Seagrass Halophila Stipulacea Research Articles

Sediment carbon storage in subtidal beds of the invasive seagrass Halophila stipulacea along an extreme water depth gradient, St. Thomas, U.S. Virgin Islands

Blue carbon ecosystems such as mangroves, salt marshes, and seagrass beds are found globally and are fundamental to fisheries production, storm surge protection, and carbon sequestration. The contribution of seagrass ecosystems to global carbon stocks is still not well understood, including in the United States Virgin Islands (USVI). No study has been published to-date assessing the sediment carbon density (SCD) in seagrass beds in the USVI. This study focused on the carbon storage ability of the invasive species, Halophila stipulacea, which is compact in size compared to common native seagrasses and has spread rapidly to become a dominant seagrass in the USVI. This species forms dense mats across a wide depth range (<1 m to 50 m) typically uninhabitable to its native counterparts (Syringodium filiforme and Thalassia testudinum). Several biotic and abiotic factors influence the carbon storage ability of seagrass, yet little is known about carbon storage sequestration along a depth gradient for H. stipulacea. This study provides the first assessment of the biological characteristics (shoot density, leaf area, leaf height, and percent cover) and carbon storage ability of H. stipulacea across a depth gradient (shallow: 5–10 m; medium: 15–20 m; deep: 25–30 m) at two sites in St. Thomas, USVI. Mean sediment carbon density (SCD) values per core reported for H. stipulacea in this study ranged from 3.88 to 15.67mgC/cm3; these were comparable to regional and global seagrass studies. Biological characteristics were not an accurate predictor of SCD. A significant interaction between water depth and site was found to affect mean SCD of H. stipulacea beds. It is likely that site-specific factors most likely account for variations seen within the data. Although carbon values in this study compared to values reported in the literature, other factors such as land use, proximity to carbon sources, sediment microbial community, and water current patterns may be driving SCD values. These findings highlight the need for site and species-specific carbon storage assessments on local to regional scales to accurately estimate current and forecasted blue carbon stocks.

Population genomics unveils the century-old invasion of the Seagrass Halophila stipulacea in the Mediterranean Sea

The tropical seagrass Halophila stipulacea invaded the Eastern Mediterranean Sea in the late nineteenth century and progressively spread throughout the basin ever since. Its spread is expected to continue north-westward as the Mediterranean Sea becomes warmer, potentially changing the seagrass biogeography of the basin. Given the power of genomics to assess invasion dynamics in non-model species, we report the first ddRAD-seq study of H. stipulacea and small-scale population genomic analysis addressing its century-old Mediterranean invasion. Based on 868 SNPs and 35 genotyped native (Red Sea) and exotic (from Cyprus, Greece, and Italy) samples, results suggest that genetic structure was high, especially between major geographic discontinuities, and that exotic populations maintain comparably lower genetic diversity than native populations, despite 130 years of invasion. The evidence of high heterozygosity excess, coupled with previously reported male-dominated and rare flowering records in the exotic range, suggests that clonal propagation likely played a pivotal role in the successful colonization and spread of H. stipulacea in the Mediterranean. This shift in reproductive strategy, particularly evident in the Italian populations located closest to the western boundary and representing more recent establishments, underscores the importance of this cost-effective mode of reproduction, especially during the initial stages of invasion, raising questions about the species future expansion trajectory. Our findings serve as a catalyst for future research into the species’ invasion dynamics, including deciphering the intricate roles of acclimatization and rapid adaptation, important for a comprehensive assessment of invasion risks and improving management strategies aimed at conserving seagrass ecosystems.

The effect of anaerobic remineralization of the seagrass Halophila stipulacea on porewater biogeochemistry in the Gulf of Aqaba

IntroductionSeagrasses 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.MethodsWe 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). We measured the changes over time in dissolved inorganic carbon (DIC), alkalinity, ferrous iron (Fe2+), hydrogen sulfide (H2S), sulfate (SO42-), and sulphur isotope ratios in sulfate within water. 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. stipulacea compartments on key microbial processes in the surrounding environment.ResultsWe show that H. stipulacea’s rhizomes had the fastest decomposition rates, followed by the young leaves, roots, and old leaves (which also indicates the preservation potential of old leaves).DiscussionHigh 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.

