For this week and some of next, we'll have some entries that Alexis put together over the summer.

Alexis writes:

Today's photo is from Wikimedia Commons (also on Flickr) and was taken by Ruth Ellison at Lake Thetis, Australia. Thanks, Ruth!

Today, we start a photo series of prehistoric plants, spanning the Cambrian period to the Cretaceous period, some extant, some long extinct, and some with living relatives.

Dating back to the Early Archaean about 3 billion years ago, stromatolites are evidence of the earliest known photosynthesizing organisms, cyanobacteria. Though cyanobacteria are also known as blue-green algae, they are not related to true algae, which are in the domain Eukaryota and not Bacteria. Modern living stromatolites can be found today in aquatic environments such as Shark Bay, Australia.

Stromatolites are made of alternating layers of cyanobacteria and sediment, and can vary in shape from mounds to pillars. To create these unique formations, the bacteria first weave around sediments to form thin mats. The ideal environment for stromatolite formation is water shallow enough to allow sufficient sunlight for photosynthesis. These can include marine environments, intertidal zones, and lakes like British Columbia's Pavilion Lake, which is the focus of much research. As more sediment gets stuck to the mats, the cyanobacteria are forced to spread vertically to maintain their access to sunlight. Sediment continues to stick to the organisms and the process continues, building layer on layer, forming a stromatolite.

In the fossil record, stromatolites are identified in cross-section by their curved laminae (via this page on stromatolites). These fossilized sediment layers (the organic bacterial mats having been long decomposed) are the only evidence that remain of the ancient cyanobacteria that we have to thank in part for generating an oxygenated atmosphere.