Operationalizing blue carbon principles in France: Methodological developments for Posidonia oceanica seagrass meadows and institutionalization.

Conservation of ecosystems is an important tool for climate change mitigation. Seagrasses, mangroves, saltmarshes and other marine ecosystems have particularly high capacities to sequester and store organic carbon (blue carbon), and are being impacted by human activities. Calls have been made to mainstream blue carbon into policies, including carbon markets. Building on the scientific literature and the French voluntary carbon standard, the 'Label Bas-Carbone', we develop the first method for the conservation of Posidonia oceanica seagrasses using carbon finance. This methodology assesses the emission reduction potential of projects that reduce physical impacts from boating and anchoring. We show how this methodology was institutionalized thanks to a tiered approach on key parameters including carbon stocks, degradation rates, and decomposition rates. We discuss future needs regarding (i) how to strengthen the robustness of the method, and (ii) the expansion of the method to restoration of seagrasses and to other blue carbon ecosystems.

Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification.

Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6-17.4 with the greatest effect at CO2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

Marine heatwave and reduced light scenarios cause species-specific metabolomic changes in seagrasses under ocean warming.

Climate change and extreme climatic events, such as marine heatwaves (MHWs), are threatening seagrass ecosystems. Metabolomics can be used to gain insight into early stress responses in seagrasses and help to develop targeted management and conservation measures. We used metabolomics to understand the temporal and mechanistic response of leaf metabolism in seagrasses to climate change. Two species, temperate Posidonia australis and tropical Halodule uninervis, were exposed to a combination of future warming, simulated MHW with subsequent recovery period, and light deprivation in a mesocosm experiment. The leaf metabolome of P. australis was altered under MHW exposure at ambient light while H. uninervis was unaffected. Light deprivation impacted both seagrasses, with combined effects of heat and low light causing greater alterations in leaf metabolism. There was no MHW recovery in P. australis. Conversely, the heat-resistant leaf metabolome of H. uninervis showed recovery of sugars and intermediates of the tricarboxylic acid cycle under combined heat and low light exposure, suggesting adaptive strategies to long-term light deprivation. Overall, this research highlights how metabolomics can be used to study the metabolic pathways of seagrasses, identifies early indicators of environmental stress and analyses the effects of environmental factors on plant metabolism and health.

Mapping and assessing seagrass meadows changes and blue carbon under past, current, and future scenarios.

Seagrasses store large amounts of blue carbon and mitigate climate change, but they have suffered strong regressions worldwide in recent decades. Blue carbon assessments may support their conservation. However, existing blue carbon maps are still scarce and focused on certain seagrass species, such as the iconic genus Posidonia, and intertidal and very shallow seagrasses (<10 m depth), while deep-water and opportunistic seagrasses have remained understudied. This study filled this gap by mapping and assessing blue carbon storage and sequestration by the seagrass Cymodocea nodosa in the Canarian archipelago using the local carbon storage capacity and high spatial resolution (20 m/pixel) seagrass distribution maps for the years 2000 and 2018. Particularly, we mapped and assessed the past, current and future capacity of C. nodosa to store blue carbon, according to four plausible future scenarios, and valued the economic implications of these scenarios. Our results showed that C. nodosa has suffered ca. 50 % area loss in the last two decades, and, if the current degradation rate continues, our estimations demonstrate that it could completely disappear in 2036 ("Collapse scenario"). The impact of these losses in 2050 would reach 1.43 MT of CO2 equivalent emitted with a cost of 126.3 million € (0.32 % of the current Canary GDP). If, however, this degradation is slow down, between 0.11 and 0.57 MT of CO2 equivalent would be emitted until 2050 ("Intermediate" and "Business-as-usual" scenarios, respectively), which corresponds to a social cost of 3.63 and 44.81 million €, respectively. If the current seagrass extension is maintained ("No Net Loss"), 0.75 MT of CO2 equivalent would be sequestered from now to 2050, which corresponds to a social cost saving of 73.59 million €. The reproducibility of our methodology across coastal ecosystems underpinned by marine vegetation provides a key tool for decision-making and conservation of these habitats.

Diverse methylotrophic methanogenic archaea cause high methane emissions from seagrass meadows.

