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Coastal vegetated ecosystems play an important role in carbon cycling and bacterial communities inhabiting coastal sediments are responsible for the remineralization and processing of organic carbon (OC). We collected 1 m-long sediment cores in Posidonia seagrass meadows from coastal and estuarine sites in Australia that differed in their sedimentary organic and inorganic carbon, nitrogen and mud contents. The metabolic diversity of sediment heterotrophic bacterial communities was characterized at different sediment depths, based on the utilization pattern of 31 individual carbon substrates using Biolog EcoPlates™. High metabolic diversity was recorded at both sites, but the carbon substrate utilization rates and the use of carbohydrates were higher at the coastal site compared to the estuarine site. The heterotrophic bacterial community in the coastal sediment appeared to metabolize a more diverse OC pool compared to the estuarine site, which might partly explain the differences in OC storage among the seagrass habitats studied. The Biolog EcoPlates™ provided a useful tool for characte rising the sediment heterotrophic bacterial communities in the meadows and sediment characteristics and biochemical composition of the organic matter played an important role in shaping heterotrophic bacterial communities and their carbon utilization rates, potentially affecting carbon accumulation and preservation within seagrass sediments. [ABSTRACT FROM AUTHOR]

Biotic and abiotic factors influence the accumulation of organic carbon (Corg) in seagrass ecosystems. We surveyed Posidonia sinuosa meadows growing in different water depths to assess the variability in the sources, stocks and accumulation rates of Corg. We show that over the last 500 years, P. sinuosa meadows closer to the upper limit of distribution (at 2-4m depth) accumulated 3- to 4-fold higher Corg stocks (averaging 6.3 kg Corg m-2) at 3- to 4-fold higher rates (12.8 g Corg m-2 yr-1) compared to meadows closer to the deep limits of distribution (at 6-8m depth; 1.8 kg Corg m-2 and 3.6 g Corg m-2 yr-1). In shallower meadows, Corg stocks were mostly derived from seagrass detritus (88% in average) compared to meadows closer to the deep limit of distribution (45% on average). In addition, soil accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0 mm yr-1 and 9 %, respectively) were approximately 2-fold higher than in deeper meadows (1.2 mm yr-1 and 5 %, respectively). The Corg stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kg Corg m-2 and 1.2 g Corg m-2 yr-1) were 3- to 11-fold lower than in P. sinuosa meadows, while fine-grained sediment content (1 %) and seagrass detritus contribution to the Corg pool (20 %) were 8- and 3-fold lower than in Posidonia meadows, respectively. The patterns found support the hypothesis that Corg storage in seagrass soils is influenced by interactions of biological (e.g., meadow productivity, cover and density), chemical (e.g., recalcitrance of Corg stocks) and physical (e.g., hydrodynamic energy and soil accumulation rates) factors within the meadow. We conclude that there is a need to improve global estimates of seagrass carbon storage accounting for biogeochemical factors driving variability within habitats. [ABSTRACT FROM AUTHOR]

The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a fourfold decrease in Corg stocks from 2-4 m to 6-8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m−2, respectively; top meter of sediment) and a fourteenfold to sixteenfold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m−2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m−2 yr−1) than in deeper meadows (2.1 g m−2 yr−1). The reduction of sedimentary Corg stocks and accretion rates along depth-related gradients of light reduction suggests that irradiance, controlling plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691-770 kg m−2 in 8-13 m thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g., eutrophication or sediment regime alterations) will lead to photoacclimation responses (i.e., reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses. [ABSTRACT FROM AUTHOR]

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species. Image 1 • Biogeochemical basis to understand organic preservation in seagrass soils. • Exceptional preservation of P. australis matter in seagrass soils over 1,700 years. • 75% of soil organic matter made of lignin, carbohydrate and non-pyrogenic aromatics. • Selective preservation of aged black carbon-like matter identified as non-pyrogenic aromatics. • Non-selective preservation of carbohydrate, lignin and protein with ageing. [ABSTRACT FROM AUTHOR]

