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Introduction: Seagrasses form oxidizing microenvironments around their roots, creating complex and strong redox gradients, thus affecting the rates of microbial carbon mineralization in their surrounding sediments. Since seagrasses are continuously being lost worldwide, a deeper understanding of the changes that occur within different seagrass sediments following the disappearance of the plants is of ecological and global importance. Methods: We conducted a slurry experiment with sediments that have different characteristics from the northern tip of Gulf of Aqaba; the different sediments included different compartments of the tropical seagrass Halophila stipulacea old and young leaves, rhizomes, or roots. These measurements were used to calculate the rate of remineralization of each seagrass compartment, allowing us to predict the potential effects of the disappearance of different H. Results: We show that H. Discussion: High concentrations of hydrogen sulfide were detected only in the slurries containing rhizomes and young leaves. High sulfide concentrations can lead to seagrass mortality and cause a positive feedback loop where the loss of seagrass due to sulfide generates further sulfide accumulation. This positive feedback loop can also be further reinforced by the loss of burrowing fauna in the sediment. This emphasizes the importance of understanding the extent of different pathways of seagrass disappearance on the surrounding environment and other geochemical feedbacks. Seagrasses are highly productive marine flowering plants angiosperms. They evolved from freshwater plants during at least three separate evolutionary events, the oldest of which is estimated to be around million years ago Les et al. Worldwide, seagrasses can be found growing in shallow m coastal waters of all continents except Antarctica Charpy-Roubaud and Sournia, ; Schubert and Demes, ; McKenzie et al. Seagrasses are usually found in soft sediments, where they may form dense meadows. Within these meadows, seagrass species diversity can vary from one dominant species to up to 14 species living in the same meadow Short et al. Seagrasses are also an efficient carbon sequestration ecosystem, and although they cover only 0. Once buried in the sediment, organic matter is oxidized to dissolved inorganic carbon DIC through the reduction of existing electron donors found in solid and dissolved phases. This process can be tracked by measuring the changes in pore water chemistry. The oxidation of organic matter follows a sequence of terminal electron acceptors according to their Gibbs free energy yield, which is the largest energy yield associated with aerobic respiration. Once oxygen is depleted, microbes use nitrate NO 3 - as the electron acceptor denitrification is then followed by manganese and iron oxides reduction. Finally, in the last reaction methanogenesis , organic matter is converted into methane CH 4 by fermentation or by CO 2 reduction Froelich et al. These reactions are not always distinctly separated, and micro niches can be formed due to the actions of different organisms e. Figure 1 Illustration of the effect seagrass has on the chemistry of the surrounding sediments and water column; shown are also the profiles of terminal electron acceptors of inorganic compounds in organic matter oxidation as a redox depth gradient in sediments. Anaerobic respiration is the process which makes use of terminal electron acceptors other than oxygen. In general, the order of consumed oxidants is oxygen O 2 aerobic respiration , followed by nitrate NO 3 - and manganese Mn and Fe oxides and eventually sulfate SO 4 Once sulfate is depleted, the dominant carbon metabolism pathway becomes methanogenesis. Based on Ugarelli et al. The rhizosphere is the subsurface zone surrounding the rhizomes and roots. The biological and chemical properties of the sediment within the rhizosphere are affected by the roots present. The interaction of seagrasses with the surrounding sediment is complex De Boer, Seagrasses release dissolved organic carbon DOC into the sediments, thus enriching the sediment with organic matter Liu S et al. This, in turn, promotes a series of microbial anaerobic respiration processes in the sediment, including microbial iron reduction and microbial sulfate reduction to sulfide Holmer et al. Sulfide is toxic to oxygen breathing- organisms and its penetration into seagrasses through their roots has been linked to seagrass mortality Koch and Erskine, ; Lamers et al. Through their lacunae, seagrasses also transport oxygen to their roots, and once in the roots, this oxygen leaks out to the sediments that are generally depleted of oxygen within millimeters Pedersen et al. This pumping of oxygen into the rhizosphere creates a new micro-niche for other organisms and promotes aerobic respiration and sulfide oxidation which then converts the toxic sulfide to its less harmful oxidized form, sulfate SO 4 Since coastal marine sediments are mostly anoxic, the microenvironment that seagrasses create by releasing oxygen through their roots is of major importance to burrowing organisms and particularly for the unique microbial communities that are associated