Ciliate-generated advective seawater transport supplies chemoautotrophic ectosymbionts

Variations of [O-2] and [H2S] in seawater surrounding laboratory-reared ses sile ciliates with ectosymbiotic chemoautotrophic bacteria were studied at high spatial and temporal resolutions using amperometric microsensors. We s how how suspension feeding by the colonial Zoothamnium niveum and the solit ary Vorticella sp, in the chemocline (O-2/H2S-interface) of near-natural an d artificial H2S-releasing substrates generates the physico-chemical microe nvironment for the ectobiotic bacteria. Continuous recordings revealed a st eep increase of [O-2] and decrease of [H2S] in the proximal region of Z, ni veum colonies during rapid stalk contraction. Hydrogen sulphide concentrati ons 2.5 mm above the substrate (upper end of the fully extended colony) inc reased when the contracted colony extended, followed by a decrease after th e colony attained the fully upright position. Multiple contractions without complete extension successively transported sulphidic seawater upwards. Th e solitary Vorticella sp, maintained high ambient [O-2] and low [H2S] 350 m um above the H2S-releasing membrane by generating a vertical flow field tha t drew seawater from above toward the ciliate. Oxygen concentration at the proximal part of Vorticella sp, did not increase during contraction, wherea s during slow extension deoxygenated seawater was transported upwards and r apidly mixed with the surrounding oxygenated seawater when the ciliate star ted to beat its cilia. In both species rapid stalk contraction and subseque nt slow extension enhanced the mixing of oxygenated and deoxygenated, H2S-c ontaining seawater; the feeding currents (toroidal vortices) drew the surro unding seawater within reach of the zooid's external surface at high speed. It is suggested that this advective fluid transport supplies the ectobioti c bacteria with O-2 and H2S simultaneously. The high fluid velocity may cau se a decrease in cell boundary layer thickness, thereby enhancing rates of nutrient uptake by the ectobiotic bacteria.