Alkalinity production in intertidal sands intensified by lugworm bioirrigation

Biogeochemical impacts of fish farming on coastal sediments: Insights into the functional role of cable bacteria

Fish farming in sea cages is a growing component of the global food industry. A prominent ecosystem impact of this industry is the increase in the downward flux of organic matter, which stimulates anaerobic mineralization and sulfide production in underlying sediments. When free sulfide is released to the overlying water, this can have a toxic effect on local marine ecosystems. The microbially-mediated process of sulfide oxidation has the potential to be an important natural mitigation and prevention strategy that has not been studied in fish farm sediments. We examined the microbial community composition (DNA-based 16S rRNA gene) underneath two active fish farms on the Southwestern coast of Iceland and performed laboratory incubations of resident sediment. Field observations confirmed the strong geochemical impact of fish farming on the sediment (up to 150 m away from cages). Sulfide accumulation was evidenced under the cages congruent with a higher supply of degradable organic matter from the cages. Phylogenetically diverse microbes capable of sulfide detoxification were present in the field sediment as well as in lab incubations, including cable bacteria (Candidatus Electrothrix), which display a unique metabolism based on long-distance electron transport. Microsensor profiling revealed that the activity of cable bacteria did not exert a dominant impact on the geochemistry of fish farm sediment at the time of sampling. However, laboratory incubations that mimic the recovery process during fallowing, revealed successful enrichment of cable bacteria within weeks, with concomitant high sulfur-oxidizing activity. Overall our results give insight into the role of microbially-mediated sulfide detoxification in aquaculture impacted sediments.

Marine heatwaves of differing intensities lead to distinct patterns in seafloor functioning

Marine heatwaves (MHWs) are increasing in frequency and intensity due to climate change. Several well-documented effects of heatwaves on community structure exist, but examples of their effect on functioning of species, communities or ecosystems remain scarce. We tested the effects of short-term, moderate and strong MHWs on macrofauna bioturbation and associated solute fluxes as examples of ecosystem functioning. We also measured macrofaunal excretion rates to assess effects of temperature on macrofauna metabolism. For this experiment, we used unmanipulated sediment cores with natural animal communities collected from a muddy location at 32 m depth in the northern Baltic Sea. Despite the mechanistic effect of bioturbation remaining unchanged between the treatments, there were significant differences in oxygen consumption, solute fluxes and excretion. Biogeochemical and biological processes were boosted by the moderate heatwave, whereas biogeochemical cycling seemed to decrease under a strong heatwave. A prolonged, moderate heatwave could possibly lead to resource depletion if primary production cannot meet the demands of benthic consumption. By contrast, decreased degradation activities under strong heatwaves could lead to a build-up of organic material and potentially hypoxia. The strong variability and the complexity of the response highlight the context dependency of these processes complicating future predictions.

Benthos response to nutrient enrichment and functional consequences in coastal ecosystems

As land use intensifies, many coastal waters are becoming enriched with otherwise limiting nutrients, leading to eutrophication. While the extreme effects of eutrophication on benthic communities are well documented, there is still a lack of knowledge about how nutrient enrichment alters biogeochemical interactions occurring at the sediment-water interface. Using ex-situ experiments, this study explores the consequences of nutrient enrichment on sediment characteristics, macrofauna community and benthic fluxes. The quantity of sedimentary organic matter and porewater concentration of NH₄⁺, NO_x and PO₄^3- increased in enriched treatments. These changes did not affect the macrobenthic community structure. However, macroinfauna buried less deep and increased their ventilation activity. As consequences, nutrient efflux increased, thereby favouring eutrophication processes. These effects were reduced in presence of seagrass, thus illustrating the buffering capacity of seagrass in the context of environmental changes, and particularly, of eutrophication. Overall, this study highlights that the functional consequences of nutrient enrichment involve interconnected processes that are variable in space and time.