Variation in nutritional quality of an invasive seagrass does not explain its low palatability to two key herbivores in a Caribbean Bay

The seagrass Halophila stipulacea continues to spread rapidly through the Caribbean. Documenting native herbivore use of this invasive plant is important for understanding its impacts on marine communities and the mechanisms favoring its expansion. This study used observational and experimental data to determine if juvenile green turtles (Chelonia mydas) and long-spine urchins (Diadema antillarum) from the US Virgin Islands consumed H. stipulacea in the presence of native seagrass. Juvenile turtles associated significantly more with beds of native seagrass than mixed (native and invasive), and monospecific H. stipulacea, beds or sand bottoms. When individuals were followed, turtles foraged significantly more within mixed beds, but selectively fed on native species within them and were never observed feeding on monospecific H. stipulacea stands. When offered the native Thalassia testudinum and Syringodium filiforme, along with H. stipulacea, sea urchins significantly preferred S. filiforme over the other two choices. Measurements of fourteen nutritional and chemical parameters in these three seagrasses showed strong interspecific differences, with fewer and less predictable seasonal variations. Expressing these nutritional data by wet mass, rather than dry mass, also decreased the number of significant seasonal contrasts. Despite these differences, no parameters explained low herbivore preference for the invasive seagrass satisfactorily because nutrient values of H. stipulacea were often similar to those of the preferred natives. Our results conflict with recent studies pointing at low nutritional quality aiding against herbivory and suggest other mechanisms, such as herbivore learning and recognition of a novel resource, could be more important than currently appreciated.

Effects of anthropogenic pressures on the seagrass Halophila stipulacea and its associated macrozoobenthic communities in the northern Gulf of Aqaba

Halophila stipulacea is a tropical seagrass species, native to the Red Sea, Persian Gulf, and Indian Ocean, while invasive to the Mediterranean and Caribbean Seas. The benthic fauna assemblages associated with H. stipulacea in its native habitats and the potential effects of anthropogenic stressors on these assemblages remain unknown. We compared meadow characteristics, associated fauna assemblages and trophic niche structures of H. stipulacea from an impacted and a pristine site in the northern Red Sea. Seagrass cover and biomass were higher in the impacted site, however, the associated fauna community was more abundant and diverse in the pristine site. Both meadows showed comparable trophic niches based on stable isotope analysis. This study provides first insights into the macrozoobenthos associated with H. stipulacea in its native habitat and highlights the importance of better understanding the relationship between seagrasses and their associated biota and the potential effects of urbanization on this relationship.

Searching for the competitive ability of the alien seagrass Halophila stipulacea with the autochthonous species Cymodocea nodosa

The tropical seagrass Halophila stipulacea (Forsskål) Ascherson, 1867 entered in the Mediterranean Sea through the Suez Canal more than 100 years ago. In coastal-marine ecosystems the spatial niche of H. stipulacea is often overlapped with that of native Mediterranean Sea seagrasses and therefore it might out-compete them. Aiming to better understand its invasiveness potential, we monitored a Southern Mediterranean shallow coastal-marine water habitat from August 2010 to August 2011, where H. stipulacea co-occurred with the native seagrass Cymodocea nodosa (Ucria) Ascherson, 1870. Besides, the year-round dynamics of H. stipulacea was also monitored in four periods. To test the hypothesis that the presence/absence of H. stipulacea may have an effect on C. nodosa density, we analyzed the shoot density of C. nodosa in 8 sites, 4 sites where H. stipulacea was present (impacted sites) and 4 where H. stipulacea was absent (control sites). The results showed significant differences in C. nodosa shoot density according to the presence/absence of H. stipulacea, with the lowest values observed in sites where it co-occurred with H. stipulacea. We hypothesize that the dense rhizome-sediment net created by H. stipulacea can interfere with C. nodosa density, pushing down its rhizomes in the anoxic layer. The leaf features of H. stipulacea were generally comparable to those of other Mediterranean populations. In January 2011 a significant decline of H. stipulacea was observed, maybe related to changes in the environmental conditions that have become unfavorable (e.g. hydrodynamics, turbidity) and, unexpectedly, the seagrass disappeared in April 2011. In January, we also observed the occurrence of the green alien alga Caulerpa cylindracea Sonder, 1945 which rapidly invaded the bare substrate left by H. stipulacea.

Superior growth traits of invaded (Caribbean) versus native (Red sea) populations of the seagrass Halophila stipulacea.

The online version contains supplementary material available at 10.1007/s10530-023-03045-z.

