Spatial heterogeneity and underlying geochemistry of phylogenetically diverse orange and white Beggiatoa mats in Guaymas Basin hydrothermal sediments

Sulfide-oxidizing bacteria of the genus Beggiatoa are found in conspicuous, colorful mats on the seafloor above active hydrothermal seeps at Guaymas Basin. Guaymas Beggiatoa filaments fall into discrete size classes representing at least five separate 16S rRNA phylotypes, and appear either white, yellow, or orange. During two R/V Atlantis cruises to Guaymas Basin, 78 temperature profiles were taken near and within 15 different orange and white Beggiatoa mats by the Alvin submersible to investigate spatial relationships between mat color and hydrothermal fluid seeps, as indicated by elevated temperatures. The surface temperatures from 78 profiles are similar to each other (on average 8–12 °C, warmer than bare sediments at 3–4 °C), indicating that Guaymas Basin Beggiatoa spp., although relying on the hydrothermal system for energy and carbon sources, live within a relatively cool temperature range. Temperatures from 40 cm below orange Beggiatoa versus white Beggiatoa are the same, at 84 °C averaged across all mat systems. However, within a single mat system, temperatures are higher beneath the predominantly orange center of the mat than beneath the white mat periphery. Push core transects across the orange-to-white color change of three Beggiatoa mats showed stronger upward compression of isotherms and metabolic zones beneath the orange mat center than beneath white mat periphery. Hydrothermal temperature gradients push the microbial processes generating carbon and energy sources for Beggiatoa mats towards the sediment surface. The resulting steep gradients of hydrothermal electron donors and carbon sources to the sediment surface, rather than the in situ temperature by itself, control the relative positioning of orange and white filaments within a Guaymas Basin Beggiatoa mat. Given the wide spectrum of temperature and hydrothermal flux regimes between different mats, the orange/white pattern represents a relative preference or even a competitive balance among different Beggiatoa types that establishes itself within each hydrothermal hot spot.