Food selectivity and processing by the cold-water coral Lophelia pertusa

Cold-water corals form prominent reef ecosystems along ocean margins that depend on suspended resources produced in surface waters. In this study, we investigated food processing of 13C and 15N labelled bacteria and algae by the cold-water coral Lophelia pertusa. Coral respiration, tissue incorporation of C and N and metabolic-derived C incorporation into the skeleton were traced following the additions of different food concentrations (100, 300, 1300 µg C/l) and two ratios of suspended bacterial and algal biomass (1:1, 3:1). Respiration and tissue incorporation by L. pertusa increased markedly following exposure to higher food concentrations. The net growth efficiency of L. pertusa was low (0.08±0.03), which is consistent with their slow growth rates. The contribution of algae and bacteria to total coral assimilation was proportional to the food mixture in the two lowest food concentrations, but algae were preferred over bacteria as food source at the highest food concentration. Similarly, the stoichiometric uptake of C and N was coupled in the low and medium food treatment, but was uncoupled in the high food treatment and indicated a comparatively higher uptake or retention of bacterial carbon as compared to algal nitrogen. We argue that behavioural responses for these small-sized food particles, such as tentacle behaviour, mucus trapping and physiological processing, are more likely to explain the observed food selectivity as compared to physical-mechanical considerations. A comparison of the experimental food conditions to natural organic carbon concentrations above CWC reefs suggests that L. pertusa is well adapted to exploit temporal pulses of high organic matter concentrations in the bottom water caused by internal waves and down-welling events.冷水珊瑚（cold-water corals）沿大洋边缘形成规模显著的礁体生态系统，其生存依赖于表层水域产生的悬浮有机质。本研究针对冷水珊瑚（cold-water corals）苍白石珊瑚（Lophelia pertusa）展开实验，探究其对经碳13（13C）、氮15（15N）标记的细菌与藻类的食物代谢过程。实验通过设置不同食物浓度（100、300、1300 μg C/L）以及两组悬浮细菌与藻类生物量配比（1:1、3:1），追踪了珊瑚的呼吸作用、碳氮在组织中的整合过程，以及代谢来源碳向骨骼的整合过程。结果显示，苍白石珊瑚的呼吸作用与组织整合效率随食物浓度升高显著提升。其净生长效率较低（0.08±0.03），这与其缓慢的生长速率相符。在两个较低食物浓度组中，藻类与细菌对珊瑚总同化作用的贡献与食物混合配比呈正相关；但在最高食物浓度组中，珊瑚更偏好藻类而非细菌作为食物来源。类似地，在低、中食物浓度处理组中，碳与氮的化学计量吸收呈耦合状态；而在高浓度处理组中二者解耦，且结果显示相较于藻类氮的吸收/留存，珊瑚对细菌碳的吸收或留存相对更高。相较于物理机械因素，我们认为针对这类小型食物颗粒的行为响应——包括触手行为、黏液捕获与生理处理过程——更能解释观测到的食物选择性。将实验食物条件与冷水珊瑚（cold-water corals，缩写CWC）礁体上方的天然有机碳浓度进行对比后发现，苍白石珊瑚已很好地适应了利用由内波与下降流事件引发的底层水中高有机质浓度的临时性脉冲。