太阳光穿透海水反射到珊瑚礁上,这些光线会穿透珊瑚、与珊瑚共生的促进光合作用的海藻,以及其他生活在珊瑚礁上的生物。那么究竟是什么原因让这些生物避免了被烤焦的厄运呢?一项新的研究表明,正是珊瑚在这里起到了遮光剂的作用——它能够吸收紫外线,并减少其对生活在珊瑚礁上的居民的伤害。
之前的研究表明,构成礁体的珊瑚的碳酸钙外骨骼能够在紫外光下发出荧光,这意味着珊瑚礁能够吸收紫外线。为了搞清这种物质是否能够保护生活在珊瑚礁上的有机体,澳大利亚昆士兰大学的海洋生物学家Ruth Reef和同事对海葵(Aiptasia pulchella)进行了研究。这些珊瑚的亲戚具有同前者类似的组织,并且同样是共生光合海藻的家。
在实验室中,研究小组将海葵放在珊瑚骨架或白色铅管的上面。与铅管不同,珊瑚骨架几乎能够吸收所有有害的紫外线,并释放出黄色的荧光。此外,与放在铅管上的海葵相比,放在珊瑚上的海葵接受的紫外辐射只是前者的1/4,并且它们遭受的脱氧核糖核酸(DNA)损伤也仅是前者的1/7。即便将这些珊瑚骨架研磨成细小的粉末,研究小组依然发现了类似的现象,这意味着这种保护作用是因为珊瑚骨架的化学构成所致,而不是其粗糙而复杂的表面对紫外线产生的散射作用。研究小组在11月25日的《科学公共图书馆·综合》(PLoS One)网络版上报告了这一研究成果。
Reef指出,生活在海洋中的许多光合生物也会形成碳酸钙,而它们也可能通过这种方式保护自己免受紫外线辐射。她说:“石灰化过程大约出现在6亿年前,当时的紫外线水平要远远高于今天。”此外,Reef强调,在大约发生于5.3亿年前的寒武纪大爆发期间,珊瑚骨骼异常丰富且多样,这可能反映了“该时期自然界对珊瑚积聚的一种迫切需求,那时许多生物体都向着较浅且富含氧气的水域迁徙,而那里的紫外线水平都很高”。摩纳哥科学中心——一所海洋学研究机构——的科学主管Denis Allemand则表示:“这种新被发现的特性是宿主针对共生生物的一种额外的、意想不到的适应。”
Reef和同事注意到,蝎子、蜘蛛,以及其他一些生物在暴露于紫外线下时也会发出荧光,这意味着遮光剂效应不止进化了一次。
原文出处:
PLoS ONE 4(11): e7995. doi:10.1371/journal.pone.0007995
Coral Skeletons Defend against Ultraviolet Radiation
Ruth Reef*, Paulina Kaniewska, Ove Hoegh-Guldberg
Centre for Marine Studies and the Austrailan Research Council (ARC) Centre of Excellence for Coral Reef Studies, The University of Queensland, St Lucia, Queensland, Australia
Background
Many coral reef organisms are photosynthetic or have evolved in tight symbiosis with photosynthetic symbionts. As such, the tissues of reef organisms are often exposed to intense solar radiation in clear tropical waters and have adapted to trap and harness photosynthetically active radiation (PAR). High levels of ultraviolet radiation (UVR) associated with sunlight, however, represent a potential problem in terms of tissue damage.
Methodology/Principal Findings
By measuring UVR and PAR reflectance from intact and ground bare coral skeletons we show that the property of calcium carbonate skeletons to absorb downwelling UVR to a significant extent, while reflecting PAR back to the overlying tissue, has biological advantages. We placed cnidarians on top of bare skeletons and a UVR reflective substrate and showed that under ambient UVR levels, UVR transmitted through the tissues of cnidarians placed on top of bare skeletons were four times lower compared to their counterparts placed on a UVR reflective white substrate. In accordance with the lower levels of UVR measured in cnidarians on top of coral skeletons, a similar drop in UVR damage to their DNA was detected. The skeletons emitted absorbed UVR as yellow fluorescence, which allows for safe dissipation of the otherwise harmful radiation.
Conclusions/Significance
Our study presents a novel defensive role for coral skeletons and reveals that the strong UVR absorbance by the skeleton can contribute to the ability of corals, and potentially other calcifiers, to thrive under UVR levels that are detrimental to most marine life.