Browsing by Subject "N-2 FIXATION"

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  • Hogfors, Hedvig; Motwani, Nisha H.; Hajdu, Susanna; El-Shehawy, Rehab; Holmborn, Towe; Vehmaa, Anu; Engström-Öst, Jonna; Brutemark, Andreas; Gorokhova, Elena (2014)
  • El-Khaled, Yusuf C.; Nafeh, Rassil; Roth, Florian; Rädecker, Nils; Karcher, Denis B.; Jones, Burton H.; Voolstra, Christian R.; Wild, Christian (2021)
    Nitrogen cycling in coral reefs may be affected by nutrient availability, but knowledge about concentrationdependent thresholds that modulate dinitrogen fixation and denitrification is missing. We determined the effects of different nitrate concentrations (ambient, 1, 5, 10 mu M nitrate addition) on both processes under two light scenarios (i.e., light and dark) using a combined acetylene assay for two common benthic reef substrates, i.e., turf algae and coral rubble. For both substrates, dinitrogen fixation rates peaked at 5 mu M nitrate addition in light, whereas denitrification was highest at 10 mu M nitrate addition in the dark. At 10 mu m nitrate addition in the dark, a near-complete collapse of dinitrogen fixation concurrent with a 76-fold increase in denitrification observed for coral rubble, suggesting potential threshold responses linked to the nutritional state of the community. We conclude that dynamic nitrogen cycling activity may help stabilise nitrogen availability in microbial communities associated with coral reef substrates.
  • Roth, Florian; Karcher, Denis B.; Radecker, Nils; Hohn, Sonke; Carvalho, Susana; Thomson, Timothy; Saalmann, Franziska; Voolstra, Christian R.; Kurten, Benjamin; Struck, Ulrich; Jones, Burton H.; Wild, Christian (2020)
    Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N-2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality. Pioneer communities exhibited high rates of C and N(2)fixation within 4-8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 mu mol C cm(-2) day(-1)and 8 to 54 nmol N cm(-2) day(-1), respectively, C and N(2)fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss. A freePlain Language Summarycan be found within the Supporting Information of this article.
  • Tilstra, Arjen; Roth, Florian; El-Khaled, Yusuf C.; Pogoreutz, Claudia; Raedecker, Nils; Voolstra, Christian R.; Wild, Christian (2021)
    Recent research suggests that nitrogen (N) cycling microbes are important for coral holobiont functioning. In particular, coral holobionts may acquire bioavailable N via prokaryotic dinitrogen (N-2) fixation or remove excess N via denitrification activity. However, our understanding of environmental drivers on these processes in hospite remains limited. Employing the strong seasonality of the central Red Sea, this study assessed the effects of environmental parameters on the proportional abundances of N cycling microbes associated with the hard corals Acropora hemprichii and Stylophora pistillata. Specifically, we quantified changes in the relative ratio between nirS and nifH gene copy numbers, as a proxy for seasonal shifts in denitrification and N-2 fixation potential in corals, respectively. In addition, we assessed coral tissue-associated Symbiodiniaceae cell densities and monitored environmental parameters to provide a holobiont and environmental context, respectively. While ratios of nirS to nifH gene copy numbers varied between seasons, they revealed similar seasonal patterns in both coral species, with ratios closely following patterns in environmental nitrate availability. Symbiodiniaceae cell densities aligned with environmental nitrate availability, suggesting that the seasonal shifts in nirS to nifH gene abundance ratios were probably driven by nitrate availability in the coral holobiont. Thereby, our results suggest that N cycling in coral holobionts probably adjusts to environmental conditions by increasing and/or decreasing denitrification and N-2 fixation potential according to environmental nitrate availability. Microbial N cycling may, thus, extenuate the effects of changes in environmental nitrate availability on coral holobionts to support the maintenance of the coral-Symbiodiniaceae symbiosis.
  • El-Khaledl, Yusuf C.; Roth, Florian; Rädecker, Nils; Kharbatia, Najeh; Jones, Burton H.; Voolstra, Christian R.; Wild, Christian (2020)
    Nitrogen (N) cycling in coral reefs is of key importance for these oligotrophic ecosystems, but knowledge about its pathways is limited. While dinitrogen (N-2) fixation is comparably well studied, the counteracting denitrification pathway is under-investigated, mainly because of expensive and relatively complex experimental techniques currently available. Here, we combined two established acetylene-based assays to one single setup to determine N-2-fixation and denitrification performed by microbes associated with coral reef substrates/organisms simultaneously. Accumulating target gases (ethylene for N-2-fixation, nitrous oxide for denitrification) were measured in gaseous headspace samples via gas chromatography. We measured N-2-fixation and denitrification rates of two Red Sea coral reef substrates (filamentous turf algae, coral rubble), and demonstrated, for the first time, the co-occurrence of both N-cycling processes in both substrates. N-2-fixation rates were up to eight times higher during the light compared to the dark, whereas denitrification rates during dark incubations were stimulated for turf algae and suppressed for coral rubble compared to light incubations. Our results highlight the importance of both substrates in fixing N, but their role in relieving N is potentially divergent. Absolute N-2-fixation rates of the present study correspond with rates reported previously, even though likely underestimated due to an initial lag phase. Denitrification is also presumably underestimated due to incomplete nitrous oxide inhibition and/or substrate limitation. Besides these inherent limitations, we show that a relative comparison of N-2-fixation and denitrification activity between functional groups is possible. Thus, our approach facilitates cost-efficient sample processing in studies interested in comparing relative rates of N-2-fixation and denitrification.