Anthony KR, Fabricius KE
Department of Marine Biology, James Cook University, Qld 4811, Townsville, Australia
J Exp Mar Biol Ecol. 2000 Sep 20;252(2):221-253. doi: 10.1016/s0022-0981(00)00237-9.
Suspended particulate matter (SPM) strongly alters the trophic environment of photosymbiotic aquatic organisms. At high particles loads, phototrophic energy gains can be diminished due to light absorption by suspended particles, and stress from particle abrasion or deposition on tissues. However, energy gains are enhanced if organisms are able to use SPM as a food source. For photosymbiotic benthic suspension feeders, increases in SPM concentrations may require both phototrophic and heterotrophic acclimation to sustain a positive energy balance. This study provides an experimental analysis of the effects of contrasting light and SPM regimes on the energy budget (scope for growth) of two zooxanthellate corals (Goniastrea retiformis and Porites cylindrica). Using a factorial design in a flow-through tank system, corals were exposed for 2 months to shaded and unshaded conditions (equivalent to 3-4 m depth at 4 and 16 mg dry weight SPM l(-1), respectively) and a range of controlled SPM loads with a natural organic content ( approximately 3% w/w). In G. retiformis, rates of particle ingestion were a linear function of SPM concentration within a broad range (1-30 mg dry weight l(-1)). After 2 months of shading, photosynthetic acclimation was significant in G. retiformis, but did not compensate for the reduced light level, as daily respiration exceeded daily photosynthesis. However, in response to the prolonged shading, G. retiformis more than doubled its rate of particle feeding. At high SPM treatments (16 mg dw l(-1)), sediment feeding by this species compensated fully for the 35-47% lower phototrophy in the shaded treatment. Due to both photo- and heterotrophic plasticity, G. retiformis gained tissue and skeletal mass at all experimental levels of light and SPM. In contrast, rates of particle intake by P. cylindrica contributed <10% to the energy budget in shaded and <3% in unshaded conditions. Feeding rates of P. cylindrica were half-saturated at approximately 3 mg dry weight l(-1), and four- to eight-fold lower than those of G. retiformis. Skeletal growth was sustained, but tissue mass and lipid contents declined in shaded and high-SPM treatments, and carbon loss due to shading by SPM was not compensated for by particle feeding. Thus, due to a lack of photo- and heterotrophic plasticity, periods of high turbidity resulted in energy deficiency in P. cylindrica, and high turbidity conditions appeared physiologically unsustainable for this species. This study is the first to show heterotrophic plasticity in a symbiotic coral, and to show that such plasticity can offset stress from high particle loads. It demonstrates that changes in the trophic mode of some coral species are a mechanism for sustaining a positive energy balance in turbid environments, thereby broadening their physiological niche.
悬浮颗粒物（SPM）会强烈改变光合共生水生生物的营养环境。在高颗粒负荷下，由于悬浮颗粒对光的吸收以及颗粒磨损或沉积在组织上造成的压力，光养能量获取可能会减少。然而，如果生物体能够将SPM用作食物来源，能量获取则会增加。对于光合共生底栖悬浮摄食者而言，SPM浓度的增加可能需要光养和异养适应来维持正能量平衡。本研究对两种虫黄藻珊瑚（网纹棘杯珊瑚和柱形鹿角珊瑚）在不同光照和SPM条件下对能量收支（生长范围）的影响进行了实验分析。在流通式水槽系统中采用析因设计，将珊瑚暴露于遮荫和非遮荫条件下2个月（分别相当于4米和16毫克干重SPM /升时3 - 4米的深度），以及一系列具有天然有机含量（约3% w/w）的可控SPM负荷条件下。在网纹棘杯珊瑚中，在较宽范围（1 - 30毫克干重/升）内，颗粒摄取率是SPM浓度的线性函数。遮荫2个月后，网纹棘杯珊瑚的光合适应显著，但无法弥补光照水平的降低，因为每日呼吸超过每日光合作用。然而，由于长期遮荫，网纹棘杯珊瑚的颗粒摄食率增加了一倍多。在高SPM处理（16毫克干重/升）下，该物种的沉积物摄食完全弥补了遮荫处理中光养作用降低35 - 47%的损失。由于光养和异养可塑性，在所有光照和SPM实验水平下，网纹棘杯珊瑚都增加了组织和骨骼质量。相比之下，柱形鹿角珊瑚的颗粒摄取率在遮荫条件下对能量收支的贡献小于10%，在非遮荫条件下小于3%。柱形鹿角珊瑚的摄食率在约合3毫克干重/升时达到半饱和，比网纹棘杯珊瑚低4至8倍。骨骼生长得以维持，但在遮荫和高SPM处理下，组织质量和脂质含量下降，且SPM遮荫造成的碳损失无法通过颗粒摄食得到弥补。因此，由于缺乏光养和异养可塑性，高浊度时期导致柱形鹿角珊瑚能量不足，高浊度条件对该物种而言在生理上似乎无法维持。本研究首次表明共生珊瑚具有异养可塑性，并表明这种可塑性可以抵消高颗粒负荷带来的压力。它证明某些珊瑚物种营养模式的变化是在浑浊环境中维持正能量平衡的一种机制，从而拓宽了它们的生理生态位。