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EcologyVolume 104, Issue 4 e4010 COVER IMAGEFree Access Cover Image First published: 03 April 2023 https://doi.org/10.1002/ecy.4010AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract COVER PHOTO: An upside-down jellyfish (Cassiopea spp.) with symbiotic photosynthetic algae lies on top of a bioturbation mound to collect sufficient light on Curaçao. The invasive seagrass Halophila stipulacea prefers the same niche as starting points of invasion. Using field experiments, Smulders et al. (Article e3980; doi:10.1002/ecy.3980) found that the invasive seagrass outcompetes the native jellyfish by overgrowing the bioturbation mounds. The authors predict this will result in dynamic seagrass meadows, unfavorable for valuable native seagrass species. Their study can be found in The Scientific Naturalist section of this issue. Photo credit: Naomi Slikboer. Volume104, Issue4April 2023e4010 RelatedInformation

Competition Between Invasive Seagrass and Native Upside‐Down Jellyfish in Caribbean Seagrass Meadows

Photo 1. On the Caribbean island of Bonaire, we observed that invasive seagrass Halophila stipulacea often uses bioturbation mounds as start locations to settle and then spread into the native Thalassia testudinum meadow. The bioturbation mounds provide space and light for this plant to succeed in colonizing new habitat. Photo credit: Fee Smulders. Photo 2. On Curacao, we observed that the invasive seagrass may be in competition with native upside-down jellyfish that prefers the same habitat. The author Naomi Slikboer, using SCUBA, is counting the number of invasive Halophila stipulacea shoots and native upside-down jellyfish individuals in control plots in Spanish water bay, Curaçao. Photo credit: Erik Wurz. Photo 3. The native upside-down jellyfish with its photosynthesizing symbionts is swimming through the water column before selecting a space to settle. In the background, the researcher and author Naomi Slikboer is taking pictures of the experimental plot. Photo credit: Erik Wurz. Photo 4. End points of two different plots. (A) After 45 days, plots where no invasive seagrass shoots had started growing throughout the experiment, were often occupied fully by native upside-down jellyfish. (B) When invasive seagrass shoots did start growing into the plot, as was the case for the majority of the plots, upside-down jellyfish were pushed out and only invasive seagrass remained. Photo credit: Naomi Slikboer. Photo 5. The apparent competition between the upside-down jellyfish and invasive seagrass leads to many interesting follow-up questions. Will the outcome always be a single-species equilibrium or can the two species cycle or co-exist on these bioturbation mounds? And how will this impact the patch dynamics in the ecosystem on the landscape scale? Photo credit: Erik Wurz. These photographs illustrate the article “Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass” by Fee O. H. Smulders, Naomi Slikboer, Marjolijn J. A. Christianen, and Jan Arie Vonk published in Ecology. https://doi.org/10.1002/ecy.3980

First record of Phytomyxid infection of the non-native seagrass Halophila stipulacea in Puerto Rico.

Halophila stipulacea is an invasive seagrass in the Caribbean Sea that also harbors a phytomyxid endoparasite. Phytomyxean parasites are known to cause disease in agricultural crops and are documented to form galls in some seagrass species. Here we make the first report of phytomyxid infection of Halophila stipulacea in the Bahía de Jobos in Salinas, Puerto Rico. We found phytomyxid infected H. stipulacea at 3 of 5 sites examined; expanding the documented range of the Marinomyxa marina phytomyxid infection by almost 400 km from where it was first documented in 2018. Presence of the endoparasite has not impeded H. stipulacea dispersal and continued expansion of H. stipulacea will likely spread both the host seagrass and the endoparasite.

The hidden invasion of the alien seagrass Halophila stipulacea (Forsskål) Ascherson along Southeastern Italy

The seagrass Halophila stipulacea (Forsskål) Ascherson (Hydrocharitaceae) is a non-indigenous species (NIS) for the MediterraneanSea. This species is expected to have a high potential for invasiveness based on models predicting the present and futuresuitability of the Mediterranean coasts with respect to the plant requirements. This species was recorded for the first time alongthe Apulian coast in the Otranto harbor in 2011. Such a record was not considered in recent publications dealing with the speciesdistribution and its forecasted invasion pathway, probably due to the scant information provided. One decade after its first recordin the area, we confirmed the presence of the H. stipulacea meadow in the same locality (Otranto), and we identified four new H.stipulacea meadows along the Ionian coast are between 1 and 30 m deep in touristic areas. Indirect evidence suggests that the plantis actively spreading along the southeastern Italian coast, and monitoring programs aiming at mapping its distribution in the area,as well as its potential effect on the native biota, are required.