Effects of sulfide availability on the metabolic activity and population dynamics of cable bacteria in freshwater sediment

Cable bacteria occur in many natural environments, and their electrogenic sulfide oxidation (e-SO_x) may influence sediment biogeochemistry. The environmental factors determining the growth and diversity of cable bacteria are poorly known, especially in freshwater sediments. We conducted a laboratory incubation experiment, using freshwater sediments with different sulfide supply levels, to study how sulfide availability in sediment affects the metabolic activity and population dynamics of cable bacteria. A moderate increase in the sulfide availability in sediment significantly promoted metabolic activity and the proliferation of the cable bacteria population, as revealed by enhanced e-SO_x intensity and increased bacteria abundance. In high-sulfide treatments there was a more significant increase in the population of cable bacteria in the deeper sediment layers, indicating that increased sulfide availability may expand the vertical scale impact of cable bacteria activities on sediment biogeochemistry. The relative proportions of co-existing species in the cable bacteria population also changed with sulfide supply levels, indicating that sulfide availability can be involved in determining the interspecies relationships of cable bacteria. Our findings provide new insight into the relationship between sediment sulfide availability and the growth, depth distribution, and species composition of cable bacteria, implying the consideration of regulating environmental sulfide availability as a potential management practice for the development of cable bacteria-based environmental biotechnologies.

Available acid consumption capacity of sediments in six water bodies in the Yangtze River Basin in China

Acid-base reactivity is a fundamental property of sediments and is responsible for sediments' multiple roles in aquatic ecosystems. However, little information currently exists about the composition, magnitude, and change of the available acid consumption capacity (AACC) of sediments. To optimize reaction conditions, we developed operational procedures to determine AACC using base titration to recover surplus acid in suspensions. We characterized the sediment AACC of Dianchi Lake (DL), Daduhe River (DR), Tuojiang River (TR), Honghu Lake (HL), Wuhan Donghu Lake (DhL), and Taihu Lake (TL) in the Yangtze River Basin, China. The procedure demonstrated that reacting 40 mL 0.1 M HCl with fresh sediments equivalent to 1.0 g dry weight for 4 h and recovering surplus acid in the suspension by NaOH titration to an endpoint pH of 3.0 could determine sediment AACC. Sediment AACC in the Yangtze River Basin had high regional variability. The mean magnitude of AACC among sites was ranked DL > DR > DhL > TR > HL > TL, which is extremely similar to their geographical location from the upper to lower reaches of the Yangtze River Basin. Qualitative results from acid titration curves showed that more components contributed to AACC in DL, DR, TR, and DhL sediments than to those in HL and TL sediments. The correlation between AACC and the total amount of multivalent cations released indicated that AACC depended significantly on labile acid-soluble minerals that contain multivalent cations (Fe³⁺, Fe²⁺, Ca²⁺, Al³⁺, Mg²⁺, and Mn²⁺) (p < 0.01). Based on the contribution percentages of multivalent cations to AACC, sediment AACC of six water bodies were divided into two types: Ca-Mg dominated (DL, DR, and TR) and Fe-Al dominated (HL, DhL, and TL). We suggest that sediment AACC complexing with pH can contribute to a better description of the acid-base characteristics of sediments.

Body size affects lethal and sublethal responses to organic enrichment: Evidence of associational susceptibility for an infaunal bivalve

Eutrophication is an increasing problem worldwide and can disrupt ecosystem processes in which macrobenthic bioturbators play an essential role. This study explores how intraspecific variation in body size affects the survival, mobility and impact on sediment organic matter breakdown in enriched sediments of an infaunal bivalve. A mesocosm experiment was conducted in which monocultures and all size combinations of three body sizes (small, medium and large) of the Sydney cockle, Anadara trapezia, were exposed to natural or organically enriched sediments. Results demonstrate that larger body sizes have higher tolerance to enriched conditions and can reduce survival of smaller cockles when grown together. Also, large A. trapezia influenced sediment organic matter breakdown although a direct link to bioturbation activity was not clear. Overall, this study found that intraspecific variation in body size influences survival and performance of bioturbators in eutrophic scenarios.

