Predicting the distribution of acid volatile sulfide in marine sediment from colour analysis of sediment-profile images

Organic enrichment of coastal sediments is a major concern worldwide. It is caused by an increase in the deposition of organic matter via terrestrial runoff (Gray et al. 2002) and aquaculture (Holmer & Kristensen 1994), or as a result of eutrophication driven by anthropogenic loading of coastal waters with phosphorus and nitrogen (Nixon 1995, Cloern 2001, Rosenberg et al. 2009). In coastal waters the majority of deposited organic carbon is mineralised by bacterial sulfate reduction because of the abundance of sulfate in the marine environement (Thode-Andersen & Jørgensen 1989, Bagarinao 1992). Hydrogen sulfide (H2S), produced during sulfate reduction, reacts with dissolved iron or reactive iron minerals to form acid-extractable iron sulfides (termed acid volatile sulfide, AVS) that contribute to the distinct black colouration of organic-rich sediment. Although there is a strong relationship between sedimentary sulfide content and organic matter input, its measurement has not been used in routine monitoring because of its laborious nature. Bull and Williamson (2001) tested a new approach to predict the sediment AVS concentration from sediment images. The authors used film photography in a laboratory and found a weak linear correlation (R2 = 0.67) between sediment colour and AVS concentration.OMV New Zealand Limite

Thin terrestrial sediment deposits on intertidal sandflats: effects on pore-water solutes and juvenile bivalve burial behaviour

Nearshore zones experience increased sedimentation due to coastal development and enhanced loads of fine terrestrial sediment (hereafter, TS) in river waters. Deposition of TS can alter seabed biogeochemical processes but the effects on benthic ecosystem functioning are unknown. The results of a past experiment with defaunated, intertidal sediment suggest that a decrease in the oxygenation of this sediment by a thin (mm) TS deposit causes substrate rejection (refusal to bury) by post-settlement juvenile recruits of the tellinid bivalve Macomona liliana. We further examined this behaviour, asking if such deposits negatively affect burial when applied to intertidal sediment that is oxygenated by bioturbation (C) or depleted of dead and living organic matter (D). We observed recruits on the surface of four treatments: C, D, and the same sediments to which we added a 1.7–1.9 mm layer of TS (CTS, DTS). The TS deposit decreased the oxygenation and the pH of the underlying intertidal sediment (CTS) confirming previous results, but significantly increased but not decreased the probability of burial, irrespectively of treatment. Juveniles more likely buried into C than into D. The mechanism that caused previously observed substrate rejection by post-settlement juvenile M. liliana remains unclear but our results suggest that contact of the recruits with the TS deposit does not cause substrate rejection. We now hypothesise that conditioning of sediment by bioturbation can mediate negative effects of TS deposits on the recruits' burial behaviour

The timing of benthic copepod emergence

We investigated emergence of marine benthic harpacticoid and cyclopoid copepods from subtidal sediment into the bottom water. Previous studies that used traps in the field found that copepods emerged from the sediment within 2 h of dusk. We examined emergence in 20-min sampling periods over 13 h from 1200 to 0100 h in a laboratory flume under conditions of constant flow speed. The light intensity was adjusted to field conditions every 20 min. We observed a peak in emergence during the 20-min sampling period after the onset of darkness, greatly increasing the accuracy with which the timing of emergence is known. This result will help to set the stage for studies of emergence cues

Sulfide assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum

We investigated the constraints on sulfide uptake by bacterial ectosymbionts on the marine peritrich ciliate Zoothamnium niveum by a combination of experimental and numerical methods. Protists with symbionts were collected on large blocks of mangrove-peat. The blocks were placed in a flow cell with flow adjusted to in situ velocity. The water motion around the colonies was then characterized by particle tracking velocimetry. This shows that the feather-shaped colony of Z. niveum generates a unidirectional flow of seawater through the colony with no recirculation. The source of the feeding current was the free-flowing water although the size of the colonies suggests that they live partly submerged in the diffusive boundary layer. We showed that the filtered volume allows Z. niveum to assimilate sufficient sulfide to sustain the symbiosis at a few micromoles per liter in ambient concentration. Numerical modeling shows that sulfide oxidizing bacteria on the surfaces of Z. niveum can sustain 100-times higher sulfide uptake than bacteria on flat surfaces, such as microbial mats. The study demonstrates that the filter feeding zooids of Z. niveum are preadapted to be prime habitats for sulfide oxidizing bacteria due to Z. niveum’s habitat preference and due to the feeding current. Z. niveum is capable of exploiting low concentrations of sulfide in near norm-oxic seawater. This links its otherwise dissimilar habitats and makes it functionally similar to invertebrates with thiotrophic symbionts in filtering organs

Deposit-feeding sea cucumbers enhance mineralization and nutrient cycling in organically-enriched coastal sediments

Bioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community

Wave-induced H2S flux sustains a chemoautotrophic symbiosis

Symbioses involving sulfur‐oxidizing bacteria and invertebrate hosts require a source of reduced sulfur, a source of O2, and transport mechanisms that ensure them a supply of both. We investigated these mechanisms using the symbiosis between the sessile ciliate Zoothamnium niveum (Hemprich and Ehrenberg 1831) and bacteria living on its surface. The stalked colonies of Z. niveum grow on peat walls around the openings of centimeter‐scale conduits created when mangrove rootlets decompose. Using in situ, time‐series measurements with fast‐responding amperometric microelectrodes, we found that the conduits were charged with H2S by diffusion from the decaying rootlets during periods of low boundary‐layer flow speed. During these times, the feeding current of the zooids transported oxygenated seawater from outside the peat wall toward the ectobiotic bacteria. During periods of high flow speed, H2S‐rich seawater from the conduits was drawn along the colonies and over the bacteria. We conclude that this symbiosis exploits a combination of two transport mechanisms: (1) venting of H2S‐rich seawater due to pulsating boundary‐layer current over ciliate groups and (2) the continuous and rapid feeding current generated by the host’s cilia. This discovery raises the possibility that other systems in which pockets of decay are exposed to pulsating flow could support similar symbioses

Seagrass Canopy Photosynthetic Response Is a Function of Canopy Density and Light Environment: A Model for Amphibolis griffithii

A three-dimensional computer model of canopies of the seagrass Amphibolis griffithii was used to investigate the consequences of variations in canopy structure and benthic light environment on leaf-level photosynthetic saturation state. The model was constructed using empirical data of plant morphometrics from a previously conducted shading experiment and validated well to in-situ data on light attenuation in canopies of different densities. Using published values of the leaf-level saturating irradiance for photosynthesis, results show that the interaction of canopy density and canopy-scale photosynthetic response is complex and non-linear, due to the combination of self-shading and the non-linearity of photosynthesis versus irradiance (P-I) curves near saturating irradiance. Therefore studies of light limitation in seagrasses should consider variation in canopy structure and density. Based on empirical work, we propose a number of possible measures for canopy scale photosynthetic response that can be plotted to yield isoclines in the space of canopy density and light environment. These plots can be used to interpret the significance of canopy changes induced as a response to decreases in the benthic light environment: in some cases canopy thinning can lead to an equivalent leaf level light environment, in others physiological changes may also be required but these alone may be inadequate for canopy survival. By providing insight to these processes the methods developed here could be a valuable management tool for seagrass conservation during dredging or other coastal developments

Intracellular Trafficking of Guanylate-Binding Proteins Is Regulated by Heterodimerization in a Hierarchical Manner

Guanylate-binding proteins (GBPs) belong to the dynamin family of large GTPases and represent the major IFN-γ-induced proteins. Here we systematically investigated the mechanisms regulating the subcellular localization of GBPs. Three GBPs (GBP-1, GBP-2 and GBP-5) carry a C-terminal CaaX-prenylation signal, which is typical for small GTPases of the Ras family, and increases the membrane affinity of proteins. In this study, we demonstrated that GBP-1, GBP-2 and GBP-5 are prenylated in vivo and that prenylation is required for the membrane association of GBP-1, GBP-2 and GBP-5. Using co-immunoprecipitation, yeast-two-hybrid analysis and fluorescence complementation assays, we showed for the first time that GBPs are able to homodimerize in vivo and that the membrane association of GBPs is regulated by dimerization similarly to dynamin. Interestingly, GBPs could also heterodimerize. This resulted in hierarchical positioning effects on the intracellular localization of the proteins. Specifically, GBP-1 recruited GBP-5 and GBP-2 into its own cellular compartment and GBP-5 repositioned GBP-2. In addition, GBP-1, GBP-2 and GBP-5 were able to redirect non-prenylated GBPs to their compartment in a prenylation-dependent manner. Overall, these findings prove in vivo the ability of GBPs to dimerize, indicate that heterodimerization regulates sub-cellular localization of GBPs and underscore putative membrane-associated functions of this family of proteins

Rollenspiel

Das Rollenspiel als Erhebungsmethode in der qualitativen Sozialforschung bietet, verbunden mit dem Auswertungsverfahren der tiefenhermeneutischen Textinterpretation, die Möglichkeit, kollektiv unbewusste Prozesse in Gruppen in ihrem Bedeutungsgehalt für die den Interaktionen zugrunde liegenden Muster zu erkennen und zu verstehen. Gruppendynamische Prozesse werden erfasst und das rollenspezifische Handeln der Gruppenteilnehmenden bezogen auf das jeweilige Erfahrungsfeld analysiert. In dem Beitrag werden die Durchführung der Rollenspiele sowie deren Dokumentation und die Auswertung der in den Rollenspielen erhobenen Daten vorgestellt, insbesondere anhand konkreter Anwendungsbeispiele die Dokumentations- und Auswertungsschritte praxisorientiert dargelegt und Limitationen der Einsatzes von Rollenspielen diskutiert