Teasing apart the host-related, nutrient-related and temperature-related effects shaping the phenology and microbiome of the tropical seagrass Halophila stipulacea

BackgroundHalophila stipulacea seagrass meadows are an ecologically important and threatened component of the ecosystem in the Gulf of Aqaba. Recent studies have demonstrated correlated geographic patterns for leaf epiphytic community composition and leaf morphology, also coinciding with different levels of water turbidity and nutrient concentrations. Based on these observations, workers have suggested an environmental microbial fingerprint, which may reflect various environmental stress factors seagrasses have experienced, and may add a holobiont level of plasticity to seagrasses, assisting their acclimation to changing environments and through range expansion. However, it is difficult to tease apart environmental effects from host-diversity dependent effects, which have covaried in field studies, although this is required in order to establish that differences in microbial community compositions among sites are driven by environmental conditions rather than by features governed by the host.ResultsIn this study we carried out a mesocosm experiment, in which we studied the effects of warming and nutrient stress on the composition of epiphytic bacterial communities and on some phenological traits. We studied H. stipulacea collected from two different meadows in the Gulf of Aqaba, representing differences in the host and the environment alike. We found that the source site from which seagrasses were collected was the major factor governing seagrass phenology, although heat increased shoot mortality and nutrient loading delayed new shoot emergence. Bacterial diversity, however, mostly depended on the environmental conditions. The most prominent pattern was the increase in Rhodobacteraceae under nutrient stress without heat stress, along with an increase in Microtrichaceae. Together, the two taxa have the potential to maintain nitrate reduction followed by an anammox process, which can together buffer the increase in nutrient concentrations across the leaf surface.ConclusionsOur results thus corroborate the existence of environmental microbial fingerprints, which are independent from the host diversity, and support the notion of a holobiont level plasticity, both important to understand and monitor H. stipulacea ecology under the changing climate.

Fine-tuned method to extract high purified proteins from the seagrass Halophila stipulacea to be used for proteome analyses

We report a fine tuned procedure to obtain high proteins yield and quality from the marine plant Halophila stipulacea. Two tissue fixation methods have been applied; sampled plants were preserved in RNAlater or frozen in liquid nitrogen. Both fixed plants have been processed following three different protein extraction protocols; we used a protocol optimized in this work, reported as Procedure 1 in which very small amount of tissue were used for the extraction in trichloroacetic acid/water, followed by trichloroacetic acid/acetone. The second protocol, reported as Procedure 2, is the well-established protocol developed for P. oceanica; following this protocol, proteins have been extracted from fivefold tissue amount than Procedure 1, without the trichloroacetic acid/acetone step. In the Procedure 3 the reverse approach between sample fixation and extraction protocols used in the previous two procedures have been applied. The lowest yield of 2.49 ± 0.18 mg/g of tissue is obtained from RNAlater preserved plants processed with the Procedure 2, while the highest protein yield of 5.88 ± 0.22 mg/g is obtained from the RNAlater preserved plants processed with the Procedure 1. These last protein samples gave the best resolved profile of peptide bands in SDS-PAGE showing higher proteins purity than those in all the other samples. The gel-based proteomics approach by the uHPLC-µESI-MS/MS analyses and bioinformatics against a customized dataset of genomic and transcriptomic seagrass sequences have been performed. Hundreds of proteins were identified from the Procedure 1 samples; lesser protein identification was obtained from Procedure 2 samples and no significant identifications have been detected from Procedure 3 samples. Statistics of classes of proteins that were identified in each procedures were also reported. Supplemental data for this article is available online at https://doi.org/10.1080/11263504.2021.2020355 .

Warming Threatens to Propel the Expansion of the Exotic Seagrass Halophila stipulacea

The spread of exotic species to new areas can be magnified when favored by future climatic conditions. Forecasting future ranges using species distribution models (SDMs) could be improved by considering physiological thresholds, because models solely based on occurrence data cannot account for plasticity due to acclimation of individuals to local conditions over their life-time or to adaptation due to selection within local populations. This is particularly relevant for the exotic seagrassHalophila stipulacea, which colonized the Mediterranean Sea a century ago and shifted its thermal niche, coping with a colder regime. Here, we used two hybrid models combining correlative SDMs with the thermal limits for growth of native and exoticH. stipulaceapopulations to predict the distribution of the species in its native (Indian Ocean and Red Sea) and exotic ranges (Mediterranean Sea and Caribbean Sea) under two scenarios forecasting limited (RCP 2.6) and severe (RCP 8.5) future climate changes by 2050 and 2100. Then, we assessed the differences between hybrid models based on native Red Sea thermal limits (niche conservatism: 17–36°C) and on exotic Mediterranean thermal limits (local adaptation: 14–36°C). At the Mediterranean exotic range, the local adaptation hybrid model accurately agreed with the present distribution of the species while the niche conservatism-based hybrid model failed to predict 87% of the current occurrences of the species. By contrast, both hybrid models predicted similar species distributions for the native range and exotic Caribbean range at present and projected thatH. stipulaceawill maintain its current worldwide under all future greenhouse gas emission scenarios. The hybrid model based on Mediterranean thermal limits projected the expansion ofH. stipulaceathrough the western Mediterranean basin (except the gulf of Leon) under the most severe scenario (RCP 8.5) by 2100, increasing its distribution by 50% in the Mediterranean. The future expansion ofH. stipulaceais related to its capacity to cope with warm waters and it may become a relevant species in the future, particularly under the projected decline of native Mediterranean seagrasses, resulting in important shifts in seagrass communities and overall ecosystem functions.

Distribution of the seagrass Halophila stipulacea: A big jump to the northwestern Mediterranean Sea

The Red Sea and Indian Ocean seagrass Halophila stipulacea entered the Mediterranean in the late 19th century via the Suez Canal. Here, we report on the discovery of a population of H. stipulacea covering 16.5 ha off the harbour of Cannes, French Riviera, France. This represents the northernmost locality of the species and a jump of 350 km to the north from its closest western Mediterranean locality (Razza di Juncu in Sardinia). At Cannes, it was found dwelling between 11 and 17 m depth on a dead matte of Posidonia oceanica. The species has most probably been introduced through mega-yacht or cruise ship anchoring since the Bay of Cannes is among the most highly-frequented places for luxury yachting and tourism. The lack of effective regulation to prevent the introduction of non-indigenous species in France and Europe is highlighted.

Fish grazing enhanced by nutrient enrichment may limit invasive seagrass expansion

The success of invasive macrophytes can depend on local nutrient availability and consumer pressure, which may interact. We therefore experimentally investigated the interacting effects of nutrient (nitrogen and phosphorus) addition, the exclusion of large herbivorous fishes and mimicked grazing on the expansion rates of the invasive seagrass Halophila stipulacea. The experiments were established on Bonaire and Aruba, two islands in the southern Caribbean, which differ in fish community structure. We observed that multiple Caribbean fish species feed on H. stipulacea. At both study sites, nutrient enrichment decreased invasive leaf carbon:nitrogen ratios. However only on Bonaire, where herbivore fish abundance was 7 times higher and diversity was 4.5 times higher, did nutrient enrichment result in a significant reduction of H. stipulacea expansion into native Thalassia testudinum meadows. This effect was likely due to increased herbivory on nutrient enriched seagrass leaves, as we found that excluding large herbivorous fish (e.g. parrotfish) doubled invasive expansion rates in bare patches on Bonaire. On Aruba, H. stipulacea expansion rates were higher overall, which coincided with lower abundances and diversity of native fishes, and were limited by mimicked fish grazing. We suggest that top-down control by the native fish community may counteract eutrophication effects by increased grazing pressure on nutrient-rich invasive seagrass leaves. We conclude that diverse and abundant herbivore communities likely play an important role in limiting invasion success and their conservation and restoration may serve as a tool to slow down seagrass invasions.

High-throughput amplicon sequencing of fungi and microbial eukaryotes associated with the seagrass Halophila stipulacea (Forssk.) Asch. from Al-Leith mangroves, Saudi Arabia

High-throughput amplicon sequencing of fungi and microbial eukaryotes associated with the seagrass Halophila stipulacea (Forssk.) Asch. from Al-Leith mangroves, Saudi Arabia

Nutrient History Affects the Response and Resilience of the Tropical Seagrass Halophila stipulacea to Further Enrichment in Its Native Habitat.

