科学家提出,在海洋表面的光合细菌可能在地球早期“单调的数十亿年”里妨碍了多细胞生物的形成,当时海洋的大部分处于大致无氧的状态。
David Johnston及其同事提出,这种细菌很可能在地球的18亿年前到8亿年前的中年时期使用硫阻止了海洋和空气中的氧的大量积累。这组作者提出,这些细菌在那个时期占据了海洋表面,产生了一个无氧区域,在那里,使用硫的细菌传播开来,在全世界创造出了具有化学毒性的海洋层。这些细菌主要使用硫化物而不是水从而获取能量。否则硫化物会与铁结合形成黄铁矿(愚人金)。在地质时代的尺度上,在海洋中循环的黄铁矿降低了硫的浓度,这限制了这种细菌的能量来源,因此也就让海洋和大气无氧。最终这种循环被打破了,把更多的铁和氧从海洋中解放出来。
这组作者说,制氧的光合细菌最终占据了海洋,这间接导致了多细胞生物的形成——最初是在海洋中,随后出现在陆地上。(生物谷bioon.com)
生物谷推荐原始出处:
PNAS September 28, 2009, doi: 10.1073/pnas.0909248106
Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age
D. T. Johnstona,b,1,2, F. Wolfe-Simona,1,2, A. Pearsona,2 and A. H. Knollb,2
aDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA, 02138; and
bDepartment of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA, 02138
Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe2+ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms.