Description: In intertidal areas, large amounts of organic matter in the form of kelp are regularly deposited on the beach. Mineralization of this organic matter leads to the release of many intermediates and end products into? the sediments. A wide variety of aerobic and anaerobic microorganisms control the mineralization process. The high input of reduced matter into the local ecosystem is apparent from elevated levels of sulfide and the abundance of white filamentous bacteria in the nearby surfaces? Or sediments?. The input of nutrients into this normally oligotrophic environment may subsequently also stimulate primary production of the microphytobenthos. We investigated the influence of kelp deposits on in situ concentrations of CO, H2, Fe(II), sulfide, CH4 and various nutrients on a beach on Helgoland. Our preliminary results show an enrichment of CO and H2 below kelp deposits. Exposure to O2 seemed essential for CO and H2 release during the process of kelp degradation, as was observed during incubation experiments. The most pronounced influence of kelp deposits was observed for Fe(II), with higher Fe(II) concentrations below kelp deposits, although large heterogeneity exists. Sulfide and CH4 were found in significant concentrations within sediments between kelp deposits and the adjacent sea. Remarkably, their levels were lower directly? below the kelp, whereas Fe(II) showed the opposite trend. We aim to define the processes responsible for the high sulfide and CH4 concentrations within the sediments, with a focus on the role of the high CO and H2 levels for sulfate reduction and methanogenesis. Through in situ measurements we will study the effects of variable oxygenation on the release of these compounds. We also aim to study and model the hydrology of the beach to assess the transport modes of the intermediates and nutrients through the permeable sands. We will furthermore determine the consequences of the nutrient input for the local microphytobenthos.

Description: We investigated light, water velocity, and CO2 as drivers of primary production in Mediterranean seagrass (Posidonia oceanica) meadows and neighboring bare sands using the aquatic eddy covariance technique. Study locations included an open-water meadow and a nearshore meadow, the nearshore meadow being exposed to greater hydrodynamic exchange. A third meadow was located at a CO2 vent. We found that, despite the oligotrophic environment, the meadows had a remarkably high metabolic activity, up to 20 times higher than the surrounding sands. They were strongly autotrophic, with net production half of gross primary production. Thus, P. oceanica meadows are oases of productivity in an unproductive environment. Secondly, we found that turbulent oxygen fluxes above the meadow can be significantly higher in the afternoon than in the morning at the same light levels. This hysteresis can be explained by the replenishment of nighttime-depleted oxygen within the meadow during the morning. Oxygen depletion and replenishment within the meadow do not contribute to turbulent O2 flux. The hysteresis disappeared when fluxes were corrected for the O2 storage within the meadow and, consequently, accurate metabolic rate measurements require measurements of meadow oxygen content. We further argue that oxygen-depleted waters in the meadow provide a source of CO2 and inorganic nutrients for fixation, especially in the morning. Contrary to expectation, meadow metabolic activity at the CO2 vent was lower than at the other sites, with negligible net primary production.

Description: El fiordo Comau en la Patagonia chilena norte se caracteriza por presentar un marcado gradiente de pH, de 7.4 a 8.1. Bajo estas condiciones, las cuales corresponden al pH pronosticado para los océanos en el año 2100, están prosperando bancos de mitílidos, bancos de braquiópodos, acumulaciones de picorocos, praderas de gorgonias y bancos de corales de aguas frías. Estos “bosques” de animales marinos forman la base estructural y funcional de un ecosistema bentónico marino muy diverso. La comunidad que domina principalmente en paredes rocosas desde los 80 m son los bancos de corales, con la especie matriz Desmophyllum dianthus. Interesantemente, D. dianthus crece en aguas de alto (sobresaturadas de aragonita) y bajo pH (insaturadas de aragonita), así como en aguas someras y profundas (desde aprox. 15 m hasta más de 400 m). Esto indica que el coral es capaz de regular y controlar su calcificación. Se incubó D. dianthus simulando futuros escenarios de acidificación oceánica y en dichas incubaciones se midió la composición isotópica del boro en su esqueleto (11B). D. dianthus presentó incrementos del pH interno de calcificación (pHcf) como respuesta reguladora frente a pH externos (pHsw) más bajos. Todavía falta una explicación fisiológica del pHcf en corales bajo diferentes pHsw. Utilizando microsensores para pH, calcio y oxígeno medimos el pHcf en D. dianthus en relación a la dinámica del calcio y la respiración a lo largo del pólipo del coral bajo diferentes pHsw. Encontramos que el pHcf y el pHsw están relacionados, pero no de forma directa debido a la gran heterogeneidad del pHcf. Esto sugiere una regulación del pHcf altamente compleja e inconsistente con los modelos anteriores, indicando que D. dianthus incrementa probablemente el pool interno de carbono y no el pH para facilitar la calcificación.

Description: Benthic cyanobacterial mats (BCMs) have increased in abundance on coral reefs worldwide. However, their species diversity and role in nitrogen fixation are poorly understood. We assessed the cyanobacterial diversity of BCMs at four coral reef sites in Curac¸ao, Southern Caribbean. In addition, nitrogen fixation rates of six common mats were measured. Microscopic examinations showed 22 cyanobacterial species, all from the order Oscillatoriales. Species diversity was similar among sites despite differences in overall BCM abundance. Dominant mats were primarily composed of Hydrocoleum glutinosum, Oscillatoria bonnemaisonii or Lyngbya majuscula. However, some mats exhibited highly variable species composition despite consistent macroscopic appearance. 16S rRNA-based phylogeny revealed similar species as those identified by microscopy, with additional sequences of unicellular (Xenococcus and Chroococcidiopsis) and heterocystous (Rivularia and Calothrix) cyanobacteria. Vice versa, morphotypes of Tychonema, Schizothrix and Dichothrix were found by microscopy only. The detection of similar species at the same sites in a study conducted 40 years ago indicates that changes in environmental conditions over these years may have favored indigenous species to bloom, rather than facilitated the introduction and proliferation of invasive species. Nitrogen fixation rates of mats were 3–10 times higher in the light than in the dark. The highest areal nitrogen fixation rate (169.1 mg N m-2 d-1) was recorded in the cyanobacterial patch dominated by O. bonnemaisonii. A scale-up of nitrogen fixation at a site with 26% BCM cover at 7 m depth yielded an aerial rate of 13 mg N m-2 reef d-1, which exceeds rates reported in open ocean blooms of Trichodesmium in the Caribbean. Our results suggest that the Caribbean basin is not only a hotspot for planktonic nitrogen fixation, but also for benthic nitrogen fixation. Because BCMs fix vast amounts of nitrogen, their proliferation will strongly alter the nitrogen budget of coral reefs.

Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 8 (2017): 15595, doi:10.1038/ncomms15595.

Description: Although increasing atmospheric nitrous oxide (N2O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N2O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N2O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N2O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N2O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N2O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N2O fluxes, especially in redox-dynamic sediments of coastal ecosystems.

Description: D.D.B. acknowledges support from the Max Planck Institute for Marine Microbiology. This work was supported by the National Science Foundation grants to W.Z. and S.D.W. (OCE-1260373) and to S.D.W. (EAR-1252161).