Microbial processing of sedimentary organic matter at a shallow LTER site in the northern Adriatic Sea: an 8-year case study

Benthic prokaryotes are the key-players in C-cycling at the sediment-seawater interface, one of the largest biologically active interfaces on Earth. Here, microbial-mediated processes, such as the degradation of organic matter and the incorporation of mobilized C into microbial biomass, depend on several factors such as environmental temperature and substrate availability, especially in shallow sediments at mid-high latitudes where seasonal fluctuations of these variables occur. In the present study, four degradative activities (β-glucosidase, lipase, chitinase and aminopeptidase), Heterotrophic C Production (HCP), Total Organic C (TOC), Total Nitrogen (TN) and Biopolymeric C (BPC) were investigated seasonally from April 2010 to April 2018 in the surface sediments of a shallow Long-Term Ecological Research (LTER) station of the northern Adriatic Sea. Significant temperature-dependences were described by Arrhenius-type equations for HCP and each of the degradative activities tested with the exception of aminopeptidase. The relatively low apparent Activation Energies suggested that these microbial-mediated processes were enhanced by the availability of palatable substrates over the study period. Nevertheless, a clear and tight dependence from such substrates was detected only for aminopeptidase, the most pronounced degradative activity observed. TN was identified by the stepwise multiple regression analysis as the environmental variable that mainly drove this exoenzymatic activity. Enhanced aminopeptidase rates mirrored peaks of TN that seemed, in turn, linked to the seasonal proliferation of benthic microalgae. By supplying prokaryotes with promptly available substrates, these autotrophs, represented mainly by diatoms, seemed to play an important role in the C-cycling regulation at the studied LTER station

Stable carbon isotopes of phytoplankton as a tool to monitor anthropogenic CO2 submarine leakages

This study aims to validate the stable carbon isotopic composition (d13C) of phytoplankton as a tool for detecting submarine leakages of anthropogenic CO2(g), since it is characterised by d13C values significantly lower than the natural CO2 dissolved in oceans. Three culture experiments were carried out to investigate the changes in d13C of the diatom Thalassiosira rotula during growth in an artificially modified medium (ASW). Three different dissolved inorganic carbon (DIC) concentrations were tested to verify if carbon availability affects phytoplankton d13C. Simultaneously, at each experiment, T. rotula was cultured under natural DIC isotopic composition (d13C DIC) and carbonate system conditions. The available DIC pool for diatoms grown in ASW was characterised by d13C DIC values (-44.2 ± 0.9‰) significantly lower than the typical marine range. Through photosynthetic DIC uptake, microalgae d13C rapidly changed, reaching significantly low values (until -43.4‰). Moreover, the different DIC concentrations did not affect the diatom d13C, exhibiting the same trend in d13C values in the three ASW experiments. The experiments prove that phytoplankton isotopic composition quickly responds to changes in the d13C of the medium, making this approach a promising and low-impact tool for detecting CO2(g) submarine leakages from CO2(g) deposits

Carbon cycle in marine systems under different anthropogenic pressure

During the past two centuries, human activities have greatly modified the exchange of carbon and nutrients between land, atmosphere, freshwater bodies, coastal zones and open ocean. This PhD thesis focuses on different aspects of carbon cycle, considering three case study for the evaluation of the effects of anthropogenic pressure and of the natural evolution of carbon cycle. The first case study focuses on anthropogenic impacts on contaminated coastal areas. These environments connect terrestrial and oceanic systems, and are highly subjected to anthropogenic pressure. An example of a contaminated coastal area is the Mar Piccolo of Taranto (Ionian Sea) and the results of two published papers are reported. In the first paper, the results of the analysis of nutrients and carbon fluxes at sediment-water interface showed that the multi-contamination of both inorganic and organic pollutants in the sediments is potentially transferable to the water column and to the aquatic trophic chain. On the other hand, in the second paper, the results of the analysis of a long time series of chemical-physical characteristics of the water column highlighted that the implementation of sewage treatment plants has positively affected the trophic status of the Mar Piccolo from being relatively eutrophic to moderately oligotrophic. The increase of atmospheric CO2 concentration has been recognised as one of the main causes of climate change. Carbon capture and storage technology (CCS) is expected to play a key role among mitigation strategies by reducing CO2 emissions into atmosphere from fossil fuel combustion. Although leakages from well-engineered storage sites are not expected, the environmental impacts related to potential CO2 seepages are a major issue for the acceptance of this approach and should be carefully monitored. Since different carbon sources have different \u3b413C values, the aim of the second case study is the evaluation of phytoplankton stable carbon isotopes as a tool for effective early warning of CO2 leakage from CCS. Two culture experiments were conducted under controlled conditions for monitoring \u3b413C changes in the diatom Thalassiosira rotula during growth in two different media. The isotopic composition of microalgae grown in natural seawater (NAT) was compared to that of diatoms grown in an artificial seawater (ASW), supplied with industrial CO2 and characterised by strongly 13C-depleted dissolved inorganic carbon values. The uptake of inorganic carbon in ASW resulted in a rapid and significant change in microalgae \u3b413C values, whereas in NAT phytoplankton \u3b413C did not show important deviations from the starting value, confirming the effectiveness of phytoplankton \u3b413C as a tool for detecting different CO2 sources. The third case study aims at a better understanding of carbon cycle, as the rate of changes not only depends on human activities, but also on natural biogeochemical processes. Marine dissolved organic matter (DOM) plays a crucial role in oceanic carbon storage, and the extent of its contribution on carbon sequestration depends on its bioavailability. In this study, a plug-flow bioreactor approach has been tested in order to evaluate DOM availability for microbial community in the Gulf of Trieste (Northern Adriatic Sea) by the analysis of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) degradation and nutrient uptake. The bioreactor approach confirmed to be useful for bioavailable DOC assessment, but further research is needed for confirming its effectiveness for defining DON and DOP bioavailability and nutrient utilisation