Eutrophication is one of the main threats to seagrass meadows, but there is limited knowledge on the interactive effects of nutrients under a changing climate, particularly for tropical seagrass species. This study aimed to detect the onset of stress in the tropical seagrass, Halophila stipulacea, by investigating the effect of in situ nutrient addition during an unusually warm summer over a 6-month period. We measured a suite of different morphological and biochemical community metrics and individual plant traits from two different sites with contrasting levels of eutrophication history before and after in situ fertilization in the Gulf of Aqaba. Nutrient stress combined with summer temperatures that surpassed the threshold for optimal growth negatively affected seagrass plants from South Beach (SB), an oligotrophic marine protected area, while H. stipulacea populations from North Beach (NB), a eutrophic and anthropogenically impacted area, benefited from the additional nutrient input. Lower aboveground (AG) and belowground (BG) biomass, reduced Leaf Area Index (LAI), smaller internodal distances, high sexual reproductive effort and the increasing occurrence of apical shoots in seagrasses from SB sites indicated that the plants were under stress and not growing under optimal conditions. Moreover, AG and BG biomass and internodal distances decreased further with the addition of fertilizer in SB sites. Results presented here highlight the fact that H. stipulacea is one of the most tolerant and plastic seagrass species. Our study further demonstrates that the effects of fertilization differ significantly between meadows that are growing exposed to different levels of anthropogenic pressures. Thus, the meadow’s “history” affects it resilience and response to further stress. Our results suggest that monitoring efforts on H. stipulacea populations in its native range should focus especially on carbohydrate reserves in leaves and rhizomes, LAI, internodal length and percentage of apical shoots as suitable warning indicators for nutrient stress in this seagrass species to minimize future impacts on these valuable ecosystems.

Seagrass (Halophila stipulacea) invasion enhances carbon sequestration in the Mediterranean Sea.

The introduction and establishment of exotic species often result in significant changes in recipient communities and their associated ecosystem services. However, usually the magnitude and direction of the changes are difficult to quantify because there is no pre-introduction data. Specifically, little is known about the effect of marine exotic macrophytes on organic carbon sequestration and storage. Here, we combine dating sediment cores (210 Pb) with sediment eDNA fingerprinting to reconstruct the chronology of pre- and post-arrival of the Red Sea seagrass Halophila stipulacea spreading into the Eastern Mediterranean native seagrass meadows. We then compare sediment organic carbon storage and burial rates before and after the arrival of H. stipulacea and between exotic (H. stipulacea) and native (C. nodosa and P. oceanica) meadows since the time of arrival following a Before-After-Control-Impact (BACI) approach. This analysis revealed that H. stipulacea arrived at the areas of study in Limassol (Cyprus) and West Crete (Greece) in the 1930s and 1970s, respectively. Average sediment organic carbon after the arrival of H. stipulacea to the sites increased in the exotic meadows twofold, from 8.4±2.5gCorg m-2 year-1 to 14.7±3.6gCorg m-2 year-1 , and, since then, burial rates in the exotic seagrass meadows were higher than in native ones of Cymodocea nodosa and Posidonia oceanica. Carbon isotopic data indicated a 50% increase of the seagrass contribution to the total sediment Corg pool since the arrival of H. stipulacea. Our results demonstrate that the invasion of H. stipulacea may play an important role in maintaining the blue carbon sink capacity in the future warmer Mediterranean Sea, by developing new carbon sinks in bare sediments and colonizing areas previously occupied by the colder thermal affinity P. oceanica.

In Vitro Evaluation of Antioxidant Potential of the Invasive Seagrass Halophila stipulacea.

Marine organisms with fast growth rates and great biological adaptive capacity might have biotechnological interests, since ecological competitiveness might rely on enhanced physiological or biochemical processes’ capability promoting protection, defense, or repair intracellular damages. The invasive seagrass Halophila stipulacea, a non-indigenous species widespread in the Mediterranean Sea, belongs to this category. This is the premise to investigate the biotechnological interest of this species. In this study, we investigated the antioxidant activity in vitro, both in scavenging reactive oxygen species and in repairing damages from oxidative stress on the fibroblast human cell line WI-38. Together with the biochemical analysis, the antioxidant activity was characterized by the study of the expression of oxidative stress gene in WI-38 cells in presence or absence of the H. stipulacea extract. Concomitantly, the pigment pool of the extracts, as well as their macromolecular composition was characterized. This study was done separately on mature and young leaves. Results indicated that mature leaves exerted a great activity in scavenging reactive oxygen species and repairing damages from oxidative stress in the WI-38 cell line. This activity was paralleled to an enhanced carotenoids content in the mature leaf extracts and a higher carbohydrate contribution to organic matter. Our results suggest a potential of the old leaves of H. stipulacea as oxidative stress damage protecting or repair agents in fibroblast cell lines. This study paves the way to transmute the invasive H. stipulacea environmental threat in goods for human health.