with the root zone e. Some of these bacterial communities in the rhizosphere are symbiotic and support the seagrass host by creating microenvironments that are beneficial to the seagrasses as well. One example being the sulfur oxidizing symbiotic bacteria found in the gills of burrowing lucinid clams in seagrass sediments which oxidize the toxic sulfide to sulfate Van Der Heide et al. These interactions affect the fitness and health of the seagrasses themselves, but also the geochemistry of their surrounding sediments which have a tremendous impact on the carbon cycle Devereux, Seagrasses are threatened by global e. Decline and loss of seagrass meadows is expected to have major consequences to coastal marine environments causing shifts of the primary producers in coastal areas and promotion of anoxic conditions in the sediments Duarte, Loss of seagrass meadows will lead to a reduction in the capacity of worldwide seagrass meadows to sequester blue carbon, resulting in higher CO 2 emissions to the water column and atmosphere through organic matter remineralization or decomposition; the breakdown of organic carbon in the sediments Githaiga et al. Massive seagrasses die-offs could have two effects on the surrounding sediments 1 the dissolved oxygen released to the pore water by the seagrass roots will stop and 2 there is an addition of organic matter residue by the now dead seagrass mostly the below-ground biomass. Both potential scenarios involved in seagrass die-offs, have been suggested to generate a positive feedback loop that will promote the generation of even more sulfide, which will then kill even more seagrasses Lee and Dunton, ; Folmer et al. Halophila stipulacea is a tropical seagrass and it is the dominant seagrass species in the Gulf of Aqaba. In the Gulf of Aqaba, H. The effect of the disappearance of the above-ground seagrass biomass on marine biogeochemical cycles in surrounding sediments might differ from the effect of losing both below- and above-ground biomasses. The remineralization rates and dynamics of different seagrass parts e. However, the extent of such changes needs to be explored for different types of seagrass species, seagrass compartments, and with different sediments. In this study, we simulated the dynamics of anaerobic remineralization of different H. For this we performed slurry experiments by incubating sediments with filtered seawater and different seagrass parts for 25 days. We tracked the changes over time in water chemistry, dissolved inorganic carbon DIC , alkalinity, sulfate, sulfide, ferrous iron, and sulfur isotope ratios in sulfate. The changes in these chemical species are indicative of the mechanism of microbial metabolism. To the best of our knowledge, this is the first time that decomposition rates of different seagrass compartments have been measured in a slurry incubation under anaerobic conditions. We used natural sediments collected from H. Although the two sites are only 7 km away from each other, they differ from one another in several aspects Mejia et al. In this site, H. Figure 2 A Map of the Gulf of Aqaba. Seagrass Halophila stipulacea was collected from the north beach in green , and sediments were collected from both the north and south beaches in blue. B Seagrass meadow at the north beach. C Various components of the native seagrass Halophila stipulacea parts. The seagrass samples for this study were collected during June from the north beach Individual intact H. Due to permit regulations, seagrass plants were only collected at the north beach site an area that is not included in the local reserve. The plants were carefully removed from the sediment to avoid damage to the roots and rhizomes during the collection and transferred to ziplock bags filled with seawater. Within an hour post collection, the seagrasses were soaked in fresh water for one hour to remove detached fauna from the plant and salts, which affect the weight and the biochemical composition of the plant previous work on H. Sand, epiphytes, and foraminifera were gently removed from the leaves, rhizome, and roots. The seagrasses were then divided into different compartments: 1 the above-ground compartment, included both young - the green leaves - in the apical shoot, and 2 old leaves - the brown-yellowish leaves in the oldest shoot, alongside below-ground plant parts that included 3 rhizomes, and 4 roots. All samples were oven-dried for 24 hours to stop any new biological activities. Sediments were obtained from both the north and the south beaches From each location south and north beaches , 5 separate sediment cores were collected adjacent to the seagrass meadows. Sediment cores were kept vertical to avoid any sediment disturbance. The water above the cores was filtered 0. The upper 5 cm of sediments from within the sediment cores were collected within a few hours. The sediment was then placed in an N 2 A total of 50 grams of homogenized sediment wet weight were then transferred to five new pre-flushed Ziplock plastic bags also filled with N 2. The sediment was homogenized in the ziplock bag under a stream of N 2 that was manually kneaded for 10 minutes. To each bag, 0. Control sediment samples were treated in the same manner but without the addition of ground seagrass sample. Two grams of each mixture sediment and plant parts were then transferred to a serum bottle containing 10 ml of anoxic filtered seawater 0. The serum bottles were filled to the top with no head space , crimp-sealed with an aluminum cap and a butyl rubber stopper. Experiments were run as single-batch experiments, and duplicates were sacrificed for sampling at each time point. Overall, 20 single-batch reactors were used for each treatment i. In the south beach sediment experiment, the amount of old leaves was insufficient. Additionally, the amount of collected roots was enough for only one replicate hence no duplicates. The laboratory experiment workflow is illustrated in Figure 3. Figure 3 The experimental design. The collected Halophila stipulacae plants Figure 2 were dried and then separated into above-ground compartments, i. Each part was then ground, filtered, and mixed with sediment and anoxic filtered seawater. Water chemistry was monitored over 25 days at a sampling resolution of days. The sampling interval was higher during the first 5 days and gradually decreased over time. At each sampling point water from the serum bottle was transferred to a syringe, filtered 0. An additional 0. Alkalinity and dissolved inorganic carbon DIC were measured immediately after sampling. Total alkalinity was measured using acid-base titration and calculated using a Gran plot. Samples were titrated with 0. Total sulfide concentrations were analyzed using a modified methylene blue assay Cline, and measured by a spectrophotometer at a wavelength of nm Agilent, Cary Measurements were performed in duplicates. Following sample filtration 0. Remineralization rates mM day -1 were calculated based on the linear regression of the DIC vs. The rates were calculated based on a slope of a linear regression between each 3 consecutive time points about days. The error of these rate is reported as the standard errors of the slopes. Sulfur has four naturally occurring isotopes with the relative abundances of each isotope shown in brackets : 32 S Stable isotope ratios e. Sulfur isotopes were measured only in the north beach sediment experiment. The loss of organic carbon in the samples was calculated by the following manner Equation 4 :. The highest increase in DIC Figures 4A , 5A and alkalinity concentrations Figures 4B , 5B were detected in the rhizomes, followed by the young leaves, while the lowest increase was observed in the old leaves and roots, in both sites. In the rhizome group, DIC concentrations increased within 25 days from 2. In the young leaves group, DIC concentrations increased from 2. Similarly, DIC for the young leaves treatment exposed to the south beach sediments increased from 2. In the old leaves group, DIC concentrations increased from 2. In the roots group, DIC concentrations increased from 2. Meanwhile, in the control group, DIC concentrations increased from 2. Rhizomes in white dots, young leaves in black dots, roots in white triangles, old leaves in black triangles, and control in black squares. Rhizomes in white dots, young leaves in black dots, roots in white triangles, and control in black squares. In the rhizome group, alkalinity concentrations increased from 2. In the control groups, DIC concentrations increased from 2. In sediments from the two sites, the highest increases in sulfide concentrations H 2 S were recorded in the rhizomes and young leaves groups Figures 4D , 5D. In the rhizome group, sulfide increased from 0 mM day 0 to 6. In the young leaves group, sulfide increased from 0 mM day 0 to 3. In the old leaves group, sulfide concentrations increased by only 0. In the roots group, by the end of the experiment day 25 sulfide had increased by only 0. The largest decreases in sulfate SO 4 2- concentrations were recorded in the rhizome compartments, followed by the young leaves Figures 4E , 5E. In the rhizome group, sulfate decrease from In the young leaves group, sulfate decreased from Similarly, in the sediments from the south beach, sulfate decreased from In the old leaves group, by the end of the experiment day 25 sulfate concentrations had decreased by only 0. In the roots group, by the end of the experiment day 25 compared to day 0 sulfide had increased by only 1 mM in sediments from both the north and south beaches. In the control groups, sulfide concentrations had increased by 1. This suggests that different seagrass parts remineralize at different rates, and that the products of this remineralization can vary in magnitude. Nitrate concentration and total dissolved Mn were lower than 0. Nitrate is the substrate for denitrification and from our results its concentration was low. Dissolved Mn concentration should increase during manganese reduction, yet this was not supported by our results which showed that Mn concentrations were less than 2uM, below the instrument detection limit Tables S1 - S9. Since sulfur isotopes were measured only in the north beach sediment experiment, we could not compare between the two sediment types Figure 4F. Both the roots and control groups showed no change over time standard deviation of 0. In the other groups old leaves, roots and control the fractionation was not significantly