A dual‐labeling method for the simultaneous measurement of dissolved inorganic carbon and phosphate uptake by marine planktonic species

Abstract

The measurement of primary production is the foundation for aquatic biogeochemistry research. The biogeochemical cycles of phosphate (P) and other biolimiting elements are tightly linked to marine primary production. We have optimized an existing method of carbon (C) and P dual isotope labeling to study the simultaneous C and P uptake by plankton species in marine environments. The two main objectives of this study were (1) to test the preservation properties of the labeled samples and the calculation methods used in separating the signals of the different radionuclides, and (2) to adapt the method to marine environments. The procedure was successfully implemented in contrasting locations within the southeast Pacific (between 146.36°W and 72.49°W). The uptake rates determined using this method ranged from 14 to 900 nM h -1 for dissolved inorganic C and from 0.03 to 4.5 nM h -1 for dissolved inorganic P in surface water. The detection limit found in ultraoligotrophic surface water was 3.33 nM h -1 and 0.01 nM h -1 for C and P, respectively. C and P assimilation fluxes in low-and high-productivity open-ocean systems may be studied using this sensitive method. We outline a protocol for marine environments that is appropriate for use under oceanographic cruise conditions. Results from the application of this method will lead to a better understanding of the interplay between carbon and phosphate biogeochemical cycles in the upper ocean.

Growth and specific P-uptake rates of bacterial and phytoplanktonic communities in the Southeast Pacific (BIOSOPE cruise)

“…The methodological problems associated with 24 h incubation experiments can be significant (Nalewajko andGarside, 1983, Harrison andHarris, 1986), especially in terms of losses. However, short incubation experiments should reduce the bias linked to such losses (see discussion in Duhamel et al, 2006). It is important to stress that even if the <0.6 µm fraction is composed of solely heterotrophic bacteria, our data set does not prove that P is turning over at the same rate as the cells.…”

“…P uptake is generally shown to be constant over 24 h (Perry andEppley, 1981, Harrison, 1983;Moutin et al, 2002) but diurnal variations in P uptake have been observed in some studies (Eppley et al, 1971;Harrison et al, 1977;Currie and Kalff, 1984). For the majority of stations, time course experiments for 33 P uptake were linear over 24 h, however there were some variations in P uptake rates at some stations along the BIOSOPE transect (Duhamel et al, 2006). The methodological problems associated with 24 h incubation experiments can be significant (Nalewajko andGarside, 1983, Harrison andHarris, 1986), especially in terms of losses.…”

“…Counting (count per minute -cpm) was carried out on a Packard Tri-Carb® 2100TR scintillation counter. In order to separate the activity due to 33 P from that of 14 C, we applied the method using the different half-lives of the two isotopes (For more details, see Duhamel et al, 2006). A second count was made a year later, samples having been preserved in the dark at room temperature.…”

“…Carbon and phosphate uptakes were determined using the 33 P/ 14 C dual labelling method (Duhamel et al, 2006). Duplicate samples (300 ml) were collected into samplerinsed, polycarbonate bottles (Nalgene) for each sampling depth.…”

“…An additional duplicate sample (300 ml) of surface water was incubated with 300 µl of HgCl 2 (20 g l −1 ) to act as a control for non-biological assimilation (Kirkwood 1992). The samples were inoculated with 1080 kBq carrierfree 33 P (<40 pmol l −1 final concentration -orthophosphate in dilute hydrochloric acid; Amersham BF 1003; half-life 25.383±0.040 days; Duhamel et al, 2006), and 3.7 MBq 14 C (bicarbonate aqueous solution; Amersham CFA3; half-life 5700±30 years; Duhamel et al, 2006). Samples were incubated under simulated conditions for 4 to 5 h. Incubation boxes equipped with light filters (nickel screens) were used to reproduce the light level at the sample depths (50 -25 -15 -7 -3 -1% of transmitted light).…”