Microbial processing of sedimentary organic matter at a shallow LTER site in the northern Adriatic Sea: an 8-year case study

Benthic prokaryotes are the key-players in C-cycling at the sediment-seawater interface, one of the largest biologically active interfaces on Earth. Here, microbial-mediated processes, such as the degradation of organic matter and the incorporation of mobilized C into microbial biomass, depend on several factors such as environmental temperature and substrate availability, especially in shallow sediments at mid-high latitudes where seasonal fluctuations of these variables occur. In the present study, four degradative activities (β-glucosidase, lipase, chitinase and aminopeptidase), Heterotrophic C Production (HCP), Total Organic C (TOC), Total Nitrogen (TN) and Biopolymeric C (BPC) were investigated seasonally from April 2010 to April 2018 in the surface sediments of a shallow Long-Term Ecological Research (LTER) station of the northern Adriatic Sea. Significant temperature-dependences were described by Arrhenius-type equations for HCP and each of the degradative activities tested with the exception of aminopeptidase. The relatively low apparent Activation Energies suggested that these microbial-mediated processes were enhanced by the availability of palatable substrates over the study period. Nevertheless, a clear and tight dependence from such substrates was detected only for aminopeptidase, the most pronounced degradative activity observed. TN was identified by the stepwise multiple regression analysis as the environmental variable that mainly drove this exoenzymatic activity. Enhanced aminopeptidase rates mirrored peaks of TN that seemed, in turn, linked to the seasonal proliferation of benthic microalgae. By supplying prokaryotes with promptly available substrates, these autotrophs, represented mainly by diatoms, seemed to play an important role in the C-cycling regulation at the studied LTER station

Taxonomic and functional macrofaunal diversity along a gradient of sewage contamination: A three-year study

International audienceWe investigated the structural and functional changes of the soft-bottom macrofaunal community following the improvement of a wastewater treatment-WWT plant. The macrofauna was collected at increasing distance from the main outfall in 2018, 2019, and 2021. Organic matter and nutrients were analysed in the water column near the outfalls to detect possible changes due to the improved treatment. We examined Functional Entities-FEs (i.e. a unique combination of species functional traits), species richness, Shannon-Wiener diversity-H′, and taxonomic and functional β-diversity. From 2018 (before the year of the treatment change), to 2021, we noted a gradual decrease of organic carbon in the water column. In contrast, sediment characteristics (i.e. grain-size) did not change before and after treatment enhancement, with the exception of redox potential. Species richness and FEs gradually increased moving far from the source of organic contamination and after wastewater treatment enhancement, especially near the outfall. We observed different phases of macrofaunal succession stage after the WWT amelioration. A ‘normal stage’, i.e. slightly lower species richness, was reflected in decreasing functional richness. Higher taxonomic β-diversity values with significant turnover components indicated that the community was subjected to broad changes in species composition. However, functional β-diversity did not follow the same pattern. After treatment improvement, modified environmental conditions led to the establishment of new species, but with the same functions. Towards 2021, the community improved its resilience by increasing functional redundancy and reduction of vulnerability, which enhanced community stability. The latter was also reflected in the well-balanced proportion of macrofaunal feeding habits after the WWT upgrade. Integrating the classical taxonomic approach with the analysis of FEs, and environmental characteristics can provide an accurate insight into macrofauna sensitivity to stressors that are likely to lead to changes in the ecological state of an area

Oxygen, carbon, and nutrient exchanges at the sediment\u2013water interface in the Mar Piccolo of Taranto (Ionian Sea, southern Italy)

