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Low functional change despite high taxonomic turnover characterizes the Ulva microbiome across a 2000-km salinity gradient
No full textThe green seaweed Ulva relies on associated bacteria for morphogenesis and is an important model to study algal-bacterial interactions. Ulva-associated bacteria exhibit high turnover across environmental gradients, leading to the hypothesis that bacteria contribute to the acclimation potential of the host. However, the functional variation of these bacteria in relation to environmental changes remains unclear. We analyzed 91 Ulva samples across a 2000-kilometer Atlantic–Baltic Sea salinity gradient using metagenomic sequencing. Metabolic reconstruction of 639 metagenome-assembled genomes revealed widespread potential for carbon, nitrogen, sulfur, and vitamin metabolism. Although the R2 value for salinity explained 70% of taxonomic variation, it accounted only for 17% of functional variation. The limited variation was attributed to typical high-salinity bacteria exhibiting enrichment in genes for thiamine, pyridoxal, and betaine biosynthesis, which likely contribute to stress mitigation and osmotic homeostasis in response to salinity variations. Our results emphasize the importance of functional profiling to understand the seaweed holobiont and its collective response to environmental change
Widespread environmental contamination from relic munitions in the southwestern Baltic Sea
No full textRelic munitions from warfare and intentional dumping contaminate coastal waters worldwide, with an estimated 300,000 tons in the German Baltic Sea alone. These contain toxic conventional explosive chemicals, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazinane (RDX), and 1,3-dinitrobenzene (DNB). Corrosion of metal munition housings in seawater releases these munition chemicals (MCs) to the marine environment. The current study performed detailed environmental sampling throughout German waters of the southwest Baltic Sea in 2017 and 2018, and measured MCs in water, suspended particles, and sediments. At least one MC was detected in nearly every water sample, from sub-pmol/L up to several thousand pmol/L. Most MC were in the dissolved phase, not on suspended particles, and MC content in sediments was patchy and generally low. TNT levels were especially high in Kiel Bay, whereas RDX and DNB concentrations were highest in Lübeck Bay, likely reflecting regional differences in munitions types. A TNT module was developed and implemented in the General Estuarine Transport Model (GETM), incorporating TNT input to the water column by dissolution and removal by microbial degradation/transformation. Simulated TNT distributions matched observed environmental patterns well, indicating good parametrization of the primary controls. Dissolved concentrations of the target MCs were generally far below acute or chronic toxicity levels for aquatic organisms, but the highest observed concentrations approached toxic levels, especially for DNB. The inventory of dissolved MC in the study region was approximately 3000 kg, implying that current contamination levels can be sustained continuously for over 800 years by existing munitions on the seafloor
Opportunities for Earth Observation to Inform Risk Management for Ocean Tipping Points
No full textAs climate change continues, the likelihood of passing critical thresholds or tipping points increases. Hence, there is a need to advance the science for detecting such thresholds. In this paper, we assess the needs and opportunities for Earth Observation (EO, here understood to refer to satellite observations) to inform society in responding to the risks associated with ten potential large-scale ocean tipping elements: Atlantic Meridional Overturning Circulation; Atlantic Subpolar Gyre; Beaufort Gyre; Arctic halocline; Kuroshio Large Meander; deoxygenation; phytoplankton; zooplankton; higher level ecosystems (including fisheries); and marine biodiversity. We review current scientific understanding and identify specific EO and related modelling needs for each of these tipping elements. We draw out some generic points that apply across several of the elements. These common points include the importance of maintaining long-term, consistent time series; the need to combine EO data consistently with in situ data types (including subsurface), for example through data assimilation; and the need to reduce or work with current mismatches in resolution (in both directions) between climate models and EO datasets. Our analysis shows that developing EO, modelling and prediction systems together, with understanding of the strengths and limitations of each, provides many promising paths towards monitoring and early warning systems for tipping, and towards the development of the next generation of climate models
Phytoplankton mean cell size and total biomass increase with nutrients are driven by both species composition and evolution of plasticity