Marine coastlines colonized by seagrasses are a net source of methane to the atmosphere. However, methane emissions from these environments are still poorly constrained, and the underlying processes and responsible microorganisms remain largely unknown. Here, we investigated methane turnover in seagrass meadows of Posidonia oceanica in the Mediterranean Sea. The underlying sediments exhibited median net fluxes of methane into the water column of ca. 106 µmol CH4 ⋅ m-2 ⋅ d-1 Our data show that this methane production was sustained by methylated compounds produced by the plant, rather than by fermentation of buried organic carbon. Interestingly, methane production was maintained long after the living plant died off, likely due to the persistence of methylated compounds, such as choline, betaines, and dimethylsulfoniopropionate, in detached plant leaves and rhizomes. We recovered multiple mcrA gene sequences, encoding for methyl-coenzyme M reductase (Mcr), the key methanogenic enzyme, from the seagrass sediments. Most retrieved mcrA gene sequences were affiliated with a clade of divergent Mcr and belonged to the uncultured Candidatus Helarchaeota of the Asgard superphylum, suggesting a possible involvement of these divergent Mcr in methane metabolism. Taken together, our findings identify the mechanisms controlling methane emissions from these important blue carbon ecosystems.

Terrestrial-type nitrogen-fixing symbiosis between seagrass and a marine bacterium.

Symbiotic N2-fixing microorganisms have a crucial role in the assimilation of nitrogen by eukaryotes in nitrogen-limited environments1-3. Particularly among land plants, N2-fixing symbionts occur in a variety of distantly related plant lineages and often involve an intimate association between host and symbiont2,4. Descriptions of such intimate symbioses are lacking for seagrasses, which evolved around 100 million years ago from terrestrial flowering plants that migrated back to the sea5. Here we describe an N2-fixing symbiont, 'Candidatus Celerinatantimonas neptuna', that lives inside seagrass root tissue, where it provides ammonia and amino acids to its host in exchange for sugars. As such, this symbiosis is reminiscent of terrestrial N2-fixing plant symbioses. The symbiosis between Ca. C. neptuna and its host Posidonia oceanica enables highly productive seagrass meadows to thrive in the nitrogen-limited Mediterranean Sea. Relatives of Ca. C. neptuna occur worldwide in coastal ecosystems, in which they may form similar symbioses with other seagrasses and saltmarsh plants. Just like N2-fixing microorganisms might have aided the colonization of nitrogen-poor soils by early land plants6, the ancestors of Ca. C. neptuna and its relatives probably enabled flowering plants to invade nitrogen-poor marine habitats, where they formed extremely efficient blue carbon ecosystems7.

Seagrass Posidonia is impaired by human-generated noise.

The last hundred years have seen the introduction of many sources of artificial noise in the sea environment which have shown to negatively affect marine organisms. Little attention has been devoted to how much this noise could affect sessile organisms. Here, we report morphological and ultrastructural changes in seagrass, after exposure to sounds in a controlled environment. These results are new to aquatic plants pathology. Low-frequency sounds produced alterations in Posidonia oceanica root and rhizome statocysts, which sense gravity and process sound vibration. Nutritional processes of the plant were affected as well: we observed a decrease in the number of rhizome starch grains, which have a vital role in energy storage, as well as a degradation in the specific fungal symbionts of P. oceanica roots. This sensitivity to artificial sounds revealed how sound can potentially affect the health status of P. oceanica. Moreover, these findings address the question of how much the increase of ocean noise pollution may contribute in the future to the depletion of seagrass populations and to biodiversity loss.

Assessment of Performance of Posidona oceanica (L.) as Biosorbent for Crude Oil-Spill Cleanup in Seawater.

The marine environment is constantly at risk of pollution by hydrocarbon spills that requires its cleanup to protect the environment and human health. Posidonia oceanica (L.) (PO) beach balls, which are characteristic of the Mediterranean Sea and abundant on the beaches, are used as biosorbent to remove hydrocarbons from the sea. The impact of several factors such as oil concentration, time sorption, and weight sorbent was investigated to determine the oil and water sorption capacity for raw and milled P. oceanica fibers. The study of kinetic models for initial crude oil concentration of 2.5, 5, 8.8, 10, 15, 20, 30, and 40 g/L revealed that crude uptake followed the pseudo-first-order model while, for isotherm models, the crude uptake onto the P. oceanica tended to fit the Langmuir model. Experiments were performed according to two systems: a pure oil and pure water system and a mixed oil/water system. For the dry system (pure oil and pure water), the maximum oil and water sorption capacity of raw and milled fibers was found to be 5.5 g/g and 14 g/g for oil and 14.95 g/g and 15.84 g/g for water, respectively, whereas, in the mixed oil/water system, the maximum oil and water sorption capacity was estimated as 4.74 g/g, 12.80 g/g and 7.41 g/g, 8.31 g/g, respectively. The results showed that, in spite of their absorbency of a lot of water, the milled fibers with grain size ranging between 0.5 mm and 1 mm might be the relevant sorbent for the elimination of crude oil from seawater thanks to its efficient sorption capacity and low cost.

Seagrass Posidonia oceanica diel pH fluctuations reduce the mortality of epiphytic forams under experimental ocean acidification.

It is hypothesized that pH fluctuations produced by seagrasses metabolism may confer marine calcifiers resistance to ocean acidification. Here, we tested this thesis by comparing the net population growth rate (NPGR) of a foraminifer species (Rosalina sp.) epiphytic of Mediterranean seagrass (Posidonia oceanica) to average current and projected pH scenarios under either stable conditions or diel fluctuations in pH of 0.3 units; variations similar to that experienced in their habitat. No significant differences were found in NPGRs between the fluctuating and stable pH treatments at current pH levels. NPGRs in treatments where pH fluctuated did not present significant differences to the treatment with high and stable pH conditions. In contrast, foraminifers exposed to stable low pH regimes experienced a steep decline in NPGR. These results suggest that diel pH fluctuations generated by P. oceanica photosynthetic activity could confer resistance to ocean acidification to Rosalina sp.

Different strategies of nitrogen acquisition in two tropical seagrasses under nitrogen enrichment.

Tropical marine seagrasses live in environments with low nutrient concentrations. However, as land development intensifies along tropical coastlines, the marine environment in which these organisms grow is becoming more nutrient-rich. Nitrogen (N) uptake, assimilation, translocation and storage under a diversity of N sources in enriched conditions were investigated in two tropical seagrass species, Cymodocea serrulata and Thalassia hemprichii, from an oligotrophic marine environment. Both seagrasses were able to take up different inorganic and organic N sources through their above- and belowground tissues when enriched with high N concentrations. The uptake rates of T. hemprichii were generally higher than C. serrulata in leaves and rhizome, whereas root uptake was systematically higher in C. serrulata. Acropetal and basipetal translocation was observed in both species. Reduction and assimilation of N, measured in terms of their nitrate reductase and glutamine synthetase activity, were correlated with nitrate and ammonium uptake rates, respectively. Cymodocea serrulata showed a tendency to immediately use the available N, whereas T. hemprichii allocated more N in assimilation and storage investment. The responses of these seagrasses to N-enrichment demonstrate their ability to adapt to over-enrichment by varying N sources in the first step of the eutrophication process.

Growth, physiological function, and antioxidant defense system responses of Lemna minor L. to decabromodiphenyl ether (BDE-209) induced phytotoxicity.

Polybrominated diphenyl ethers (PBDEs), represent one of the new types of persistent organic pollutants (POPs) that are currently found in ambient aquatic ecosystems. Lemna minor L. is a floating freshwater plant, which is widely employed for phytotoxicity studies of xenobiotic substances. For this study, we investigated the growth, physiological functions, and antioxidant capacities of L. minor, which were exposed to 0-20 mg L-1 decabromodiphenyl ether (BDE-209) for 14 days. A logistic model was suitable for describing the growth of L. minor when the BDE-209 concentration was in the range of from 0 to 15 mg L-1. When exposed to 5 and 10 mg L-1 BDE-209, the growth of L. minor was significantly increased, where the intrinsic rate (r) and the maximum capacity of the environment (K) of L. minor were significantly higher than those of the control. In this case, the chlorophyll content and soluble proteins were also markedly increased. Moreover, the photosynthetic function (Fv/Fm, PI) was enhanced. However, for 15 mg L-1 BDE-29 treated group, the growth of L. minor was significantly inhibited, with decreases in chlorophyll and the soluble protein content, until the L. minor yellowed and expired under a concentration of 20 mg L-1. Photosynthetic functions were also negatively correlated with increasing increments of BDE-209 (15 and 20 mg L-1). The malondialdehyde (MDA), superoxide anion radical (O2̄·) content, and permeability of the plasma membranes increased with higher BDE-209 concentrations (0-20 mg L-1). The superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities of L. minor increased when the BDE-209 concentration ranged from 0 to 10 mg L-1; however, the activities of SOD and POD were decreased. Only the CAT activity remained higher in contrast to the control group under 15-20 mg L-1 BDE-209. These results demonstrated that 15 mg L-1 BDE-209 imparted high toxicity to L. minor, which was a consequence of the overproduction of reactive oxygen species (ROS), which conveyed oxidative damage to plant cells. This study provided a theoretical understanding of BDE-209 induced toxicity as relates to the physiology and biochemistry of higher hydrophytes.

Dispersion aided tenside disintegration of seagrass Syringodium isoetifolium: Towards biomethanation, kinetics, energy exploration and evaluation.

In this study, an attempt was made to enhance the biomethanation potential of seagrass (Syringodium isoetifolium) by the aid of disperser-tenside (polysorbate 80) disintegration for the first time in literature. A disperser rpm of 10,000 for 20 min and PS 80 dose of 0.000864 g/g TS were selected as ideal parameters for effectual seagrass biomass disintegration. Dispersion aided tenside disintegration (DTD) with a disperser energy consumption of 349 kJ/kg TS, was observed to be efficacious with a biomass lysis rate of 25.6%. The impact of DTD on bioacidification and biomethanation assay with respect to volatile fatty acids concentration (1100 mg/L) and methane generation (0.256 g/g COD), was greater than dispersion disintegration (DD) (800 mg/L; 0.198 g/g COD). Thus, S. isoetifolium is considered as a promising substrate to attain the third generation biofuel goals in the near future.

Marine vegetation analysis for the determination of volatile methylsiloxanes in coastal areas.

Volatile methylsiloxanes (VMSs) are massively produced chemicals that comprise a wide range of industrial and household applications. The presence of cyclic and linear VMSs in several environmental matrices and ecosystems indicates persistence associated with a potential of (bio)accumulation and food web transfer with possible toxicological effects. Due to the high anthropogenic pressure in its vicinities particularly in summer, coastal areas in Southern European countries are potential hotspots for the presence of VMSs. The massive afflux of tourists and consequent increase of the use of personal care products (PCPs) with VMSs in their formulations highlight the importance of VMSs assessment in such areas. In this study, different species of marine vegetation (algae and seaweed) were collected in three different geographical areas, covering the Atlantic Ocean (North coast of Portugal), as well as the Mediterranean Sea (coasts of the Region of Murcia, Spain and of the city of Marseille, France). Samples were analysed for the determination of 4 cyclic (D3, D4, D5, D6) and 3 linear (L3, L4, L5) VMSs employing a QuEChERS extraction methodology, followed by gas chromatography/mass spectrometry (GC/MS) quantification. VMSs were detected in 92% of the 74 samples analysed, with the sum of the concentrations per sample ranging from below the limit of detection (LOD) to 458 ±â€¯26 ng·g-1dw (dry weight). A strong predominance of cyclic VMSs over linear ones was verified in almost all samples studied, with D5 and D6 found at higher concentrations. Seasonal variation was also assessed and despite higher levels of VMSs being identified mostly in summer months, clear seasonal trends were not perceived. It was also noted that generally the higher incidence of VMSs occurred in samples from urban and industrialized areas or in the vicinities of WWTPs, suggesting a direct input from these sources in the levels of siloxanes observed.

Seagrass litter decomposition: an additional nutrient source to shallow coastal waters.

Seagrass ecosystems are vital for its regulatory services yet, highly threatened by degradation due to human pressures. Decomposition of two tropical seagrass species (Cymodocea serrulata and Cymodocea rotundata) was studied and compared, to understand their potential in generating additional nutrients to coastal waters. Release of carbon, nitrogen and phosphorus during the decomposition process of seagrass wracks was estimated in bacteria-active (non-poisoned) and bacteria-inhibited (poisoned) conditions from shore-washed fresh seagrass, sampled from Palk Bay, India. Incubation experiments for 25 days indicated a near three times higher concentration of dissolved organic carbon (DOC) in bacteria-inhibited flasks compared to bacteria-active conditions for both species. The maximum leaching rates of DOC, TDN and TDP were found to be 294, 65.1 and 11.2 µM/g dry wt/day, respectively. Further, higher release of dissolved inorganic nitrogen (DIN) (> 1.3 times) was documented from the bacteria-active flask, highlighting the significance of microbial process in generating bio-available nutrients from decaying seagrass. Faster decomposition (0.014 ± 0.004 day-1) in the initial stages (up to 8 days) compared to the later stages (0.005 ± 0.001 day-1) indicated a rapid loss of biomass carbon during the initial leaching process and its relative importance in the decomposition pathway. The decomposition rate is best described by a single-stage exponential decay model with a half-life of 41 days. It is estimated that the total seagrass litter available along the Palk Bay coast is about ~ 0.3 Gg with high potential of additional nitrogen (0.9 ± 0.5 Mg) and phosphorus (0.3 ± 0.1 Mg) supply to the adjacent coastal waters.

Carbon stocks and accumulation rates in Red Sea seagrass meadows.

Seagrasses play an important role in climate change mitigation and adaptation, acting as natural CO2 sinks and buffering the impacts of rising sea level. However, global estimates of organic carbon (Corg) stocks, accumulation rates and seafloor elevation rates in seagrasses are limited to a few regions, thus potentially biasing global estimates. Here we assessed the extent of soil Corg stocks and accumulation rates in seagrass meadows (Thalassia hemprichii, Enhalus acoroides, Halophila stipulacea, Thalassodendrum ciliatum and Halodule uninervis) from Saudi Arabia. We estimated that seagrasses store 3.4 ± 0.3 kg Corg m-2 in 1 m-thick soil deposits, accumulated at 6.8 ± 1.7 g Corg m-2 yr-1 over the last 500 to 2,000 years. The extreme conditions in the Red Sea, such as nutrient limitation reducing seagrass growth rates and high temperature increasing soil respiration rates, may explain their relative low Corg storage compared to temperate meadows. Differences in soil Corg storage among habitats (i.e. location and species composition) are mainly related to the contribution of seagrass detritus to the soil Corg pool, fluxes of Corg from adjacent mangrove and tidal marsh ecosystems into seagrass meadows, and the amount of fine sediment particles. Seagrasses sequester annually around 0.8% of CO2 emissions from fossil-fuels by Saudi Arabia, while buffering the impacts of sea level rise. This study contributes data from understudied regions to a growing dataset on seagrass carbon stocks and sequestration rates and further evidences that even small seagrass species store Corg in coastal areas.

Photoacclimation strategies in northeastern Atlantic seagrasses: Integrating responses across plant organizational levels.

Seagrasses live in highly variable light environments and adjust to these variations by expressing acclimatory responses at different plant organizational levels (meadow, shoot, leaf and chloroplast level). Yet, comparative studies, to identify species' strategies, and integration of the relative importance of photoacclimatory adjustments at different levels are still missing. The variation in photoacclimatory responses at the chloroplast and leaf level were studied along individual leaves of Cymodocea nodosa, Zostera marina and Z. noltei, including measurements of variable chlorophyll fluorescence, photosynthesis, photoprotective capacities, non-photochemical quenching and D1-protein repair, and assessments of variation in leaf anatomy and chloroplast distribution. Our results show that the slower-growing C. nodosa expressed rather limited physiological and biochemical adjustments in response to light availability, while both species of faster-growing Zostera showed high variability along the leaves. In contrast, the inverse pattern was found for leaf anatomical adjustments in response to light availability, which were more pronounced in C. nodosa. This integrative plant organizational level approach shows that seagrasses differ in their photoacclimatory strategies and that these are linked to the species' life history strategies, information that will be critical for predicting the responses of seagrasses to disturbances and to accordingly develop adequate management strategies.

Frond transformation system mediated by Agrobacterium tumefaciens for Lemna minor.

The Lemnaceae, known as duckweed, the smallest flowering aquatic plant, shows promise as a plant bioreactor. For applying this potential plant bioreactor, establishing a stable and efficient genetic transformation system is necessary. The currently favored callus-based method for duckweed transformation is time consuming and genotype limited, as it requires callus culture and regeneration, which is inapplicable to many elite duckweed strains suitable for bioreactor exploitation. In this study, we attempted to establish a simple frond transformation system mediated by Agrobacterium tumefaciens for Lemna minor, one of the most widespread duckweed species in the world. To evaluate the feasibility of the new transformation system, the gene CYP710A11 was overexpressed to improve the yield of stigmasterol, which has multiple medicinal purposes. Three L. minor strains, ZH0055, D0158 and M0165, were transformed by both a conventional callus transformation system (CTS) and the simple frond transformation system (FTS). GUS staining, PCR, quantitative PCR and stigmasterol content detection showed that FTS can produce stable transgenic lines as well as CTS. Moreover, compared to CTS, FTS can avoid the genotype constraints of callus induction, thus saving at least half of the required processing time (CTS took 8-9 months while FTS took approximately 3 months in this study). Therefore, this transformation system is feasible in producing stable transgenic lines for a wide range of L. minor genotypes.

