Dark uptake of inorganic¹⁴C in oligotrophic oceanic waters

Dark uptake of inorganic 14 C by offshore plankton was measured at two depths at 36 stations in the Atlantic Ocean from 52°S to 26°N, mainly along 30°W. The samples were incubated for 2 h with and without inhibition of biological activity with HgCl 2 . In addition, six time course experiments were performed. The mean dark uptake rate varied from 0.68 to 4.82 (µmol C m −3 h −1 over the transect and showed a significant positive relationship with chlorophyll a. The dark uptake was usually >5% of the maximum photosynthetic capacity ( Pm ), and higher values relative to Pm were associated with low values of P m and not with high absolute dark values. A linear relationship between dark uptake and P m was found with a background value (y-axis intercept) of 0.51 (µmol C m −3 h −1 and a slope of 0.77% of Pm . A major fraction of the dark signal, 66–80% of the total signal, persisted in bottles treated with HgCl2, indicating that most of the dark signal was independent of biological activity. Time course experiments showed a linear dark uptake with time for the first hours, whereafter the uptake ceased. At stations with low concentrations of inorganic nitrogen [>1 (µmol (NH 4+ +NO 3− )], a second stage was observed after 3–8 h, probably due to an increase in bacterial activity. The results suggest three mechanisms for the dark value in short-term incubations in oligotrophic waters. A background value independent of biomass and incubation time which was the dominant part of the dark signal in samples with very low phytoplankton biomass (>0.3 p-g Chi a 1“). Another important part was residuals of 14 C associated with plankton, probably adsorbed to compounds inside the cells. This fraction was dominant in short-term incubations at chlorophyll concentrations >0.3 p.g Chi a H. Active uptake by living cells (total minus ‘HgCl2 uptake‘) was only a minor part of the dark signal in short-term incubations, but dominated at longer incubation time (>3–9 h), probably driven by an increase in bacterial activity. A significant enhancement of the non-photosynthetic uptake of 14 C was observed in light, probably associated with a carbon-concentrating mechanism in phytoplankton or light stimulation of β-carboxylation activity. The results strongly suggest that dark values should be subtracted from the light uptake. This correction is particularly important when photosynthetic rates are low, e.g. at low light or in short-term incubations where a time-zero background becomes a significant part of the total uptake in light.