No full textCommunity trait variability can arise from the species, genotypic, or individual plastic level. Trait changes on these levels can occur simultaneously, interact, and potentially translate to community functioning. Thus, they are crucial to realistically predict community functional changes. Using a phytoplankton model community comprising a diatom and a coccolithophore each with nine genotypes varying in cell size, we conducted a selection experiment over 130 generations towards nutrient availability. According to our expectations, mean community cell size and total biomass increased with increasing nutrient availability. Interspecifically, these community level changes were driven by shifts in species composition towards the larger diatom. Changes caused by intraspecific shifts did not result from sorting according to genotypes' standing variation in cell size in the first place. Instead, intraspecific changes likely resulted from the selection for a highly plastic diatom genotype, which led to a phenotypic distribution with larger cells in high and smaller cells in lower nutrient concentrations. We suggest that besides interspecific species sorting, the evolution of size plasticity through genotype selection represented an intraspecific contribution to mean community size increase with increasing nutrient availability that ultimately translated to increased total biomass. Our results demonstrate that all three levels on which trait changes can occur, regulate phytoplankton community-level functional changes and thus should be considered when predicting community change on ecological time scales
Sill stacking in subseafloor unconsolidated sediments and control on sustained hydrothermal systems: evidence from IODP drilling in the Guaymas Basin, Gulf of California
No full textMagma emplacement in the top unconsolidated sediments of rift basins is poorly constrained in terms of mechanics and associated hydrothermal activity. Our study compares two shallow sills from the Guaymas Basin (Gulf of California) using core data and analyses from IODP Expedition 385, and high-resolution 2D seismic data. We show that magma stalling in the top uncemented sediment layer is controlled by the transition from siliceous claystone to uncemented silica-rich sediment, promoting flat sill formation. Space is created through a combination of viscous indentation, magma-sediment mingling and fluidization processes. In low magma input regions, sills form above the opal-A/CT diagenetic barrier, while high magma input leads to upward stacking of sills, forming funnel-shaped intrusions near the seafloor. Our petrophysical, petrographic, and textural analyses show that magma-sediment mingling creates significant porosity (up to 20%) through thermal cracking of the assimilated sediment. Stable isotope data of carbonate precipitates indicate formation temperatures of 70−90°C, consistent with the current background geothermal gradient at 250−325 m depth. The unconsolidated, water-rich host sediments produce little thermogenic gas through contact metamorphism, but deep diagenetically formed gas bypasses the low-permeability top sediments via hydrothermal fluids flowing through the magma plumbing system. This hydrothermal system provides a steady supply of hydrocarbons at temperatures amendable for microbial life, acting as a major microbial incubator. Similar hydrothermal systems are expected to be abundant in magma-rich young rift basins and play a key role in sustaining subseafloor ecosystems
Smoother sea ice in a more dynamic Arctic: 30 years of airborne pressure ridge observations
No full textPressure ridges, formed by sea ice deformation, affect momentum transfer in the Arctic Ocean and support a larger biomass than the surrounding-level ice. Although trends in Arctic sea ice thickness and concentration are well documented, changes in ridge morphology remain unclear. This study provides airborne-based evidence of a shift towards a smoother ice surface, with fewer pressure ridges and reduced surface drag, attributed to the loss of old ice. Furthermore, an increase in seasonal ice cover enhances overall deformation in the Arctic and acts as a negative feedback mechanism on pan-Arctic ridge morphology: the greater the proportion of seasonal ice, the higher the pan-Arctic mean ridge rate, dampening an overall decline in ridges with age. While thinner and less frequent ridges benefit industries such as shipping, these changes are likely to have profound impacts on the energy and mass balance and the ecosystem of the Arctic Ocean
Ocean alkalinity enhancement in an open-ocean ecosystem: biogeochemical responses and carbon storage durability