In the shallow environment, the nutrient and carbon exchanges at the sediment\u2013water interface contribute significantly to determine the trophic status of the whole water column. The intensity of the allochthonous input in a coastal environment subjected to strong anthropogenic pressures determines an increase in the benthic oxygen demand leading to depressed oxygen levels in the bottom waters. Anoxic conditions resulting from organic enrichment can enhance the exchange of nutrients between sediments and the overlying water. In the present study, carbon and nutrient fluxes at the sediment\u2013water interface were measured at two experimental sites, one highly and one moderately contaminated, as reference point. In situ benthic flux measurements of dissolved species (O2, DIC, DOC, N-NO3 12, N-NO2 12, N-NH4+, P-PO43 12, Si-Si(OH)4, H2S) were conducted using benthic chambers. Furthermore, undisturbed sediment cores were collected for analyses of total and organic C, total N, and biopolymeric carbon (carbohydrates, proteins, and lipids) as well as of dissolved species in porewaters and supernatant in order to calculate the diffusive fluxes. The sediments were characterized by suboxic to anoxic conditions with redox values more negative in the highly contaminated site, which was also characterized by higher biopolymeric carbon content (most of all lipids), lower C/N ratios and generally higher diffusive fluxes, which could result in a higher release of contaminants. A great difference was observed between diffusive and in situ benthic fluxes suggesting the enhancing of fluxes by bioturbation and the occurrence of biogeochemically important processes at the sediment\u2013water interface. The multi-contamination of both inorganic and organic pollutants, in the sediments of the Mar Piccolo of Taranto (declared SIN in 1998), potentially transferable to the water column and to the aquatic trophic chain, is of serious concern for its ecological relevance, also considering the widespread fishing and mussel farming activities in the area

Structural and Functional Analyses of Motile Fauna Associated with Cystoseira brachycarpa along a Gradient of Ocean Acidification in a CO2-Vent System off Panarea (Aeolian Islands, Italy)

Ocean acidification (OA), one of the main climate-change-related stressors linked to increasing CO2 concentration in the atmosphere, is considered an important threat to marine biodiversity and habitats. Studies on CO2-vents systems, naturally acidified environments that mimic future ocean scenarios, help to explore the sensitivity of species and to understand how benthic communities rearrange their structure and functioning under the pressure of OA. We addressed this problem by studying the benthic invertebrates associated with a habitat-forming brown alga (Cystoseira brachycarpa) in the Bottaro crater vents system off Panarea island (Tyrrhenian Sea, Italy), by sampling along an OA gradient from the proximity of the main venting area (station B3, pH 7.9) to a control zone (B1 station, pH 8.1). Samples were collected in September 2016 and 2018. A total of 184 taxa and 23 different functional traits have been identified, considering feeding habit, motility, size, reproductive and developmental biology, and occurrence of calcareous structures. Invertebrates are distributed according to the distance from the high venting zone and low pH levels and results very consistent between the two investigated years. In the low-pH area (B3), 43% of the species are selected. The functional traits of the fauna mirror this zonation pattern, mainly changing the relative proportion of the number of individuals of the various functional guilds along the OA gradient. Invertebrates inhabiting the low-pH zone are mainly composed of weakly or non-calcified species, with small size, burrower/tubicolous habit, omnivorous or suspension feeders, and with direct development and brooding habit. In the other stations, heavily calcified forms, herbivore and herbivore/detritivore, and with medium (1–5 cm) and large (>5 cm) sizes prevail, showing indirect benthic and planktic development. The taxonomic analysis, coupled with functional aspects, increases our prediction of which traits could be potentially more advantageous for species to adapt to the hypothesized scenarios of OA, and identify present and future winner and/or loser organisms in the future ocean of the Anthropocene

Structural and Functional Analyses of Motile Fauna Associated with <i>Cystoseira brachycarpa</i> along a Gradient of Ocean Acidification in a CO₂-Vent System off Panarea (Aeolian Islands, Italy)

Ocean acidification (OA), one of the main climate-change-related stressors linked to increasing CO2 concentration in the atmosphere, is considered an important threat to marine biodiversity and habitats. Studies on CO2-vents systems, naturally acidified environments that mimic future ocean scenarios, help to explore the sensitivity of species and to understand how benthic communities rearrange their structure and functioning under the pressure of OA. We addressed this problem by studying the benthic invertebrates associated with a habitat-forming brown alga (Cystoseira brachycarpa) in the Bottaro crater vents system off Panarea island (Tyrrhenian Sea, Italy), by sampling along an OA gradient from the proximity of the main venting area (station B3, pH 7.9) to a control zone (B1 station, pH 8.1). Samples were collected in September 2016 and 2018. A total of 184 taxa and 23 different functional traits have been identified, considering feeding habit, motility, size, reproductive and developmental biology, and occurrence of calcareous structures. Invertebrates are distributed according to the distance from the high venting zone and low pH levels and results very consistent between the two investigated years. In the low-pH area (B3), 43% of the species are selected. The functional traits of the fauna mirror this zonation pattern, mainly changing the relative proportion of the number of individuals of the various functional guilds along the OA gradient. Invertebrates inhabiting the low-pH zone are mainly composed of weakly or non-calcified species, with small size, burrower/tubicolous habit, omnivorous or suspension feeders, and with direct development and brooding habit. In the other stations, heavily calcified forms, herbivore and herbivore/detritivore, and with medium (1–5 cm) and large (>5 cm) sizes prevail, showing indirect benthic and planktic development. The taxonomic analysis, coupled with functional aspects, increases our prediction of which traits could be potentially more advantageous for species to adapt to the hypothesized scenarios of OA, and identify present and future winner and/or loser organisms in the future ocean of the Anthropocene