3 results for Pohn, M

  • Flow microenvironment of two marine peritrich ciliates with ectobiotic chemoautotrophic bacteria

    Vopel, K; Reick, CH; Arlt, G; Pohn, M; Ott, JA (2011-07-14)

    Journal article
    Auckland University of Technology

    The flow microenvironment of 2 marine peritrich ciliates, Vorticella sp. and Zoothamnium niveum, with ectobiotic sulfur bacteria was studied with frame-by-frame analyses of video sequences and a microsensor for fluid velocity. Both species populate the chemocline above H2Sreleasing mangrove peat. Vorticella sp. moves the surrounding seawater up to a horizontal and vertical distance of at least 400 μm with a maximum flow velocity of 18 mm s–1 close to its peristomial edge. The feather-shaped colonies of Z. niveum generate a unidirectional flow of seawater passing the colony perpendicular to the stalk; the convex side of the feather faces upstream. The flow velocity increased exponentially towards the colony, up to 11 mm s–1 at a distance of 100 μm. Contraction of the stalk forces the zooids of Vorticella sp. and Z. niveum towards the substrate at a high velocity of 71 and 520 mm s–1, respectively. During contraction of Vorticella sp., only little seawater is dragged along towards the surface to which the ciliates are attached whereas the contraction of Z. niveum resulted in a clear increase in the velocity of the seawater both surrounding the colony and above the substrate. Extension of the species proceeds 700 to 1000 times more slowly than contraction, and the surrounding seawater sticks to the cells and therefore is dragged along. The measurements given here support our earlier data indicating the importance of the feeding current for the bacteria-ciliate association, i.e. the cilia beat drives H2S- and O2-containing seawater toward the zooid at high velocity and thus, supports the growth of the ectobiotic sulfide-oxidizing bacteria. Rapid movement, shrinkage (Vorticella sp.) and bunching (Z. niveum) of the zooids during stalk contraction apparently cause sufficient shear stress to abrade ectobiotic bacteria that, once suspended, could enter the feeding currents.

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  • Microclimate of the brown alga Feldmannia caespitula interstitium under zero-flow conditions

    Pohn, M; Vopel, K; Grunberger, E; Ott, J (2011-07-14)

    Journal article
    Auckland University of Technology

    The microclimate of the brown alga Feldmannia caespitula (J. Agardh) Knoepffler-Peguy interstitium was studied using microelectrode techniques. Zero water flow and irradiances of 170 and 1500 mu mol photons m(-2) s(-1) cause steep O-2 gradients peaking 3 to 4 mm below the outer surface of the tufts at 310 and 506% atmospheric saturation, respectively. The mean fluxes of O-2 from the interstitium to the surrounding bulk water were 87 +/- 21 and 262 +/- 68 nmol cm(-2) h(-1) at low and high quantum flux density. Except for the outer 2 to 4 mm thick margin, the alga interstitium became anoxic within 52 min after abrupt darkening. The rate of dark oxygen uptake was 52 +/- 5 nmol cm(-2) h(-1). The tufts were populated by 9 metazoan taxa: nematodes, harpacticoid copepods, ostracods, gastropods, bivalves, polychaetes, amphipods, isopods and halacarids. Our results suggest that the interstitium of fine-textured algal thalli is a microhabitat of variable water chemistry with temporary anoxia and hyperoxia in an otherwise relatively stable water column. Although the tufts are attractive for meiofauna by providing food and protection from currents and predators, rapid fluctuations in oxygen concentration probably cue temporal emigration of the algal infauna.

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  • Ciliate-generated advective seawater transport supplies chemoautotrophic ectosymbionts

    Vopel, K; Pohn, M; Sorgo, A; Ott, J (2011-07-14)

    Journal article
    Auckland University of Technology

    Variations of [O2] and [H2S] in seawater surrounding laboratory reared sessile ciliates with ectosymbiotic chemoautotrophic bacteria were studied at high spatial and temporal resolutions using amperometric microsensors. We show how suspension feeding by the colonial Zoothamnium niveum and the solitary Vorticella sp. in the chemocline (O2/H2S-interface) of near-natural and artificial H2S-releasing substrates generates the physico-chemical microenvironment for the ectobiotic bacteria. Continuous recordings revealed a steep increase of [O2] and decrease of [H2S] in the proximal region of Z. niveum colonies during rapid stalk contraction. Hydrogen sulphide concentrations 2.5 mm above the substrate (upper end of the fully extended colony) increased when the contracted colony extended, followed by a decrease after the colony attained the fully upright position. Multiple contractions without complete extension successively transported sulphidic seawater upwards. The solitary Vorticella sp. maintained high ambient [O2] and low [H2S] 350 μm above the H2S-releasing membrane by generating a vertical flow field that drew seawater from above toward the ciliate. Oxygen concentration at the proximal part of Vorticella sp. did not increase during contraction, whereas during slow extension deoxygenated seawater was transported upwards and rapidly mixed with the surrounding oxygenated seawater when the ciliate started to beat its cilia. In both species rapid stalk contraction and subsequent slow extension enhanced the mixing of oxygenated and deoxygenated, H2S-containing seawater; the feeding currents (toroidal vortices) drew the surrounding seawater within reach of the zooid’s external surface at high speed. It is suggested that this advective fluid transport supplies the ectobiotic bacteria with O2 and H2S simultaneously. The high fluid velocity may cause a decrease in cell boundary layer thickness, thereby enhancing rates of nutrient uptake by the ectobiotic bacteria.

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