Conspicuous Veils Formed by Vibrioid Bacteria on Sulfidic Marine Sediment

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Abstract
We describe the morphology and behavior of a hitherto unknown bacterial species that forms conspicuous veils (typical dimensions, 30 by 30 mm) on sulfidic marine sediment. The new bacteria were enriched on complex sulfidic medium within a benthic gradient chamber in oxygen-sulfide countergradients, but the bacteria have so far not been isolated in pure culture, and a detailed characterization of their metabolism is still lacking. The bacteria are colorless, gram-negative, and vibrioid-shaped (1.3- to 2.5- by 4- to 10-μm) cells that multiply by binary division and contain several spherical inclusions of poly-β-hydroxybutyric acid. The cells have bipolar polytrichous flagella and exhibit a unique swimming pattern, rotating and translating along their short axis. Free-swimming cells showed aerotaxis and aggregated at ca. 2 μM oxygen within opposing oxygen-sulfide gradients, where they were able to attach via a mucous stalk, forming a cohesive whitish veil at the oxic-anoxic interface. Bacteria attached to the veil kept rotating and adapted their stalk lengths dynamically to changing oxygen concentrations. The joint action of rotating bacteria on the veil induced a homogeneous water flow from the oxic water region toward the veil, whereby the oxygen uptake rate could be enhanced up to six times, as shown by model calculations. The veils showed a pronounced succession pattern. New veils were generated de novo within 24 h and had a homogeneous whitish translucent appearance. Bacterial competitors or eukaryotic predators were apparently kept away by the low oxygen concentration prevailing at the veil surface. Frequently, within 2 days the veil developed a honeycomb pattern of regularly spaced holes. After 4 days, most veils were colonized by grazing ciliates, leading to the fast disappearance of the new bacteria. Several-week-old veils finally developed into microbial mats consisting of green, purple, and colorless sulfur bacteria.