Microorganisms facilitate uptake of dissolved organic nitrogen by seagrass leaves.

Microorganisms play a critical role in nitrogen cycling by mineralising dissolved organic nitrogen (DON) compounds into bioavailable inorganic forms (DIN). Although DIN is crucial for seagrass growth, the hypothesis that seagrass leaf associated-microorganisms could convert DON to forms available for plant uptake has never been tested. We conducted a laboratory-based experiment in which seagrass (Posidonia sinuosa) leaves were incubated with 15N-amino acids (aa), with and without associated microorganisms. Samples were collected after 0.5, 2, 6 and 12 h. Both bulk stable isotope and nanoscale secondary ion mass spectrometry (NanoSIMS) analysis showed high accumulation of 15N within seagrass leaf tissues with an associated microbiota, but not in plants devoid of microorganisms. These results significantly change our understanding of the mechanisms of seagrass nitrogen use and provide evidence that seagrass microbiota increase nitrogen availability for uptake by seagrass leaves by mineralising aa, thus enhancing growth and productivity of these important coastal ecosystems.

Comparative assessment of trace element accumulation and bioindication in seagrasses Posidonia oceanica, Cymodocea nodosa and Halophila stipulacea.

Accumulation and bioindication of trace elements were compared in three seagrasses growing in the Mediterranean Sea: Posidonia oceanica, Cymodocea nodosa and Halophila stipulacea. The levels of the elements As, Cd, Cr, Cu, Hg, Mn, Ni, Pb and Zn were investigated in water, sediments, and roots, rhizomes and leaves of seagrasses. Results showed that seagrasses can accumulate comparable levels of trace elements, but P. oceanica and C. nodosa showed higher mean values of element accumulation. Moreover, P. oceanica and C. nodosa may accumulate high element concentrations in their leaves, whereas in H. stipulacea restricted with the bulk of trace elements in roots and rhizomes. Seagrasses reflected to a different degree the levels of several trace elements in sediments, especially P. oceanica and C. nodosa, whose use as bioindicators is recommended. The future step for an effective use of seagrasses as bioindicators of marine pollution is to set up biomonitoring networks on a large scale.

Na⁺-Dependent High-Affinity Nitrate, Phosphate and Amino Acids Transport in Leaf Cells of the Seagrass Posidonia oceanica (L.) Delile.

Posidonia oceanica (L.) Delile is a seagrass, the only group of vascular plants to colonize the marine environment. Seawater is an extreme yet stable environment characterized by high salinity, alkaline pH and low availability of essential nutrients, such as nitrate and phosphate. Classical depletion experiments, membrane potential and cytosolic sodium measurements were used to characterize the high-affinity NO3-, Pi and amino acids uptake mechanisms in this species. Net uptake rates of both NO3- and Pi were reduced by more than 70% in the absence of Na⁺. Micromolar concentrations of NO3- depolarized mesophyll leaf cells plasma membrane. Depolarizations showed saturation kinetics (Km = 8.7 ± 1 µM NO3-), which were not observed in the absence of Na⁺. NO3- induced depolarizations at increasing Na⁺ also showed saturation kinetics (Km = 7.2 ± 2 mM Na⁺). Cytosolic Na⁺ measured in P. oceanica leaf cells (17 ± 2 mM Na⁺) increased by 0.4 ± 0.2 mM Na⁺ upon the addition of 100 µM NO3-. Na⁺-dependence was also observed for high-affinity l-ala and l-cys uptake and high-affinity Pi transport. All together, these results strongly suggest that NO3-, amino acids and Pi uptake in P. oceanica leaf cells are mediated by high-affinity Na⁺-dependent transport systems. This mechanism seems to be a key step in the process of adaptation of seagrasses to the marine environment.