Nitrogen is essential for seagrass productivity but excesses in nitrogen exposure contribute to declines in meadow health. This study reports baseline data of bulk nitrogen loadings and contents in surficial sediments and seagrass tissues to determine the extent of nitrogen inputs in meadows of Sungai Pulai estuary (Johor, Malaysia). The sediment contained relatively low nitrogen loadings (mean range of 91–94 g N m−2) with likely origins from land-based sources. At the meadow-level, Enhalus acoroides, Cymodocea serrulata and Thalassia hemprichii are the most important species as nitrogen sinks. The highest δ15N values of seagrass tissues were recorded for T. hemprichii (10.7 ± 0.4‰), which indicated an elevated capacity for internal recycling of nitrogen. The data demonstrates the provision of ecosystem services by the meadows in mitigating excess nitrogen imported into the estuary. Seagrasses health, however, needs to be at optimum levels for the effectiveness of the meadow as a nutrient sink. Image 1 • Coastal and environmental changes place the meadows at risk to excess N inputs. • Reference baselines into contemporary N loadings are presented. • The meadow location makes it susceptible in receiving N from upstream sources. • Foundation seagrass species balances N recycling and N sink capacity. • Quantifying N at the meadow-scale assists in conceptualising budget models. [ABSTRACT FROM AUTHOR]

The capacity of macroalgae to absorb, hold, and release the relative amounts of carbon (C) and nitrogen (N) in its environment varies. C and N stable isotopes can be used to gain a better understanding of the ecosystem services of nutrient cycling by quantifying its usage in macroalgae. In this study, we conducted an in situ δ13C and δ15N tracer study of the macroalgae Ulva reticulata and Jania adhaerens in the seagrass meadows of Sungai Pulai estuary, Johor. Bulk tissues were labelled with enriched sodium bicarbonate (NaH13CO 3) and ammonium chloride (15NH 4 Cl). Within 1 to 18 days post-enrichment, the macroalgae were measured for C N contents and stable isotope signatures. U. reticulata showed more bulk losses for excess 13C and 15N compared to J. adhaerens. J. adhaerens tended to retain greater %C and %N incorporated in its tissues. There was a clear distinction between the amount of C and N retained and lost in the tissues of each species. The amount of nutrients, especially C, that remained at the end of the experiment demonstrated that both macroalgae species are capable of short-term carbon sequestration, with implications for meadow-scale nutrient subsidies. Higher N uptake and retention in the biomass of the macroalgae, especially in U. reticulata , suggest their relatively high growth capacity. Understanding the C and N dynamics and loss rate of the whole plant provides an estimation of the nutrient uptake and retention capacity of the macroalgae. Such insights are important as a means to formulating steps in preserving the ecological balance of marine ecosystems. [Display omitted] • C and N isotope signatures of Jania adhaerens and Ulva reticulata are reported. • After pulsing with 13C and 15N, these macroalgae were placed in a seagrass meadow. • Variations of δ13C and δ15N in their tissues were measured over 18 days. • J. adhaerens retained more of the initial 13C and 15N tracers than U. reticulata. • C and N changes allow insights into ecological roles of nutrient cycling. [ABSTRACT FROM AUTHOR]

Coralline macroalgae are globally distributed rhodopyhtes that remove carbon from their immediate environment and transform it into carbonate sediments through the senescence of their calcified tissues. In this study, the calcium carbonate (CaCO 3) stocks in the tissue of Jania adhaerens and sediments in Tanjung Adang Shoal, Johor were quantified for a 13-month study period. The detailed maps of the geographical distribution based on the spatial and temporal variations of biomass and CaCO 3 were also assessed. The highest amount of biomass, CaCO 3 and organic carbon (OC) stocks in the tissues showed the highest in May 2018 and May 2019. The biomass values ranged from 65 to 143 g DW m−2, which contained 53–147 g CaCO 3 m−2 and 3–11 g OC m−2. These findings provided insights into the biogeochemical cycling of these inputs, which can be used to estimate the overall carbon budget of the macrophyte meadow. • Monsoon events affect the variability of Jania adhaerens distribution. • Drift accumulation of J. adhaerens was highest during the inter-monsoon season. • Senescence of J. adhaerens contributes to carbonate deposition. • The calcified tissues of J. adhaerens are susceptible to ocean acidification. • Measuring CaCO 3 stocks of J. adhaerens helps in understanding local carbon budgets. [ABSTRACT FROM AUTHOR]