Campbell S E
Orig Life. 1979 Sep;9(4):335-48. doi: 10.1007/BF00926826.
A cyanophyte dominated mat, desert crust, forms the ground cover in areas measuring hundreds of square meters in Utah and smaller patches in Colorado. The algal mat shows stromatolitic features such as sediment trapping and accretion, a convoluted surface, and polygonal cracking. Sand and clay particles are immobilized by a dense network of filaments of the two dominating cyanophyte species, Microcoleus vaginatus and M. chthonoplastes, which secrete sheaths to which particles adhere. These microorganisms can tolerate long periods of desiccation and are capable of instant reactivation and migration following wetting. Migration occurs in two events: 1. immediately following wetting of dry mat, trichomes are mechanically expelled from the sheath as it swells during rehydration, and 2. subsequently, trichomes begin a self-propelled gliding motility which is accompanied by further production of sheath. The maximum distance traveled on solid agar by trichomes of Microcoleus vaginatus during a 12 hour period of light was 4.8 cm. This corresponds to approximately 500 times the length of the fastest trichome, and provides a measure of the potential for spreading of the mat in nature via the motility of the trichomes. Dehydration resistence of the sheath modifies the extracellular environment of the trichomes and enables their transition to dormancy. Following prolonged wetting and evaporative drying of the mat in the laboratory, a smooth wafer-like crust is formed by the sheaths of Microcleus trichomes that have migrated to the surface. Calcium carbonate precipitates among the algal filaments under experimental conditions, indicating a potential for mat lithification and fossilization in the form of a caliche crust. It is suggested that limestones containing tubular microfossils may, in part, be of such an origin. The formation of mature Precambrian soils may be attributable to soil accretion, stabilization, and biogenic modification by blue-green algal land mats similar to desert crust.
一种以蓝藻为主的地被物，即沙漠结皮，在犹他州数百平方米的区域以及科罗拉多州较小的地块上形成地面覆盖物。藻席呈现出叠层石特征，如沉积物捕获和堆积、表面呈褶皱状以及多边形开裂。沙子和黏土颗粒被两种主要蓝藻物种——鞘丝藻和厚壁鞘丝藻——密集的丝状网络固定，这两种蓝藻会分泌鞘，颗粒附着在鞘上。这些微生物能够耐受长时间的干燥，并且在湿润后能够立即重新激活并迁移。迁移过程分两个阶段：1. 干燥的藻席湿润后，丝状体在再水化过程中随着鞘膨胀而被机械性地从鞘中排出；2. 随后，丝状体开始自主滑动运动，并伴随着鞘的进一步产生。在光照12小时期间，鞘丝藻的丝状体在固体琼脂上移动的最大距离为4.8厘米。这大约相当于最快丝状体长度的500倍，为藻席在自然界中通过丝状体的运动进行扩散的潜力提供了一种衡量方式。鞘的抗脱水能力改变了丝状体的细胞外环境，并使其进入休眠状态。在实验室中，经过长时间的湿润和蒸发干燥后，迁移到表面的鞘丝藻的鞘形成了光滑的薄片状结皮。在实验条件下，碳酸钙在藻类细丝之间沉淀，表明藻席有可能以钙质结皮的形式石化和形成化石。有人认为，含有管状微化石的石灰岩可能部分起源于此。成熟的前寒武纪土壤的形成可能归因于类似于沙漠结皮的蓝藻地被物对土壤的堆积、稳定和生物成因改造。