Calcium carbonate alters the functional response of coastal sediments to eutrophication-induced acidification

Coastal ocean acidification research is dominated by laboratory-based studies that cannot necessarily predict real-world ecosystem response given its complexity. We enriched coastal sediments with increasing quantities of organic matter in the field to identify the effects of eutrophication-induced acidification on benthic structure and function, and assess whether biogenic calcium carbonate (CaCO₃) would alter the response. Along the eutrophication gradient we observed declines in macrofauna biodiversity and impaired benthic net primary productivity and sediment nutrient cycling. CaCO₃ addition did not alter the macrofauna community response, but significantly dampened negative effects on function (e.g. net autotrophy occurred at higher levels of organic matter enrichment in +CaCO₃ treatments than −CaCO₃ (1400 vs 950 g dw m⁻²)). By identifying the links between eutrophication, sediment biogeochemistry and benthic ecosystem structure and function in situ, our study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO₃ in moderating responses.

The application of bioturbators for aquatic bioremediation: Review and meta-analysis

Human activities introduce significant contamination into aquatic systems that impact biodiversity and ecosystem function. Many contaminants accumulate, and remediation options are now required worldwide. One method for bioremediation involves the application of macrofauna to stimulate microbial ecosystem processes including contaminant removal. However, if we are to confidently apply such a technique, we need clarity on the effect of bioturbators on different contaminants and how these vary under different environmental scenarios. Here we used a systematic review and meta-analysis to analyse current knowledge on the activities of bioturbating macrofauna in contaminated sediments and quantify how bioturbation-bioremediation changes depend on the taxonomic group, the aquatic ecosystem and important environmental variables. Three common contaminant classes were reviewed and analysed: metals, nutrients (i.e. ammonia and phosphorous) and polycyclic aromatic hydrocarbons (PAH). In addition, meta-regressions were calculated to estimate the effect of environmental and experimental design variables on effect sizes. Meta-analytic results revealed that deeper burrowing and more active sediment surface animals (e.g. polychaetes) increased metal release from sediments, nutrients and oxygen uptake by microbial fractions in comparison to bioturbators that inhabit shallower depths in sediments. In addition, there was a different effect of bioturbators on response variables in different aquatic systems. Finally, bioturbator effects on nutrient and metal release appeared modulated by context-specific variables such as temperature, pH, sediment grain size, animal density and experimental duration. Our findings highlight critical knowledge gaps such as field applications, less studied macrobenthic fauna and the incorporation of molecular approaches. Our results provide the first quantitative synthesis of the effects of bioturbators on contaminant fate and the variables that need to be considered for the optimization of this method as a viable approach for sediment remediation and contaminant management in aquatic systems.

Increasing densities of an invasive polychaete enhance bioturbation with variable effects on solute fluxes

Bioturbation is a key process affecting nutrient cycling in soft sediments. The invasive polychaete genus Marenzelleria spp. has established successfully throughout the Baltic Sea increasing species and functional diversity with possible density-dependent effects on bioturbation and associated solute fluxes. We tested the effects of increasing density of M. arctia, M. viridis and M. neglecta on bioturbation and solute fluxes in a laboratory experiment. Benthic communities in intact sediment cores were manipulated by adding increasing numbers of Marenzelleria spp. The results showed that Marenzelleria spp. in general enhanced all bioturbation metrics, but the effects on solute fluxes varied depending on the solute, on the density and species identity of Marenzelleria, and on the species and functional composition of the surrounding community. M. viridis and M. neglecta were more important in predicting variation in phosphate and silicate fluxes, whereas M. arctia had a larger effect on nitrogen cycling. The complex direct and indirect pathways indicate the importance of considering the whole community and not just species in isolation in the experimental studies. Including these interactions provides a way forward regarding our understanding of the complex ecosystem effects of invasive species.

Olivine Dissolution in Seawater: Implications for CO2 Sequestration through Enhanced Weathering in Coastal Environments

Enhanced weathering of (ultra)basic silicate rocks such as olivine-rich dunite has been proposed as a large-scale climate engineering approach. When implemented in coastal environments, olivine weathering is expected to increase seawater alkalinity, thus resulting in additional CO₂ uptake from the atmosphere. However, the mechanisms of marine olivine weathering and its effect on seawater-carbonate chemistry remain poorly understood. Here, we present results from batch reaction experiments, in which forsteritic olivine was subjected to rotational agitation in different seawater media for periods of days to months. Olivine dissolution caused a significant increase in alkalinity of the seawater with a consequent DIC increase due to CO₂ invasion, thus confirming viability of the basic concept of enhanced silicate weathering. However, our experiments also identified several important challenges with respect to the detailed quantification of the CO₂ sequestration efficiency under field conditions, which include nonstoichiometric dissolution, potential pore water saturation in the seabed, and the potential occurrence of secondary reactions. Before enhanced weathering of olivine in coastal environments can be considered an option for realizing negative CO₂ emissions for climate mitigation purposes, these aspects need further experimental assessment.

