Department of Geosciences, University of Montana, Missoula, MT, USA.
Geobiology. 2010 Jan;8(1):1-23. doi: 10.1111/j.1472-4669.2009.00220.x. Epub 2009 Oct 23.
When cyanobacteria originated and diversified, and what their ancient traits were, remain critical unresolved problems. Here, we used a phylogenomic approach to construct a well-resolved 'core' cyanobacterial tree. The branching positions of four lineages (Thermosynechococcus elongatus, Synechococcus elongatus, Synechococcus PCC 7335 and Acaryochloris marina) were problematic, probably due to long branch attraction artifacts. A consensus genomic tree was used to study trait evolution using ancestral state reconstruction (ASR). The early cyanobacteria were probably unicellular, freshwater, had small cell diameters, and lacked the traits to form thick microbial mats. Relaxed molecular clock analyses suggested that early cyanobacterial lineages were restricted to freshwater ecosystems until at least 2.4 Ga, before diversifying into coastal brackish and marine environments. The resultant increases in niche space and nutrient availability, and consequent sedimentation of organic carbon into the deep oceans, would have generated large pulses of oxygen into the biosphere, possibly explaining why oxygen rose so rapidly. Rapid atmospheric oxidation could have destroyed the methane-driven greenhouse with simultaneous drawdown in pCO(2), precipitating 'Snowball Earth' conditions. The traits associated with the formation of thick, laminated microbial mats (large cell diameters, filamentous growth, sheaths, motility and nitrogen fixation) were not seen until after diversification of the LPP, SPM and PNT clades, after 2.32 Ga. The appearance of these traits overlaps with a global carbon isotopic excursion between 2.2 and 2.1 Ga. Thus, a massive re-ordering of biogeochemical cycles caused by the appearance of complex laminated microbial communities in marine environments may have caused this excursion. Finally, we show that ASR may provide an explanation for why cyanobacterial microfossils have not been observed until after 2.0 Ga, and make suggestions for how future paleobiological searches for early cyanobacteria might proceed. In summary, key evolutionary events in the microbial world may have triggered some of the key geologic upheavals on the Paleoproterozoic Earth.
当蓝藻起源和多样化时，以及它们的古代特征是什么，仍然是关键的未解决问题。在这里，我们使用系统基因组学方法构建了一个分辨率良好的“核心”蓝藻树。四个谱系（伸长鱼腥藻、 elongatus 聚球藻、 elongatus 聚球藻和海洋阿卡利亚藻）的分支位置存在问题，可能是由于长分支吸引伪影造成的。使用共识基因组树通过祖先状态重建（ASR）研究特征进化。早期蓝藻可能是单细胞的、淡水的，细胞直径较小，并且缺乏形成厚微生物席的特征。放松的分子钟分析表明，早期蓝藻谱系至少在 24 亿年前就局限于淡水生态系统，然后才多样化为沿海半咸水和海洋环境。由此产生的生态位空间和营养物质可用性的增加，以及有机碳随之沉降到深海中，会导致大量氧气进入生物圈，这可能解释了为什么氧气会如此迅速上升。快速的大气氧化可能破坏了甲烷驱动的温室效应，同时使 pCO2 下降，导致“雪球地球”条件。直到 LPP、SPM 和 PNT 进化分支多样化之后，即 23.2 亿年前之后，才出现与厚层、分层微生物席形成相关的特征（大细胞直径、丝状生长、鞘、运动性和固氮作用）。这些特征的出现与 22 亿至 21 亿年前之间的全球碳同位素偏移重叠。因此，海洋环境中复杂分层微生物群落的出现可能导致了生物地球化学循环的大规模重新排序，从而导致了这种偏移。最后，我们表明 ASR 可能解释了为什么直到 20 亿年前之后才观察到蓝藻微生物化石，并且为未来如何进行早期蓝藻的古生物学搜索提出了建议。总之，微生物世界中的关键进化事件可能引发了古元古代地球的一些关键地质剧变。
