Carbon isotope fractionation by a marine diatom:dependence on the growth-rate-limiting resource

Abstract

The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (E,,) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variab&ty requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potent~ally influence %, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate ( p ) and CO2 concentration ([CO2aqj) in controlling E,,. Experimental examination of the relationship of E, with p and [C02aq] in studies using different experimental approaches, however, has ylelded inconsistent results. Here we directly compare new and previously published data on c, as a function of CO2 concentration and growth rate for the marine diatom Phaeodactylum tncornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), E,, of P. tn'cornutum decreases with increasing growth rate. In contrast, under N-replete conditions E, values are considerably lower at comparable growth rates and CO2 concentrations and are largely insensitive to a 3-fold increase in growth rate due to increasing photon flux density. In both experimental approaches, E, shows a relatively small CO2 sensitivity in the range of CO2 concentrations naturally occurring in the ocean (8 to 25 p 0 1 kg-'). Below ca 5 pm01 CO2 kg-', a strong decline in E,, with decreasing [C02aq] is observed. The apparent difference in E, responses between nitrate-limited and light-controlled cultures of P. tricornutum suggests a principal difference in carbon acquisition for different growth-rate-limiting resources. A mechanistic explanation is proposed and potential implications for the interpretation of phytoplankton carbon isotope fractionation are discussed.

Effect of phosphorus limitation on elemental composition and stable carbon isotope fractionation in a marine diatom growing under different CO2 concentrations

“…In addition, p may also be strongly influenced by the factor limiting phytoplankton growth rate (e.g., nutrients vs. light; Riebesell et al 2000). All these results were achieved by steady state laboratory experiments in chemostats or dilute batch cultures with low algal biomass.…”

Isotope‐based Fluvial Organic Carbon (ISOFLOC) Model: Model formulation, sensitivity, and evaluation

“…While the relationship between partial pressure of CO 2 and e Fix is still not clearly defined [Bade et al, 2006], ISOFLOC assumes an exponential decay for e Fix as a function of the inverse of C DIC since findings of Riebesell et al [2000] suggest low sensitivity of e Fix at moderate-high C DIC and a steep decline for decreasing DIC at low C DIC , reminiscent of an exponential decay relationship. ISO-FLOC simulates d 13 C FPOC-Bed as …”

“…where e Fix varies temporally and spatially since previous studies have shown that enrichment factors at low concentrations of aqueous CO 2 are significantly lower than at high aqueous CO 2 concentrations [e.g., Riebesell et al, 2000]. While the relationship between partial pressure of CO 2 and e Fix is still not clearly defined [Bade et al, 2006], ISOFLOC assumes an exponential decay for e Fix as a function of the inverse of C DIC since findings of Riebesell et al [2000] suggest low sensitivity of e Fix at moderate-high C DIC and a steep decline for decreasing DIC at low C DIC , reminiscent of an exponential decay relationship.…”

Stable isotopes from diatom silica

“…A number of other factors including cell shape and size, metabolic pathways, water temperature, and other inter-species vital effects may also lead to spatial and temporal changes in δ 13 C diatom (Laws et al, 1995;Popp et al, 1998;Burkhardt et al, 1999Burkhardt et al, , 2001Riebesell et al, 2000; Jacot des Combes et al 2008). However, provided the influence of these factors on δ 13 C diatom can be accounted for, changes in δ 13 C diatom can be employed to reconstruct changes in paleoproductivity and CO 2(aq) , and so changes in pCO 2 .…”