different than zero p-values are: 0. Using the changes in DIC concentrations over time, the rates of remineralization were calculated during the experiment for both the north and south beaches sediments Figure 6. These calculations showed that rhizomes had the highest remineralization rate among all groups in the first five days, followed by the young leaves and then the roots, old leaves, and the control. The rhizomes start at a faster remineralization rate 1. However, after day 5, the remineralization rates of young leaves bypassed that of the rhizomes and reached 1. Although the remineralization rate of the leaves continued and actually bypassed the remineralization rate of the rhizomes between days 5 to 12 the amount of inorganic material released at the beginning was high enough to determine that the rhizome decomposed the fastest. The remineralization rate of the old leaves and roots was similar and lower than the young leaves and rhizomes for the entire duration of the experiment. In all experimental groups, the decomposition rates drop dramatically from day 10 young leaves or day 15 rhizomes. Starting at 0. Figure 6 Remineralization rate mM day -1 of the different seagrass parts in sediment from the north beach A and the south beach B at the Gulf of Aqaba. Rates were calculated based on the change slope of DIC vs. When plotting the changes of DIC vs. At the north beach, the regression analysis for alkalinity vs. DIC yielded slopes of 1. The data from the south beach sediment yielded incredibly similar slopes of 1. However, overall, these slopes were not significantly different than 1 p-value equals to 0. In sediments from both the north and south beaches, a significant increase in alkalinity concentration was recorded with little change in sulfate concentrations Figures 7B, D. After sulfate concentrations started to decrease during the sulfate reduction stage which started after five days the alkalinity was doubled for every decrease in sulfate. The regression analysis for alkalinity vs. The carbon C content was highest in the rhizomes The nitrogen N content however was highest in the young leaves 1. The C:N ratios were Figure 7 Alkalinity vs. DIC concentrations change over time A and Alkalinity vs. SO 4 2- B from slurry with sediment from the north beach at the Gulf of Aqaba. Alkalinity vs. DIC concentrations change over time C and Alkalinity vs. SO 4 2- D from slurry with sediment from the south beach at the Gulf of Aqaba. The broken lines in the figure represent general trends rather than regression lines. Understanding the remineralization rates of different seagrass plant compartments may have implications for calculations of carbon stocks and sequestration rates. The remineralization rates in this study were calculated based on the DIC accumulation over time Figures 4A , 5A using equation 1 and shown in Figure 7. During the first 5 days of the experiment, in sediments from both the north and south beaches, the remineralization rates of the rhizomes were the highest followed by the remineralization rate of the young leaves Figure 7. A decrease in rates at the later stage of the experiment starting at 7 to 15 days was observed in all treatments, indicating a change of the remaining organic carbon quality. The roots and the old leaves showed rather similar behavior, indicating their similarity in the recalcitrance of their organic carbon composition. These findings provide a significant insight for the estimation of seagrass carbon stocks and the possible impacts of seagrass decline on these stocks. The slow decomposition rate and low percentage of carbon loss in the roots compared to the young leaves and rhizomes, indicate that the below-ground biomass plays a critical role in the long-term carbon storage of seagrass meadows even while declining. The remineralization rate is related to the biochemical composition of the different parts of the seagrass and is a proxy for their organic content. Previous work suggested that the C:N ratio of the organic matter is correlated with the decomposition rates e. However, in H. This is shown both in our results Table S1 and in a previous study Beca-Carretero et al. This, somehow, contradicts our results as there is a mismatch between the relationship between the C:N ratio and the remineralization rates. Hence, the C:N ratio of the organic carbon by itself cannot be a proxy for the rates of decomposition. Similar observations were made in a recent study comparing the decomposition rates of seagrass compartments under aerobic conditions and showed that the above-ground biomass decomposed faster than the below-ground biomass Satoh and Shigeki, ; Wada et al. This is likely since there are some labile e. Therefore, in order to understand the remineralization rates of each seagrass part, a more detailed description of the seagrass biochemistry is needed. In tropical seagrass species, it was found that in the Caribbean seagrass Thalassia hemprichii , the rhizomes had the highest concentration of sugars and starch followed by the leaves and then the roots, which concurs well with our experimental results. This could therefore suggest that a higher fraction of soluble sugars results in faster decomposition rates and a lower fraction of remaining organic carbon. Additionally, it is evident that there is a wide variation between the sugars conten and organic matter between young and old roots Waldron et al. Although in our study we separated between the leaves according to their age, young vs old , we did not differentiate between the different age of roots since the root biomass was significantly lower than other parts. Since the decomposition rates in all groups decrease with time, we hypothesize that the remineralization rate would continue to be slower until all the non-refractory organic matter is consumed. As discussed earlier, we also do not expect the last reaction in the anaerobic respiration chain in sediments i. The slurry experiment method used in our study provides insights into the complex dynamics involved in the anaerobic remineralization of seagrass, as observed through changes in various water chemistry components Chuan et al. In the absence of oxygen, a redox cascade of the main electron acceptors in the system starting with denitrification, followed by manganese reduction, iron reduction, sulfate reduction and finally methanogenesis. These dynamics should be reflected in water chemistry. Our results indicate that organic matter remineralization primarily occurs through the reduction of iron oxides after 3 days and sulfate after 5 days , while the remineralization occurring through nitrate NO 3 - and Mn oxides are negligible Tables S1 - S9 and Figures 4 , 5. This is in line with a previous observation from the Gulf of Aqaba and other oligotrophic environments showing that sulfate reduction is the dominant reaction in coastal sediments Kristensen et al. However, a recent study conducted on seagrass meadows of Posidonia oceanica revealed the occurrence of methane production during seagrass die-off from methylated compounds Schorn et al. Microbial iron reduction leads to an increase in dissolved ferrous iron concentrations, and microbial sulfate reduction results in a decrease in sulfate concentrations and a corresponding rise in dissolved sulfide e. Dissolved sulfide can react quickly with free dissolved ferrous iron to form iron monosulfide or pyrite e. The results of our experiments shown in Figures 4 , 5 can be broadly divided into two stages: 1 microbial iron reduction dominates, and 2 microbial sulfate reduction becomes dominant. This dichotomy is particularly evident for the rhizomes and the young leaves treatments. The roots, old leaves, and the control treatments, seemed to be mostly dominated by bacterial iron reduction throughout the experiment while the rhizomes and young leaves treatments are dominated by microbial sulfate reduction. Bigger grain size will improve the conductivity between the seawater and pore water, which results in ventilation of the pore water. Surrounded by some of the driest deserts in the world, within the oligotrophic Gulf, much of the available nutrients that reaches the the sediment comes from airborne dust which impact many of the biological processes. Therefore, in our case, granulometry also implies on the minerology. Overall, there were no significant differences observed between sediments from the north and south beaches in most measurements See correlation table; Table S The only major difference between the experiments were the results with ferrous iron concentrations, which can be attributed to differences in sediment source and the higher sedimentary reactive iron concentration in the north beach compared to the south beach sediments Blonder et al. Although observations of biogeochemical trends over time provide valuable information, cross-plotting different components can enhance our understanding of reactions stoichiometries and provide a fresh perspective on the mechanism of microbial respiration. For instance, cross-plotting alkalinity vs. DIC indicates the mechanism of microbial respiration since it reflects the stoichiometry between the carbonate system species and proton donors both of which change in a predictable way for each reaction in the sediment- Soetaert et al. Typically, microbial sulfate reduction produces one equivalent of alkalinity per one mole of DIC, while microbial iron reduction is closer to a ratio of one to eight e. In our experiment, most Alkalinity vs. DIC data points fell close to line Figures 7A, C , suggesting a strong dominance of microbial sulfate reduction. Evidence for the dominance of microbial sulfate reduction can also be gained by plotting the changes in alkalinity vs. During microbial sulfate reduction, it is expected that for every loss of one sulfate molecule, two equivalents of alkalinity would be generated e. Total Alkalinity expresses the balance between proton acceptors and proton donors in the solution measured in equivalents per liter. Therefore, an addition of species such as bicarbonate HCO 3 - , carbonate CO 3 2- , bisulfide HS - and sulfide S 2- will result in change in alkalinity. The oxidation of organic material by sulfate reduction follows the generalized reaction Equation 5 :. Hence, for a reduction of one mole of sulfate SO 4 2- we anticipate an increase of 2 equivalents in alkalinity. In this study, sediments from both the north and south beaches, showed