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

Abstract

This study has examined the effect of low seawater pH values (induced by an increased CO₂ partial pressure) on the rates of photosynthesis, as well as on the carbon budget and carbon translocation in the scleractinian coral species <i>Stylophora pistillata</i>, using a new model based on ¹³C labelling of the photosynthetic products. Symbiont photosynthesis contributes to a large part of the carbon acquisition in tropical coral species, and it is thus important to know how environmental changes affect this carbon acquisition and allocation. For this purpose, nubbins of <i>S. pistillata</i> were maintained for six months at two pH_<i>T</i>s (8.1 and 7.2, by bubbling seawater with CO₂). The lowest pH value was used to tackle how seawater pH impacts the carbon budget of a scleractinian coral. Rates of photosynthesis and respiration of the symbiotic association and of isolated symbionts were assessed at each pH. The fate of ¹³C photosynthates was then followed in the symbionts and the coral host for 48 h. Nubbins maintained at pH_<i>T</i> 7.2 presented a lower areal symbiont concentration, and lower areal rates of gross photosynthesis and carbon incorporation compared to nubbins maintained at pH_<i>T</i> 8.1. The total carbon acquisition was thus lower under low pH. However, the total percentage of carbon translocated to the host as well as the amount of carbon translocated per symbiont cell were significantly higher under pH_<i>T</i> 7.2 than under pH_<i>T</i> 8.1 (70% at pH_<i>T</i> 7.2 vs. 60% at pH_<i>T</i> 8.1), such that the total amount of photosynthetic carbon received by the coral host was equivalent under both pHs (5.5 to 6.1 μg C cm⁻² h⁻¹). Although the carbon budget of the host was unchanged, symbionts acquired less carbon for their own needs (0.6 compared to 1.8 μg C cm⁻² h⁻¹), explaining the overall decrease in symbiont concentration at low pH. In the long term, such decrease in symbiont concentration might severely affect the carbon budget of the symbiotic association

Species‐specific photosynthetic responses of symbiotic zoanthids to thermal stress and ocean acidification

“…Finally, clade A symbionts in the coral Stylophora pistillata had lower gross photosynthesis but higher translocation at pH 7.2 compared with 8.1, resulting in similar amounts of fixed carbon received by the host (Tremblay et al . ).…”

“…; Tremblay et al . ) in the form of glycerol, glucose, amino acids and other organic acids (Muscatine ; Trench ; Gates et al . ; Whitehead & Douglas ), and providing as much as 100% or more of the host's daily carbon requirement for respiration (Muscatine et al .…”

Nitrogen cycling in corals: the key to understanding holobiont functioning?

“…It is thus important to test the established understanding of the distribution of genes related to nitrogen cycling between the coral host and its symbiotic partners through incorporation of data from multiple species and under different conditions. Furthermore, the emerging use of stable isotope analysis and isotopic labeling provides an excellent tool to identify the fate of metabolic products in corals [47,48,91]. In combination with NanoSIMS technology [27,29,53,92], such approaches will enable us to understand how nitrogen uptake and nitrogen fixation affect the various symbiotic partners within the coral holobiont.…”

Heterotrophy promotes the re-establishment of photosynthate translocation in a symbiotic coral after heat stress

“…However, evidence in the literature indicates that bleaching can either stimulate the loss of organic matter394142 or its uptake35. Finally, corals can increase translocation of photosynthates by the remaining symbionts to sustain the host metabolism43 or translocate carbon to their symbionts to enhance their growth and photosynthesis, which in turn will increase carbon translocation38. This final mechanism of carbon exchange between the symbiotic partners is poorly understood.…”