Spatial and Temporal Variations in Stable Carbon (δ13C) and Nitrogen (δ15N) Isotopic Composition of Symbiotic Scleractinian Corals

Abstract

Tropical scleractinian corals are considered autotrophic as they rely mainly on photosynthesis-derived nutrients transferred from their photosymbionts. Corals are also able to capture and ingest suspended particulate organic matter, so heterotrophy can be an important supplementary trophic pathway to optimize coral fitness. The aim of this in situ study was to elucidate the trophic status of 10 coral species under contrasted environmental conditions in a French Polynesian lagoon. Carbon (δ13C) and nitrogen (δ15N) isotopic compositions of coral host tissues and photosymbionts were determined at 3 different fringing reefs during wet and dry seasons. Our results highlighted spatial variability in stable isotopic compositions of both coral host tissues and photosymbionts. Samples from the site with higher level of suspended particulate matter were 13C-depleted and 15N-enriched relative to corals and photosymbionts from less turbid sites. However, differences in both δ13C and δ15N between coral host tissues and their photosymbionts (Δhost-photosymbionts 13C and Δhost-photosymbionts 15N) were small (0.27 ± 0.76‰ and 1.40 ± 0.90‰, respectively) and similar at all sites, thus indicating no general increases in the heterotrophic pathway. Depleted δ13C and enriched δ15N values of coral host tissues measured at the most turbid site were explained by changes in isotopic composition of the inorganic nutrients taken up by photosymbionts and also by changes in rate of isotopic fractionation with environmental conditions. Our results also highlighted a lack of significant temporal variations in δ13C and δ15N values of coral host and photosymbiont tissues and in Δhost-photosymbionts 13C and Δhost-photosymbionts 15N values. This temporal stability indicated that corals remained principally autotrophic even during the wet season when photosymbiont densities were lower and the concentrations of phytoplankton were higher. Increased coral heterotrophy with higher food availability thus appears to be species-specific.

Among-species variation in the energy budgets of reef-building corals: scaling from coral polyps to communities

“…However, differentiation in host and symbiont δ 13 C for Acropora and Galaxea, species that had low CHAR values, indicates that δ 15 N may be a more reliable indicator of coral heterotrophic feeding than δ 13 C. Nevertheless, carbon translocation from symbiont to host can vary with particle feeding rates (Hughes et al, 2010;Tremblay et al, 2014) and a decrease in the amount of carbon translocated to the host when feeding rates are low and nutrient supply is limited could drive differentiation in host and symbiont δ 13 C. Finally, we note that the δ 13 C values for symbiont and host observed here are more negative compared with other studies (e.g. −11 to −14‰ in Nahon et al, 2013;or −13 and −16‰ in Swart, 1983). Likely explanations for this observation include the relatively low light levels under which corals were grown in our study: several studies have found decreasing δ 13 C with depth (e.g.…”

“…However, studies using stable isotope techniques have shown that corals in deeper waters consume more plankton than corals in shallow habitats (Muscatine et al, 1989;Alamaru et al, 2009). Similarly, there is evidence of among-reef variation in the carbon and nitrogen isotopic composition of coral tissues (Heikoop et al, 2000), and a recent study has demonstrated that spatial variation in coral tissue composition was associated with variation in turbidity (Nahon et al, 2013). Nevertheless, it remains unclear whether such variation in tissue composition is due to corals in different areas consuming different amounts or types of particulate food, or whether corals in different areas have differential reliance on photosynthesis versus particulate feeding.…”

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery

“…M. capitata and P. compressa δ 15 N values were within the range of δ 15 N‐nitrate values in Kāne‘ohe Bay (4–5‰; Supporting Information Table S2) and higher at HIMB relative to other sites. Similar patterns of higher δ 15 N values in southern Kāne‘ohe Bay were also seen in juvenile brown stingray ( Dasyatis lata ) known to have a fairly constant diet (Dale et al ), indicating spatial variability in the sources and isotopic values of DIN δ 15 N values that permeate the food web of Kāne‘ohe Bay (Heikoop et al ; Nahon et al ). These spatial effects are expected to result from a combination of greater subterranean groundwater discharge in northern Kāne‘ohe Bay (Dulai et al ), high stream input (30% of bay total), and legacy effects of sewage dumping (1951–1978) in southern Kāne‘ohe Bay (Smith et al ).…”

Stable isotopes as tracers of trophic interactions in marine mutualistic symbioses

“…For example, low ∆δ 13 C host-symbiont was recorded in coral species from surface waters of Moorea Island (from −1.44 ± 0.23‰ to 2.98 ± 0.58‰), which are corals that entirely rely on their symbionts for their energetic needs (Nahon et al, 2013). The contribution of the symbionts to the diet of their host is inversely proportional to the ∆δ 13 C host-symbiont , or, in other words, the contribution will decrease from 100% to lower values when ∆δ 13 C host-symbiont increases from zero to higher values.…”