a significant increase in alkalinity concentration over time, with little change in sulfate concentrations in the initial stage of our experiment first five days. However, after sulfate concentrations started to decrease during the sulfate reduction stage which started after five days the alkalinity was doubled for every decrease in sulfate Figures 7B, D. We suggest that the initial stage of the incubations was dominated by bacterial iron reduction, yet during this stage we recorded a change of 5mEq kg -1 of alkalinity Figures 4 , 5. In addition, this change in alkalinity was observed in both sediments from the north and south beaches while the iron reduction in the south beach was much less pronounced Figures 4C , 5C. The lack of correlation between alkalinity and iron can be also demonstrated through a cross-plot See Figure S1. This can indicate that no substantial dissolution of carbonate minerals occurred, which could have affected alkalinity without any change in sulfate concentration. This production of organic alkalinity in the form of volatile fatty acids could later fuel other microbial processes Sansone and Martens, ; Glombitza et al. We assume that the volatile fatty acids formed in the first stage of the experiment were used by sulfate reducers as electron donor for microbial sulfate reduction Glombitza et al. Taken together, we suggest that microbial iron reduction and fermentation, dominated the first days of the experiment, followed by microbial sulfate reduction in the rhizome and young leaves Figure 4 that took place in the later days of the experiment. Sulfate is the second most abundant anion in the ocean with a concentration of 32mM at the Gulf of Aqaba water column e. Seagrass sediments exhibit relatively high rates of microbial sulfate reduction Blackburn et al. In our experiments, the fastest decrease in sulfate concentrations were recorded in the slurry that included rhizomes, followed by the one that included young leaves. In comparison, the slurry that included roots or old leaves showed only a slight decrease, with the control staying relatively constant Figure 4E. Similar observations were recorded in the south beach sediments Figure 5. Correspondingly, the highest increase in sulfide concentrations were also measured in the rhizome and in the leaves, and with no detectable sulfide in the roots, old leaves, and control groups for seagrasses exposed to sediments from both sites Figure 4D. These results suggest a strong dominance of microbial sulfate reduction with the rhizome and the young leaves and its absence in the roots. Sulfur isotope ratios are a powerful tool to study microbial sulfate reduction. As microbial sulfate reduction progresses, the sulfur isotope composition of sulfate typically increases monotonically Harrison and Thode, ; Kaplan and Rittenberg, ; Rees, This is because the enzymatic steps tend to prefer the lighter sulfur isotope 32 S and leave behind the heavier 34 S, resulting in the slow distillation of 32 S into the external sulfide pool Rees, ; Brunner and Bernasconi, Furthermore, earlier studies found a positive correlation between the magnitude of sulfur isotope fractionation and sulfate reduction rate Kaplan and Rittenberg, ; Rees, ; Chambers et al. This has been demonstrated in pure culture and batch culture experiments, as well as in situ using pore fluid profiles Aharon and Fu, ; Canfield et al. Specifically, slower sulfate reduction rates corresponded to higher sulfur isotope fractionation e. In our experiments, we found that sulfate reduction was only pronounced in the experiment with the rhizomes and the young leaves. This provides further evidence of the different remineralization rates of these two parts, which is also consistent with our calculated rates Figure 6. The fractionation was shown to be also dependent on the organic matter substrate, where an easier degradable substrate by microbes results in lower fractionation Sim et al. Our results demonstrate that rhizomes and young leaves of H. Furthermore, while both rhizomes and roots are commonly referred to as belowground biomass Collier et al. The seagrass roots are typically embedded in the sediment depth, which is characterized by reduced oxygen levels, leading to the dominance of anaerobic reactions, yet our results show that the decomposition of the roots does not produce high concentrations of sulfide, thus mitigating the risk of toxic sulfide intruding the roots of neighboring seagrasses and potentially killing them. The rhizomes, however, do yield high sulfide concentrations, yet their proximity to the sediment surface reduces the changes of sulfide reaching to deeper sediment layers where other seagrasses roots are located. Seagrass remineralization plays a critical role in the biogeochemistry of marine sediments, yet our understanding of the pathways of this remineralization is still limited. The decline of seagrasses is caused by a complex array of factors, including changes in water quality, eutrophication, climate change, coastal development, and overfishing Jackson et al. Organic matter enrichment in the sediments, which results in high sulfide concentrations was also found to be a major factor for seagrass decline Carlson et