Cell density regulates cellular reversal frequency in Myxococcus xanthus.

Myxococcus xanthus is a Gram-negative bacterium that aggregates to form fruiting bodies when nutrients are limiting. Previous studies showed that the frz mutants that are defective in chemotaxis exhibited irregular and infrequent patterns of cellular reversal. In contrast, wild-type cells, when examined individually, reverse relatively frequently, about once every 6 min. It is not known how the change of reversal frequency effects cellular aggregation during fruiting body formation in M. xanthus. In this study, we stained cells with a tetrazolium dye so that we could track the reversal frequencies of single cells and cells in groups. We found that developmental cells in large groups reverse much less than cells in small groups or as single cells. This reduced cellular reversal frequency is related to the frz signal transduction system and correlated with the methylation of FrzCD (a methyl-accepting chemotaxis protein). Cells containing a mutation in the frz genes or in the genes required for social motility do not respond in this way. The reduction in cellular reversals as developmental cells accumulate in groups suggests a simple hypothesis for the aggregation of cells into discrete mounds during fruiting body formation. We also found that M. xanthus cells glide with equal frequency in the forward or reverse directions, indicating that cells do not contain a "head" or "tail."

Fast or slow melting of the Marinoan snowball Earth? The cap dolostone record

The end of the Neoproterozoic era is punctuated by two global glacial events marked by the presence of glacial deposits overlaid by cap carbonates. Duration of glacial intervals is now consistently constrained to 3-12 million years but the duration of the post-glacial transition is more controversial due to the uncertainty in cap dolostone sedimentation rates. Indeed, the presence of several stratabound magnetic reversals in Brazilian cap dolostones recently questioned the short sedimentation duration (a few thousand years at most) that was initially suggested for these rocks. Here, we present new detailed magnetostratigraphic data of the Mirassol d`Oeste cap dolostones (Mato Grosso, Brazil) and ""bomb-spike"" calibrated AMS (14)C data of microbial mats from the Lagoa Vermelha (Rio de Janeiro, Brazil). We also compile sedimentary, isotopic and microbiological data from post-Marinoan outcrops and/or recent depositional analogues in order to discuss the deposition rate of Marinoan cap dolostones and to infer an estimation of the deglaciation duration in the snowball Earth aftermath. Taken together, the various data point to a sedimentation duration in the range of a few 10(5) years. (C) 2010 Elsevier B.V. All rights reserved.