Multielement stoichiometry in aquatic invertebrates: when growth dilution matters

Element concentrations in organisms can be variable, often causing deviations from otherwise consistent, taxon-specific multielement stoichiometries. Such variation can have considerable ecological consequences, yet physiological mechanisms remain unclear. We tested the influence of somatic growth dilution (SGD) on multiple element concentrations under different bioenergetic conditions. SGD occurs when rapid individual growth causes a disproportional gain in biomass relative to gain of a specific element. SGD can strongly affect elements in various organisms, but we lack a general framework to unify results across studies and assess its overall importance. We derived the general conditions that trigger SGD from an element accumulation model. We parameterized the model with bioenergetic and element-specific rates summarized from the literature to compare SGD effects on 15 elements (nonessential metals, essential trace elements, macronutrients) in three aquatic invertebrate taxa. For all taxa, we found that SGD: (1) occurs to some degree for all 15 elements over realistic ranges of growth and ingestion rates, and (2) has the greatest effect on elements with low efflux (excretion) rates, including certain nonessential metals (e.g., MeHg, Po), essential trace elements and macronutrients (e.g., N, Fe). Thus, SGD can strongly affect concentrations of a spectrum of elements under natural conditions. These results provide a framework for predicting variation in elemental composition of animals.

生物体内的元素浓度存在波动，此类波动往往会偏离原本稳定一致的类群特异性多元素化学计量比。此类变异会产生显著的生态效应，但其背后的生理机制仍不明晰。我们在不同生物能学条件下，探究了体细胞生长稀释（somatic growth dilution, SGD）对多种元素浓度的影响。当个体快速生长时，相较于特定元素的积累量，生物量会出现不成比例的增长，此时便会发生体细胞生长稀释。体细胞生长稀释可对多种生物体内的元素产生显著影响，但目前学界尚未能构建统一的研究框架，以整合不同研究的结果并评估其整体重要性。我们通过元素积累模型推导了触发体细胞生长稀释的通用条件。我们结合文献中总结的生物能学参数与元素特异性速率对该模型进行参数化，以此对比体细胞生长稀释对三类水生无脊椎动物类群内15种元素的影响，涉及元素包括非必需金属、必需微量元素与大量营养素。针对所有受试类群，我们发现体细胞生长稀释呈现如下特征：(1) 在符合实际的生长与摄食速率范围内，15种元素均会出现一定程度的体细胞生长稀释效应；(2) 对低外排（排泄）速率的元素影响最为显著，这类元素包括部分非必需金属（如MeHg、Po）、必需微量元素以及大量营养素（如N、Fe）。由此可见，在自然条件下，体细胞生长稀释可显著影响一系列元素的浓度。本研究结果可为预测动物元素组成的变异提供统一的理论框架。