The anatomy and morphology of a plant shoot changes over the course of its development. Although many traits (e.g., leaf size) vary gradually, other traits change abruptly at predictable times in shoot development (Hackett and Murray 1993;Greenwood 1995;Poethig 2003). Two such changes occur during the postembryonic growth of the shoot. The juvenile-to-adult transition (vegetative phase change) occurs early in the life of the shoot, and is marked by differences in the anatomy, morphology, and chemistry of leaves and internodes produced before and after this transition. The second transition (reproductive phase change) occurs during the adult phase, and results in the production of flowers and flower-bearing branches in place of vegetative shoots.
Highly acidic (pH 0-1) biofilms, known as 'snottites', form on the walls and ceilings of hydrogen sulfide-rich caves. We investigated the population structure, physiology and biogeochemistry of these biofilms using metagenomics, rRNA methods and lipid geochemistry. Snottites from the Frasassi cave system (Italy) are dominated (470% of cells) by Acidithiobacillus thiooxidans, with smaller populations including an archaeon in the uncultivated 'G-plasma' clade of Thermoplasmatales (415%) and a bacterium in the Acidimicrobiaceae family (45%). Based on metagenomic evidence, the Acidithiobacillus population is autotrophic (ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBisCO), carboxysomes) and oxidizes sulfur by the sulfide-quinone reductase and sox pathways. No reads matching nitrogen fixation genes were detected in the metagenome, whereas multiple matches to nitrogen assimilation functions are present, consistent with geochemical evidence, that fixed nitrogen is available in the snottite environment to support autotrophic growth. Evidence for adaptations to extreme acidity include Acidithiobacillus sequences for cation transporters and hopanoid synthesis, and direct measurements of hopanoid membrane lipids. Based on combined metagenomic, molecular and geochemical evidence, we suggest that Acidithiobacillus is the snottite architect and main primary producer, and that snottite morphology and distributions in the cave environment are directly related to the supply of C, N and energy substrates from the cave atmosphere.
Little Salt Spring (Sarasota County, FL, USA) is a sinkhole with groundwater vents at ~77 m depth. The entire water column experiences sulfidic (~50 μM) conditions seasonally, resulting in a system poised between oxic and sulfidic conditions. Red pinnacle mats occupy the sediment-water interface in the sunlit upper basin of the sinkhole, Furthermore, our observations of the production of 2-methyl hopanoids by Cyanobacteria under conditions of low oxygen and low light are consistent with the recovery of these structures from ancient black shales as well as their paucity in modern marine environments.
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