Carbon exchange and 14C tracer methods in a nitrogen-limited diatom, Thalassiosira pseudonana

Abstract

A simple carbon-exchange model accounted for the kinetics of I4C uptake and release by the diatom 73alassiosira pseudonana in nitrogen-limited chernostat culture. The model treats the cells as consisting of 2 pools of carbon; an exchanging pool which carries out photosynthesis, respiration and excretion, and a synthetic pool which does not exchange, but accumulates carbon from the exchanging pool. The model fitted well to observed I4C kinetics over a 10-fold range of growth rates and was demonstrably superior to 2 alternate models which have been prevalent in the theory and application of 14C methodology in primary production studies. The exchanging pool was small (4 to 15 % of cell carbon) and rapidly cycled (90 % turnover time of 1 to 12 h) in all steady-state cultures, but was larger (21 %) and more slowly cycled (15 h) in a chemostat deprived of its limiting nitrogen supply for 24 h. In all cultures, the observed kinetics indicated that usual I4C estimates of phytoplankton production should be close to net production rates, but that short-term 14C estimates of excretion should be too low for slow-growing populations.

Net and gross productivity: weighing in with 14C

“…Alternatively, C can also be sequestered via synthetic pathways (Smith & Platt 1984). Both refixation and sequestration via synthetic pathways can explain the data, although there is extensive evidence for refixation of respired CO 2 .…”

“…That C assim- O gross production values are from K. Grande & M. Bender (unpubl. data) ilation measured using the 14 C method is linear over time has been repeatedly found, both in natural populations and in the laboratory (Li & Goldman 1981, Smith & Platt 1984, Marra et al 1988. Dring & Jewson (1982), using both tracer kinetic theory and observations, analyzed C assimilation time courses.…”

Growth‐compensating phenomena in continuous cultures of Dunaliella tertiolecta limited simultaneously by light and nitrate

“…To estimate the photosynthetic rate P, DIC data and one isotopic discrimination coefficient (6%) were used (Steemann Nielsen 1952). The gross or net nature of P has not yet been clearly established (Jespersen et al 1992;Peterson 1980;Smith and Platt 1984). We have assumed that the sum of 14C retained by the cells during a relatively short incubation time (DIC assimilation) and 14C present in the medium [dissolved organic carbon (DOC) excretion] was more representative of gross production.…”

Dissolved and particulate primary production and bacterial production in offshore Antarctic waters during austral summer: coupled or uncoupled?

“…To avoid these potential artifacts, time-course (kinetic) experiments of 14 C incorporation into the dissolved and particulate fractions were used (Smith 1982, Smith & Platt 1984. Partitioning of phytoplanktonic organic carbon production into dissolved and particulate compartments is obviously needed in order to determine whether the Antarctic microbial loop functions differently from that in other pelagic ecosystems, as suggested by Bird & Karl (1999).…”