No full textOcean alkalinity enhancement (OAE) is considered for the long-term removal of gigatonnes of carbon dioxide (CO2) from the atmosphere to achieve our climate goals. Little is known, however, about the ecosystem-level changes in biogeochemical functioning that may result from the chemical sequestration of CO2 in seawater and how stable the sequestration is. We studied these two aspects in natural plankton communities under carbonate-based, CO2-equilibrated OAE of up to a doubling of ambient alkalinity (+2400 µeq kg−1, Ωaragonite∼10) in the nutrient-poor North Atlantic. During our month-long mesocosm experiment, the majority of biogeochemical pools, including inorganic nutrients, particulate organic carbon and phosphorus, and biogenic silica, remained unaltered across all OAE levels. Noticeable exceptions were a minor decrease in particulate organic nitrogen and an increase in the carbon-to-nitrogen ratio (C:N) of particulate organic matter in response to OAE. Thus, in our nitrogen-limited system, nitrogen turnover processes appear more susceptible than those of other elements, which could lead to decreased food quality and increased organic carbon storage. However, alkalinity and chemical CO2 sequestration were not stable at all levels of OAE. Two weeks after alkalinity addition, we measured a loss of added alkalinity and of the initially stored CO2 in the mesocosm where alkalinity was highest. The loss rate in this mesocosm accelerated over time and amounted to ∼10 % of stored CO2 within 4 weeks after alkalinity enhancement. Additional tests showed that such secondary precipitation can be initiated by particles acting as precipitation nuclei and that this process can occur even at lower levels of OAE. In conclusion, in scenarios like our study with carbonate-based OAE, where the carbon is already sequestered, the risk of major and sustained impacts on biogeochemical functioning may be low in the nutrient-poor ocean. However, the durability of carbon sequestration using OAE could be constrained by alkalinity loss in supersaturated waters with precipitation nuclei present. Our study provides an evaluation of the ecosystem impacts of an idealised OAE deployment for monitoring, reporting, and verification in an oligotrophic system. Whether biogeochemical functioning is resilient to more technically simple and economically viable approaches that induce stronger water chemistry perturbations remains to be seen
Spatial distribution of permeability in carbonate fault damage zones
No full textFault conduits often localized fluid flow at specific sites related to fault segment growth and linkage. Understanding these mechanisms is essential for assessing geofluid pathways within reservoirs or leaks to the biosphere. We study here a segmented fault zone with strike-slip kinematics and pluri-decametric displacement, affecting carbonate rocks (Pag island, Croatia). This fault zone has multiple core zones surrounded by a damage zone (DZ), composed of different structures, including wall and link damage. To build discrete fracture networks (DFNs) of these structures, we conducted high-resolution fracture mapping and measurement of aperture in five areas around the main fault system. We also analyzed rock samples from each damage structure using the same method. Fluid flow simulations were performed through the DFNs to quantify the permeability and its anisotropy. We show that link damage is about 102 more permeable than the background damage, and 2 to 5 times more permeable than the wall damage. DZ permeability can be approximated by a tensor at the decametric-scale, but not at the centimetric-scale due to the strong permeability heterogeneity inherent to this scale. In the DZ, decametric-scale fracture patterns are 10–65 times more permeable than the centimetric-scale fractures, providing conduits for fluid flow. Finally, the maximum permeability strongly correlates with the product of mean aperture and connectivity, suggesting that these parameters could be used as proxy of the permeability in fault DZ. These results allow better estimation of fault zone permeability, providing constraints for flow modelling in various applications in the energy transition
Eukaryotic phytoplankton drive a decrease in primary production in response to elevated CO 2 in the tropical and subtropical oceans
No full textOcean acidification caused by increasing anthropogenic CO 2 is expected to impact marine phytoplankton productivity, yet the extent and even direction of these changes are not well constrained. Here, we investigate the responses of phytoplankton community composition and productivity to acidification across the western North Pacific. Consistent reductions in primary production were observed under acidified conditions in the North Pacific Subtropical Gyre and the northern South China Sea, whereas no significant changes were found at the northern boundary of the subtropical gyre. While prokaryotic phytoplankton showed little or positive responses to high CO 2 , small (<20 µm) eukaryotic phytoplankton which are primarily limited by low ambient nitrogen drove the observed decrease in community primary production. Extrapolating these results to global tropical and subtropical oceans predicts a potential decrease of about 5 Pg C y −1 in primary production in low Chl- a oligotrophic regions, which are anticipated to experience both acidification and stratification in the future