al. We conducted slurry experiments with sediments from the north and south beaches of the Gulf of Aqaba, sediments that differ in their granometry, minerology and organic matter content, to investigate the effects of different plant compartments from the tropical seagrass H. Although our experiment represents a situation where seagrass parts are buried in their entirety, it can provide valuable insights into the pathways of seagrass remineralization in anoxic conditions. Our findings indicate that sulfide is generated only during the anaerobic remineralization of young leaves and rhizomes, likely due to their biochemical composition. Notably, we did not observe high concentrations of sulfide during the anaerobic remineralization of roots, old leaves, or in the controls. This agrees with the remineralization rates of each seagrass plant part. The sulfidic feedback loop, described as a positive feedback loop, where the generation of sulfide cause seagrass mortality, then, the remaining seagrass parts act as a substrate for sulfate reduction, resulting in generation of more sulfide. This positive feedback loop can also be further reinforced by the loss of burrowing fauna in the sediment such as mollusks and polychaetas and even lead to the production of methane e. Seagrass increases oxygen concentration and pH levels in the water column Job et al. Therefore, their decline may negatively affect neighboring ecosystems such as coral reefs Waycott et al. Although the seagrass rhizome is considered part of the below-ground biomass, in H. Thus, the rhizome does not necessarily participate in anaerobic respiration processes in the sediment. Our results suggest that in the disappearance scenario of H. This may suggest that H. This possible outcome highlights the importance of understanding the different pathways of seagrass remineralization. Overall, the anaerobic remineralization rates of H. Carbon dioxide removal from the atmosphere requires not only its fixation into organic form but also its burial, preferably in the sediment. Therefore, to store organic carbon in the sediment, the rates of remineralization of buried organic carbon must be low. Our findings suggest that H. Evidently, when the seagrass dies, the buried biomass within the sediment decomposes and releases DIC back to the water column at a slower phase. This decelerated decomposition pattern emphasizes H. For example, a possible change in the biochemical composition of the seagrass and loss of intracellular materials due to leaking caused by being soaked in fresh water for an hour. However, no stress effect is expected from such short-term exposures to low salinity Oscar et al. To prevent potential artifacts in future experiments, fauna could be removed under a stream of seawater and the seagrass samples could be washed under fresh water for a maximum of 5 minutes to remove salts. In future experiments, we intend to request authorization and collocate a higher quantity of seagrasses. We also aim to replicate the conducted experiments using seagrass samples from the south beach and conduct the experiments under aerobic conditions as well. Seagrass samples can also be collected and examined in different seasons, to test possible effect of seasonally variable factors. Additionally, we intend to broaden the scope of the current study and repeat the experiment with other seagrass species. Despite the potential limitations to our method, there are still important insights we can make from such an experiment. Further inquiries can be directed to the corresponding author. NS and GA were involved in setting up the experiments, sediment and seagrass collection, and performing the chemical measurements. NS wrote the first draft of the manuscript. All authors contributed to the manuscript revision, read, and approved the submitted version. We would like to acknowledge Prof. Orit Sivan, Dr. Avner Gross and Dr. We also would like to thank the two reviewers whose constructive comments immensely improved the quality of this manuscript. NS would like to thank Shany Barkan for her exceptional assistance in proofreading this manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. 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Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction. Geology 29 7 , — Keywords: Halophila stipulacea , Gulf of Aqaba Eilat , sulfur, seagrass, blue carbon, sediments. The use, distribution or reproduction in other forums is permitted, provided the original author s and the copyright owner s are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher. Top bar navigation. About us About us. Sections Sections. About journal About journal. Article types Author guidelines Editor guidelines Publishing fees Submission checklist Contact editorial office. The effect of anaerobic remineralization of the seagrass Halophila stipulacea on porewater biogeochemistry in the Gulf of Aqaba. Keywords: Halophila stipulacea , Gulf of Aqaba Eilat , sulfur, seagrass, blue carbon, sediments Citation: Soto N, Winters G and Antler G The effect of anaerobic remineralization of the seagrass Halophila stipulacea on porewater biogeochemistry in the Gulf of Aqaba.

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