Autotrophic carbon budget in coral tissue: a new 13C-based model of photosynthate translocation

Abstract

SUMMARYCorals live in symbiosis with dinoflagellates of the genus Symbiodinum. These dinoflagellates translocate a large part of the photosynthetically fixed carbon to the host, which in turn uses it for its own needs. Assessing the carbon budget in coral tissue is a central question in reef studies that still vexes ecophysiologists. The amount of carbon fixed by the symbiotic association can be determined by measuring the rate of photosynthesis, but the amount of carbon translocated by the symbionts to the host and the fate of this carbon are more difficult to assess. In the present study, we propose a novel approach to calculate the budget of autotrophic carbon in the tissue of scleractinian corals, based on a new model and measurements made with the stable isotope 13 C. Colonies of the scleractinian coral Stylophora pistillata were incubated in H 13 CO 3 --enriched seawater, after which the fate of 13 C was followed in the symbionts, the coral tissue and the released particulate organic carbon (i.e. mucus). Results obtained showed that after 15min, ca. 60% of the carbon fixed was already translocated to the host, and after 48h, this value reached 78%. However, ca. 48% of the photosynthetically fixed carbon was respired by the symbiotic association, and 28% was released as dissolved organic carbon. This is different from other coral species, where <1% of the total organic carbon released is from newly fixed carbon. Only 23% of the initially fixed carbon was retained in the symbionts and coral tissue after 48h. Results show that our 13 C-based model could successfully trace the carbon flow from the symbionts to the host, and the photosynthetically acquired carbon lost from the symbiotic association.

Breakdown of the coral-algae symbiosis: towards formalising a linkage between warm-water bleaching thresholds and the growth rate of the intracellular zooxanthellae

“…New and emerging techniques offer great hope for improved experimentation on the intact symbiosis (see e.g. Tremblay et al, 2012). A resignation to experiment on the host or zooxanthellae in isolation will never deliver the necessary energy-budget relations needed to support (or falsify) the integrated coral-algae bleaching model outlined here (Figs.…”

Vortical ciliary flows actively enhance mass transport in reef corals

“…For this holobiont to thrive, the coral animal must support the metabolic requirements of its symbionts by supplying nutrients and eliminating toxic byproducts, such as excess oxygen accumulated as a byproduct of the symbionts' photosynthetic activity (2)(3)(4). The algal symbionts, in return, provide the coral with organic carbon (2,5), and their activity underpins the calcification and skeletal growth that is at the basis of the coral reef ecosystem (6,7). These processes and other key metabolic processes involve the continuous exchange of nutrients, inorganic carbon and dissolved oxygen between the coral and the surrounding seawater.…”

Photosynthate translocation increases in response to low seawater pH in a coral–dinoflagellate symbiosis

“…For this purpose, a new model of carbon translocation (Tremblay et al, 2012b), based on the isotopically labelled 13 C bicarbonate, was used. Although the low pH value chosen in this study is lower than that expected under future climate scenarios, it is useful to examine the physiological response of corals to environmental hypercapnia (Barry et al, 2010).…”

“…The experiments were performed according to Tremblay et al (2012b). Briefly, corals were placed in H 13 CO − 3 (NaH 13 CO 3 98 atom % 13 C, #372382, Sigma-Aldrich, StLouis, MO, USA) enriched seawater and then transferred into non-enriched seawater for various chase periods.…”

“…Three nubbins (one per colony) were removed after 0, 2, 4, 24 and 48 h and immediately frozen at −20 • C. Six control nubbins per condition (two per colony; incubated from the beginning in 200 mL non-enriched seawater) were run in parallel and two were sampled after 0, 24 and 48 h and immediately frozen at −20 • C. The pH N BS was controlled after addition of the 13 C bicarbonate and during the whole experiment to ensure that it remained at the desired level, especially for the low pH condition. All nubbins were treated according to Tremblay et al (2012b). Briefly, tissue was detached from the skeleton using an air brush in 0.2 µm-filtered seawater.…”