Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae.
Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae.
复制标题
DOI：
10.1371/journal.pbio.2000735
复制
发表时间：
2017-03
期刊：
影响因子：
9.8
通讯作者：
Whitehouse M
中科院分区：
文献类型：
--
作者：
Bengtson S;Sallstedt T;Belivanova V;Whitehouse M
印度中部下文地统约1.6Ga的Tirohan白云岩中含有磷化的叠层石微生物岩。我们从那里报道了保存完好的独特化石，被解释为可能的冠群视藻(红藻)。丝状型几丁质拉斐氏菌n.gen，n.sp.有单列的大细胞，通过弥漫分布的隔膜生长。每个细胞都有一个中央悬挂的明显的菱形圆盘，被解释为肾小球。细胞之间的隔膜具有中心结构，可能代表凹坑连接和凹坑塞子。另一种丝状细菌Denaricion mendax n.Gen.，n.sp.具有硬币状细胞，类似于大型硫氧化细菌中的细胞，但比后者充满液泡的细胞更顽固。虽然后者的细胞体积要小得多，但它与振荡纲蓝藻有相似之处。Denaricion的更广泛的亲和力是不确定的。新种Ramathallus lobatus n.Gen.，n.sp.是一种叶状无柄藻类，具假薄壁组织的叶状体、“细胞喷泉”和顶端生长，暗示着花的亲和力。如果这些推断是正确的，Rafatazia和Ramathallus代表了冠群多细胞视紫藻类，将化石记录中最古老的以前被接受的红藻提前了大约4亿年。现代真核生物的最后一个共同祖先通常被认为生活在中元古代，大约在16亿到10亿年前，或者可能更早一些。我们研究了印度中部约16亿年前的沉积岩中保存完好的化石群落，这些沉积岩代表了以光合作用生物为特征的浅水海洋环境。我们在广阔的蓝藻沼泽中发现了丝状和分叶生物群，这些生物群与现代真核藻，尤其是红藻有着显著的共同特征。岩石主要由钙和镁碳酸盐组成，但微生物垫和化石保存在磷酸钙中，使我们能够使用同步辐射X射线断层扫描显微镜观察细胞和亚细胞的三维结构。丝状细胞中最明显的内部物体是菱形血小板，我们将其解释为红藻光合作用机制的一部分。分叶状从一个共同的中心放射状球状或指状突起生长。这些化石比之前公认的最早的红藻早了大约4亿年，这表明真核生物的历史可能比人们通常认为的更长。
The ~1.6 Ga Tirohan Dolomite of the Lower Vindhyan in central India contains phosphatized stromatolitic microbialites. We report from there uniquely well-preserved fossils interpreted as probable crown-group rhodophytes (red algae). The filamentous form Rafatazmia chitrakootensis n. gen, n. sp. has uniserial rows of large cells and grows through diffusely distributed septation. Each cell has a centrally suspended, conspicuous rhomboidal disk interpreted as a pyrenoid. The septa between the cells have central structures that may represent pit connections and pit plugs. Another filamentous form, Denaricion mendax n. gen., n. sp., has coin-like cells reminiscent of those in large sulfur-oxidizing bacteria but much more recalcitrant than the liquid-vacuole-filled cells of the latter. There are also resemblances with oscillatoriacean cyanobacteria, although cell volumes in the latter are much smaller. The wider affinities of Denaricion are uncertain. Ramathallus lobatus n. gen., n. sp. is a lobate sessile alga with pseudoparenchymatous thallus, “cell fountains,” and apical growth, suggesting florideophycean affinity. If these inferences are correct, Rafatazmia and Ramathallus represent crown-group multicellular rhodophytes, antedating the oldest previously accepted red alga in the fossil record by about 400 million years. The last common ancestor of modern eukaryotes is generally believed to have lived during the Mesoproterozoic era, about 1.6 to 1 billion years ago, or possibly somewhat earlier. We studied exquisitely preserved fossil communities from ~1.6 billion-year-old sedimentary rocks in central India representing a shallow-water marine environment characterized by photosynthetic biomats. We discovered amidst extensive cyanobacterial mats a biota of filamentous and lobate organisms that share significant features with modern eukaryotic algae, more specifically red algae. The rocks mainly consist of calcium and magnesium carbonates, but the microbial mats and the fossils are preserved in calcium phosphate, letting us view the cellular and subcellular structures in three dimensions with the use of synchrotron-radiation X-ray tomographic microscopy. The most conspicuous internal objects in the cells of the filamentous forms are rhomboidal platelets that we interpret to be part of the photosynthetic machinery of red algae. The lobate forms grew as radiating globular or finger-like protrusions from a common centre. These fossils predate the previously earliest accepted red algae by about 400 million years, suggesting that eukaryotes may have a longer history than commonly assumed.