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A recent numerical cell model, which explains observed light and carbonate system effects on particulate organic and inorganic carbon (POC and PIC) production rates under the assumption of internal pH homeostasis, is extended for stable carbon isotopes ( 12 C, 13 C). Aim of the present study is to mechanistically understand the stable carbon isotopic fractionation signal ( ε ) in POC and PIC and furthermore the vital effect(s) included in measured ε PIC values. The virtual cell is divided into four compartments, for each of which the 12 C as well as the 13 C carbonate system kinetics are implemented. The compartments are connected to each other via trans-membrane fluxes. In contrast to existing carbon fractionation models, the presented model calculates the disequilibrium state for both carbonate systems and for each compartment. It furthermore calculates POC and PIC production rates as well as ε POC and ε PIC as a function of given light conditions and the compositions of the external carbonate system. Measured POC and PIC production rates as well as ε PIC values are reproduced well by the model (comparison with literature data). The observed light effect on ε POC (increase of ε POC with increasing light intensities), however, is not reproduced by the basic model set-up, which is solely based on RubisCO fractionation. When extending the latter set-up by assuming that biological fractionation includes further carbon fractionation steps besides the one of RubisCO, the observed light effect on ε POC is also reproduced. By means of the extended model version, four different vital effects that superimpose each other in a real cell can be detected. Finally, we discuss potential limitations of the ε PIC proxy. [ABSTRACT FROM AUTHOR]

Abstract: It has been known since the 19th century that coral skeletons are built of aragonite crystals with taxonomy-linked arrangements, but the way by which each coral species controls this crystallization process remains an unsolved question. The problem became still more intriguing when it was shown that isotopic compositions of coral aragonite were subject to taxonomy-linked influences (the “vital effect”). On the other hand, presence of an organic component in coral skeletons is also long known, but localization of these compounds is admittedly restricted to particular structures called “centres of calcifications.” Fibres, the largely predominant part of the coral skeletons, are usually considered as purely mineral units. In this paper, it is shown that in both “centres of calcification” and fibres, organic compounds are associated with the mineral material at a deep structural level. A series of variously scaled observations and localized measurements allow recognition of the presence of an organic component at the nanometre scale. Far from being a freely operating process, crystallization of coral fibres is thus permanently controlled by the polyp basal ectoderm through a cyclic two-step process acting at the micrometre-scale. The biomineralization cycle begins by secretion of a proteoglycan matrix. As the composition of these sugars–proteins assemblages has been shown taxonomy dependent, the hypothesis can be made that multiple and long recognized specificities of coral skeletons are linked to this biochemically driven crystallization process. Additionally, this new concept of the biomineralization process in coral skeletons provides us with an access to the long term evolution of the Scleractinia. Remarkably, results of a skeleton-based approach using microstructural criteria (i.e., the spatial relationships of “centres of calcification” and the three-dimensional arrangements of fibres), are consistent with a molecular phylogenetic analysis carried out on the same species. Clearly, at the overall ontogenic level, the two-step growth mode of coral skeletons is also a valuable tool to reconstruct the evolutionary history of Scleractinia. [Copyright &y& Elsevier]