Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes.

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m-2 d-1 in mangroves and 57 ± 104 mmol m-2 d-1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.

Sedimentary organic carbon and nitrogen stocks of intertidal seagrass meadows in a dynamic and impacted wetland: Effects of coastal infrastructure constructions and meadow establishment time.

Seagrass meadows, through their large capacity to sequester and store organic carbon in their sediments, contribute to mitigate climatic change. However, these ecosystems have experienced large losses and degradation worldwide due to anthropogenic and natural impacts and they are among the most threatened ecosystems on Earth. When a meadow is impacted, the vegetation is partial- or completely lost, and the sediment is exposed to the atmosphere or water column, resulting in the erosion and remineralisation of the carbon stored. This paper addresses the effects of the construction of coastal infrastructures on sediment properties, organic carbon, and total nitrogen stocks of intertidal seagrass meadows, as well as the size of such stocks in relation to meadow establishing time (recently and old established meadows). Three intertidal seagrass meadows impacted by coastal constructions (with 0% seagrass cover at present) and three adjacent non-impacted old-established meadows (with 100% seagrass cover at present) were studied along with an area of bare sediment and two recent-established seagrass meadows. We observed that the non-impacted areas presented 3-fold higher percentage of mud and 1.5 times higher sedimentary organic carbon stock than impacted areas. Although the impacted area was relatively small (0.05-0.07 ha), coastal infrastructures caused a significant reduction of the sedimentary carbon stock, between 1.1 and 2.2 Mg OC, and a total loss of the carbon sequestration capacity of the impacted meadow. We also found that the organic carbon stock and total nitrogen stock of the recent-established meadow were 30% lower than those of the old-established ones, indicating that OC and TN accumulation within the meadows is a continuous process, which has important consequences for conservation and restoration actions. These results contribute to understanding the spatial variability of blue carbon and nitrogen stocks in coastal systems highly impacted by urban development.

Interaction between Ammonium Toxicity and Green Tide Development Over Seagrass Meadows: A Laboratory Study.

Eutrophication affects seagrasses negatively by increasing light attenuation through stimulation of biomass of fast-growing, bloom-forming algae and because high concentrations of ammonium in the water can be toxic to higher plants. We hypothesized nevertheless, that moderate amounts of nitrophilic macroalgae that coexists with seagrasses under eutrophic conditions, can alleviate the harmful effects of eutrophication on seagrasses by reducing ammonium concentrations in the seawater to non-toxic levels because such algae have a very large capacity to take up inorganic nutrients. We studied therefore how combinations of different ammonium concentrations (0, 25 and 50 µM) and different standing stocks of macroalgae (i.e. 0, 1 and 6 layers of Ulva sp.) affected survival, growth and net production of the seagrass Zostera noltei. In the absence of Ulva sp., increasing ammonium concentrations had a negative influence on the performance of Z. noltei. The presence of Ulva sp. without ammonium supply had a similar, but slightly smaller, negative effect on seagrass fitness due to light attenuation. When ammonium enrichment was combined with presence of Ulva sp., Ulva sp. ameliorated some of negative effects caused by high ammonium availability although Ulva sp. lowered the availability of light. Benthic microalgae, which increased in biomass during the experiment, seemed to play a similar role as Ulva sp.--they contributed to remove ammonium from the water, and thus, aided to keep the ammonium concentrations experienced by Z. noltei at relatively non-toxic levels. Our findings show that moderate amounts of drift macroalgae, eventually combined with increasing stocks of benthic microalgae, may aid seagrasses to alleviate toxic effects of ammonium under eutrophic conditions, which highlights the importance of high functional diversity for ecosystem resistance to anthropogenic disturbance.

Evidence for a plasmalemma-based CO2 concentrating mechanism in Laminaria saccharina.

A kinetic analysis of the photosynthesis inhibition by buffers allowed quantification of some components of the carbon concentrating mechanism (CCM) of the brown macroalga Laminaria saccharina. The CCM was based on the presence of acid regions outside the plasma membrane that increased the CO(2) concentration available for photosynthesis by 10-20 times above that of the bulk medium at alkaline pH. Furthermore, the results suggested that the CCM is located mainly on the cell membrane and not in the chloroplast, as suggested for most macroalgae. The degree of dissipation of the acid regions by a buffer was related to the buffer anion concentration (B(-)), estimated from the titration of the buffer from bulk medium pH to a pH endpoint value close to the first pK (a) of the carbonic acid system. A kinetic model describing the relationship between inhibition of photosynthesis by a buffer and B(-) was developed suggesting that buffers act as competitive inhibitors with IC(50) (the concentration of the buffer anion which reduces the reaction velocity by half) of 5.0 mol m(-3). This model can be used to estimate the inhibitory effect of any buffer on the photosynthesis of L. saccharina. Nevertheless, some buffers tested showed a lower effect than that predicted from the hyperbolic model suggesting that their strength as inhibitors depended on: (1) the pK (a) in relation to the first pK (a) of the carbonic acid system and (2) its molecular weight (i.e. its mobility).

Growth, carbon allocation and proteolytic activity in the seagrass Zostera noltii shaded by Ulva canopies.

The effects of light reduction [100%, 25%, 10% and 1% mean daily-integrated photon irradiance (I0)] by Ulva rigida C. Agardh canopies on carbon balance, sugar-related enzymes and proteolytic activities of the seagrass Zostera noltii Hornem. were investigated. Shaded plants showed negative net growth and starch was mobilized in both above- and below-ground tissues. Sucrose declined in below-ground parts under severe light deprivation (10% and 1% I0), but was accumulated in above-ground parts. Mobilization of the non-structural carbohydrates (sucrose and starch) was explained by changes in activities of sucrose synthase (SuSy, EC 2.4.1.13) and sucrose-phosphate synthase (SPS, EC 2.4.2.24). Under severe light reduction, the capacity of above-ground tissues for sucrose formation and export declined, indicated by the lowest SPS activity. In contrast, severe light reduction increased the 'sink strength' of below-ground tissues, demonstrated by the highest SuSy activities, and diminished the capacity for sucrose resynthesis from starch breakdown, as the lowest SPS activity was observed under low light. These results suggest a cessation of sucrose transport throughout the plant under extreme light limitation, the carbon supply being dependent on the starch breakdown in each tissue. The response of Z. noltii to gradual light reduction was co-ordinated at the whole-plant level, since an enhancement of proteolytic activities induced by carbon starvation in both above- and below-ground tissues was also recorded during prolonged light deprivation. Therefore, carbon mobilization was accompanied by enhanced protein turnover and changes in metabolic pathways.