Giant clam nitrogen isotopic data and code for N isotope model

Giant clams are reef-dwelling bivalves that reach unusual sizes through a
partnership with photosymbiotic algae. To date, no shell-based
biogeochemical proxy has been found which directly records the
photosymbiotic development and health of these animals. We present new
results showing a size-related decline in nitrogen isotopic values of
shell-bound organic matter from the hinge layers of giant clams from the
Northern Red Sea. In three of four tested shells, δ15N values decline from
> 4‰ at the juvenile stage to between 0 and -2.5‰ at maturity,
consistent with a transition from heterotrophic nutrition early in the
bivalve’s life to receiving most of their nutrition from photosynthetic
symbionts and external dissolved inorganic nitrogen at maturity. We find
less extreme declines or no change within the outer shell layer, with more
inter-individual variability, which is likely related to the greater
influence of the symbionts in the adjacent siphonal mantle of the animals.
We use a von Bertalanffy-linked trophic model that uses δ15N of nitrate
and particulate organic matter to corroborate and explain the trophic
transition in the ontogeny of the clams, and propose that high-resolution
δ15N measurements in bivalves could be used as a proxy for photosymbiosis
and reef paleoenvironmental conditions in the fossil record.

巨蛤（Giant clams）是栖息于珊瑚礁环境的双壳类软体动物（bivalves），通过与光合共生藻类（photosymbiotic algae）形成互利共生关系，得以生长至异常庞大的体型。迄今为止，学界尚未发现可直接记录这类动物光合共生发育与健康状况的基于壳体的生物地球化学代用指标（biogeochemical proxy）。本研究呈现全新实验结果：红海北部巨蛤铰合层（hinge layers）的壳体结合有机质的δ15N值随体型增大呈现显著下降趋势。在4枚测试壳体中的3枚里，δ15N值从幼体阶段（juvenile stage）的大于4‰下降至成熟阶段（maturity）的0‰至-2.5‰区间，这与该双壳类生命早期依赖异养营养（heterotrophic nutrition）、成熟阶段主要通过光合共生藻类及外界溶解无机氮（dissolved inorganic nitrogen）获取营养的营养模式转变相一致。研究团队同时发现，壳体外层（outer shell layer）的δ15N值下降幅度更小甚至无变化，且个体间变异性（inter-individual variability）更强，这大概率与动物相邻虹吸管外套膜（siphonal mantle）中共生藻类的更强影响有关。本研究采用结合冯·贝塔朗菲（von Bertalanffy）生长模型的营养模型（trophic model），以硝酸盐与颗粒有机质（particulate organic matter）的δ15N值为参数，验证并阐释了巨蛤个体发育（ontogeny）过程中的营养模式转变，并提出：双壳类壳体的高分辨率δ15N测量结果，可作为化石记录（fossil record）中光合共生状况与珊瑚礁古环境（paleoenvironmental conditions）的代用指标。