increasing concentrations of CO2 will not have an impact on rates of primary production. 

O. Schofield, B.M.A. Kroon, B.B. Prezelin (1995). Impact of ultraviolet-B radiation on photosystem II activity and its relationship to the inhibition of carbon fixation rates for Antarctic ice algae communities. J. Phycol, 31, 703-715. 

Applying the flux equation presented above, a nitrate flux of 0.56 0.16 mol a is computed. Using the average biological C N ratio of 6.6, this leads to a carbon fixation rate of 3.7 + 1.0molm a . The estimate thus obtained is a local, annual-scale measure of new production. 

In addition, cyanobacteria possess a second carbon fixation mechanism through phosphoenolpyruvate carboxylase (PPC) that is responsible for close to 25% of CO2 fixation [102]. PPC fixes HCOg" rather than CO2 and combines it with phosphoenolpyruvate to form oxaloacetate and inorganic phosphate. An in silico modeling study has proposed to couple PPC with the core C4 plant carbon fixation cycle to Increase the overall carbon fixation rate compare to the CBB cycle [103]. 

Depsite the apparently normal structure of the corolla chloroplasts they uere able to carry out only 2S% of the rate of electron transport as compared to leaf-chloroplast (1). Their light carbon fixation rate uas only 2 % per unit chlorophyll than that of the leaf (table 1). An attempt uas made to elucidate the factors determining this lou ligiit fixation efficiency. 

Triose phosphates produced by the Calvin cycle in bright sunlight, as we have noted, may be stored temporarily in the chloroplast as starch, or converted to sucrose and exported to nonphotosynthetic parts of the plant, or both. The balance between the two processes is tightly regulated, and both must be coordinated with the rate of carbon fixation. Five-sixths of the triose... 

It is assumed that the reaction catalyzed by RubisCO is a rate-limiting step in reactions of photosynthetic carbon fixation expressed as follows in the lump ... 

Welsh, D. T., Bourgues, S., deWit, R., and Auby, I. (1997). Effect of plant photosynthesis, carbon sources and ammonium availability on nitrogen fixation rates in the rhizosphere of Zostera noltii. Aquatic Mkrob. Ecol. 12, 285-290. 

These earlier and more recent field measures all demonstrate the importance of these DDAs on the local conditions, however, in most experiments the collection of samples used towed nets, and thus are rather disruptive. Two experiments by Mague etal. (1974, 1977) found that preparing samples by concentration caused a significant (17—29%) reduction in acetylene reduction activity. It seems that more attention or creative sampling schemes need to be developed to accurately measure the N2 (and likely carbon) fixation by these DDAs. Studies similar to those presented by Zehr et al. (2007) and Needoba et al. (2007), which combine uptake rates with quantitative PCR approaches for the target diazotrophs are a plausible alternative since assays are run on bulk water. 

MulhoUand, M. R., and Bernhardt, P. W. (2005). The effect of growth rate, phosphorus concentration, and temperature on N2 fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS 101. Limnol. Oceanogr. 50(3), 839—849. 

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