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Unformatted text preview: life forms are the archaebacteria (the archaea in modern usage). This group is today restricted to marginal environments. Recent discoveries of bacteria at mid-ocean ridges add yet another possible origin for life: at these mid-ocean ridges where heat and molten rock rise to the earth's surface. Many of the ancient phototrophs and heterotrophic bacteria lived in colonial associations known as stromatolites. Cyanobacteria are on the outer surface, with other photosynjthetic bacteria (anoxic) below them. Below these phottrophs are layers of heterotrophic bacteria. The layers in the stromatolites are alternating biogenic and sedimentologic in origin. A modern day stromatolite is shown in Figure 20....
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- Fall '10