Electrogenic sulfur oxidation in a northern saltmarsh (St. Lawrence Estuary, Canada)

Measurements of porewater O2, pH, and H2S microprofiles in intact sediment cores collected in a northern saltmarsh in the St. Lawrence Estuary (Quebec, Canada) revealed the occurrence of electrogenic sulfur oxidation (e-SOx) by filamentous “cable” bacteria in submerged marsh pond sediments in the high marsh. In summer, the geochemical fingerprint of e-SOx was apparent in intact cores, while in fall, cable bacteria were detected by fluorescence in situ hybridization and the characteristic geochemical signature of e-SOx was observed only upon prolonged incubation. In exposed, unvegetated creek bank sediments sampled in the low marsh in summer, cable bacteria developed only in repacked cores of sieved (500 μm), homogenized sediments. These results suggest that e-SOx is suppressed by the activity of macrofauna in exposed, unvegetated marsh sediments. A reduced abundance of benthic invertebrates may promote e-SOx development in marsh ponds, which are dominant features of subarctic saltmarshes as in the St. Lawrence Estuary.

Effects of Bioadvection by Arenicola marina on Microphytobenthos in Permeable Sediments

We used hyperspectral imaging to study short-term effects of bioturbation by lugworms (Arenicola marina) on the surficial biomass of microphytobenthos (MPB) in permeable marine sediments. Within days to weeks after the addition of a lugworm to a homogenized and recomposed sediment, the average surficial MPB biomass and its spatial heterogeneity were, respectively, 150-250% and 280% higher than in sediments without lugworms. The surficial sediment area impacted by a single medium-sized lugworm (~4 g wet weight) over this time-scale was at least 340 cm2. While sediment reworking was the primary cause of the increased spatial heterogeneity, experiments with lugworm-mimics together with modeling showed that bioadvective porewater transport from depth to the sediment surface, as induced by the lugworm ventilating its burrow, was the main cause of the increased surficial MPB biomass. Although direct measurements of nutrient fluxes are lacking, our present data show that enhanced advective supply of nutrients from deeper sediment layers induced by faunal ventilation is an important mechanism that fuels high primary productivity at the surface of permeable sediments even though these systems are generally characterized by low standing stocks of nutrients and organic material.

Rapid redox signal transmission by "Cable Bacteria" beneath a photosynthetic biofilm

Recently, long filamentous bacteria, belonging to the family Desulfobulbaceae, were shown to induce electrical currents over long distances in the surface layer of marine sediments. These "cable bacteria" are capable of harvesting electrons from free sulfide in deeper sediment horizons and transferring these electrons along their longitudinal axes to oxygen present near the sediment-water interface. In the present work, we investigated the relationship between cable bacteria and a photosynthetic algal biofilm. In a first experiment, we investigated sediment that hosted both cable bacteria and a photosynthetic biofilm and tested the effect of an imposed diel light-dark cycle by continuously monitoring sulfide at depth. Changes in photosynthesis at the sediment surface had an immediate and repeatable effect on sulfide concentrations at depth, indicating that cable bacteria can rapidly transmit a geochemical effect to centimeters of depth in response to changing conditions at the sediment surface. We also observed a secondary response of the free sulfide at depth manifest on the time scale of hours, suggesting that cable bacteria adjust to a moving oxygen front with a regulatory or a behavioral response, such as motility. Finally, we show that on the time scale of days, the presence of an oxygenic biofilm results in a deeper and more acidic suboxic zone, indicating that a greater oxygen supply can enable cable bacteria to harvest a greater quantity of electrons from marine sediments. Rapid acclimation strategies and highly efficient electron harvesting are likely key advantages of cable bacteria, enabling their success in high sulfide generating coastal sediments.