Jet-suspended, Calcite-ballasted Cyanobacterial. Waterwarts in a Desert

We describe a population of colonial cyanobacteria (waterwarts) that develops as the dominant primary producer in a bottom-fed, warm spring in the Cuatro Cienegas karstic region of the Mexican Chihuahuan Desert. The centimeter-sizedwaterwarts were suspendedwithin a central, conicallyshaped, 6-m deep well by upwelling waters. Waterwarts were built by an unicellular cyanobacterium and supported a communityof epiphytic filamentouscyanobacteria and diatoms but were free of heterotrophic bacteria inside. Sequence analysis of genes revealed that this cyanobacterium is only distantly related to several strains of other unicellular teria Cyanothece, Waterwarts contained orderly arrangements of mineral made up of microcrystallinelow-magnesiumcalcitewith high levels of strontium and sulfur. were 95.9% glycan, 2.8% cells, and 1.3% mineral grains and had a buoyant density of 1.034 An analysis of the hydrological properties of the spring well and the waterwarts demonstrated that both large colony size and the presence of controlled amounts of mineral ballast are required to prevent the population from being washed out of the well. The unique hydrological characteristics of the spring have likely selectedfor both traits. The mechanismsbywhich controlled nucleation of extracellular calcite is achieved remain to be explored.