Sponge bioerosion on changing reefs: ocean warming poses physiological constraints to the success of a photosymbiotic excavating sponge Scientific Reports

Excavating sponges are prominent bioeroders on coral reefs that in comparison to other benthic organisms may suffer less or may even benefit from warmer, more acidic and more eutrophic waters. Here, the photosymbiotic excavating sponge Cliona orientalis from the Great Barrier Reef was subjected to a prolonged simulation of both global and local environmental change: future seawater temperature, partial pressure of carbon dioxide (as for 2100 summer conditions under "business-as-usual" emissions), and diet supplementation with particulate organics. The individual and combined effects of the three factors on the bioerosion rates, metabolic oxygen and carbon flux, biomass change and survival of the sponge were monitored over the height of summer. Diet supplementation accelerated bioerosion rates. Acidification alone did not have a strong effect on total bioerosion or survival rates, yet it co-occurred with reduced heterotrophy. Warming above 30 degrees C (+2.7 degrees C above the local maximum monthly mean) caused extensive bleaching, lower bioerosion, and prevailing mortality, overriding the other factors and suggesting a strong metabolic dependence of the sponge on its resident symbionts. The growth, bioerosion capacity and likelihood of survival of C. orientalis and similar photosymbiotic excavating sponges could be substantially reduced rather than increased on end-of-the-century reefs under "business-as-usual" emission profiles.

挖掘海绵是珊瑚礁上的主要生物侵蚀生物，与其他底栖生物相比，它们在水温升高、海水酸化加剧以及富营养化程度提升的环境中，受影响程度可能更低，甚至可能从中获益。本研究针对来自大堡礁的光合共生挖掘海绵——东方穿贝海绵（Cliona orientalis），开展了全球与局地环境变化的长期模拟实验，模拟因子包括未来海水温度、二氧化碳分压（对应"照常排放"情景下2100年夏季的环境条件）以及颗粒有机物补充投喂。研究在盛夏期间监测了这三个因子的单独及联合作用对该海绵的生物侵蚀速率、代谢氧与碳通量、生物量变化以及存活率的影响。颗粒有机物补充投喂可提升生物侵蚀速率。单独的海水酸化对总生物侵蚀速率或存活率未产生显著影响，但酸化会伴随异养作用的减弱。当水温升高至30℃以上（较当地月均最高温高出2.7℃）时，海绵会出现大规模褪色、生物侵蚀速率下降以及普遍死亡的现象，该效应掩盖了其他因子的影响，表明该海绵的代谢活动高度依赖其内生共生体。在"照常排放"情景下，本世纪末的珊瑚礁环境中，东方穿贝海绵及其他类似光合共生挖掘海绵的生长、生物侵蚀能力以及存活概率都将大幅降低，而非提升。