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Among living tree species, Acacia raddiana (Savi) and Acacia tortilis (Forssk), species of the legume family, populate some of the hottest and driest places on Earth. Our research investigates the physiological processes underlying the unique survival of trees in extreme environmental conditions. We measured Acacia trees in their natural habitat together with a controlled experiment under scenarios of drought and low N on a lysimeters system to unravel their water use strategies and growth dynamics. In the field, temperature positively influenced the growth rate of the trees, daily and annual gas-exchange curves showed higher gas exchange during noon and in summer, when temperature and radiation are maximal (44°C, 2000 µmol m-2 s-1), and the air is dry (21% RH). Furthermore, in the controlled experiment, Acacia saplings keep transpiring water (180 g per day), especially at noontime (0.08 gwater/gplant/ min), and therefore continue growing in low soil water content of 5%. These findings suggest a strong potential for acacia trees to contribute to ecosystem carbon sequestration in warming and drying climates.

The nature and strength of interactions between native and invasive species can determine invasion success. Species interactions can drive, prevent or facilitate invasion, making understanding the nature and outcome of these interactions critical. We conducted mesocosm experiments to test the outcome of interactions between Halophila stipulacea, a seagrass that invaded the Mediterranean and Caribbean Seas, and native seagrasses (Cymodocea nodosa and Syringodium filiforme, respectively) to elucidate mechanisms explaining the successful invasions. Mesocosms contained intact cores with species grown either mixed or alone. Overall, in both locations, there was a pattern of the invasive growing faster with the native than when alone, while also negatively affecting the native, with similar patterns for shoot density, aboveground and belowground biomass. In the Caribbean, H. stipulacea increased by 5.6 ± 1.0 SE shoots in 6 weeks when grown with the native while, when alone, there was a net loss of −0.8 ± 1.6 SE shoots. The opposite pattern occurred for S. filiforme, although these differences were not significant. While the pattern in the Mediterranean was the same as the Caribbean, with the invasive grown with the native increasing shoots more than when it grew alone, these differences for shoots were not significant. However, when measured as aboveground biomass, H. stipulacea had negative effects on the native C. nodosa. Our results suggest that a seagrass that invaded two seas may drive its own success by both negatively affecting native seagrasses and benefiting from that negative interaction. This is a novel example of a native seagrass species facilitating the success of an invasive at its own cost, providing one possible mechanism for the widespread success of this invasive species.

The seagrass Halophila stipulacea is native to the Red Sea. It invaded the Mediterranean over the past century and most of the Caribbean over the last two decades. Understanding the main drivers behind the successful invasiveness of H. stipulacea has become crucial. We performed a comprehensive study including field measurements, a mesocosm experiment, and a literature review to identify ‘superior traits’ that can potentially explain the success story of H. stipulacea. We assessed meadow characteristics and plant traits of three invasive H. stipulacea populations growing off the Island of Sint Eustatius (eastern Caribbean). We compared similar parameters between native (Eilat, Red Sea) and invasive H. stipulacea plants in a common-garden mesocosm. Lastly, we also compared our field measurements with published data. The newly arrived H. stipulacea plants from St. Eustatius were characterized by higher percent cover, higher below- and above-ground biomasses, more apical shoots, and faster leaf turnover rates than those measured in both native and older invaded habitats. These results were further confirmed by mesocosm experiments where the invasive H. stipulacea plants grew faster and developed more apical shoots than the native plants. Results suggest that increased growth vigour is one of the main invasive traits that characterize successful invasive H. stipulacea populations in the Caribbean and potentially in other invaded areas. We encourage long-term monitoring of H. stipulacea in both native and invaded habitats to better understand the future spread of this species and its impacts on communities and their ecosystem functions and services.

Seagrasses and benthic foraminifera are present in a wide range of habitats over the globe. These organisms are considered ecosystem engineers and bio-indicators of coastal marine environments. In tropical waters, seagrass meadows are a favorable habitat for benthic foraminifera that benefit from this unique environment. Although each one of these organisms has been studied extensively and has been shown to be sensitive to changes within their environment, much less is known about their association with each other, particularly with respect to climate change and anthropogenic pressures. This study was designed to ecologically characterize the benthic foraminiferal assemblages associated with the meadows of the tropical seagrass Halophila stipulacea as a model ecosystem for monitoring anthropogenic and natural changes in the Gulf of Aqaba-Eilat. The study was based on 11 sampling campaigns (January 2020- April 2021) in two sites that differ in their conditions: the North Beach (NB) site (10 m depth) representing a meadow near to a heavily urbanized coast, and the South beach (SB) site (10 m depth) which is found in an undisturbed area of the shoreline The ecological examination included characterization of the foraminiferal assemblages from three microhabitats at each site: leaf microhabitat (L), seagrass sediment (sediment inside meadows- SIM), and non-seagrass sediment (sediment outside meadows- SOM). In addition, sediment grain size distribution and leaf surface area were measured as the environmental parameters that characterize each site. Our study provides multiple ecological pieces of evidence that highlight the biological importance of H. stipulacea seagrass meadows as a favored habitat for tropical benthic foraminifera. First, the foraminiferal biomass and diversity within the seagrass environments were significantly higher than that found outside the seagrass meadows. Specifically, the highest numbers of individuals were found on the leaf microhabitats, which were populated by epiphytic foraminifera species that were considerably less common or rare within sediment microhabitats. The anthropogenic footprint of the urban area in the northern shore (the NB Site) was reflected by 1) The significantly lower abundances of the total living foraminifera at the NB Site compared to the SB Site, 2). The very low abundances of the two common GoA taxa, soritids, and A. lobifera, at the NB Site, and 3). An exceptionally higher abundance of O. ammonoides and Ammonia spp. (known to be affiliated with eutrophic environments) at the NB Site relative to the SB Site.Our study demonstrates the importance of seagrass meadows as both autogenic and allogenic ecosystem engineers for benthic foraminifera by providing a substrate and other ecological resources. However, this association is delicate and could be damaged by anthropogenic stressors, as indicated by the decreased abundances of foraminifera in the NB impacted site.

Aims In plant eco-physiology, less negative (enriched) carbon 13 (13C) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, 13C is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in δ 13C from leaves (photosynthetic organs) to branches, stems and roots (non-photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought. Methods We measured leaf-to-root 13C in three closely related desert acacia species (Acacia tortilis, A. raddiana and A. pachyceras). We measured δ 13C in leaf tissues from mature trees in southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0 or 4.0 L day−1 was conducted in an experimental orchard. At the end of the experiment, growth parameters and δ 13C were measured in leaves, branches, stems and roots. Important Findings The δ 13C in leaf tissues sampled from mature trees was ca. −27‰, far more depleted than expected from a desert tree growing in one of the Earth’s driest and hottest environments. Across acacia species and compartments, δ 13C was not enriched at all irrigation levels (−28‰ to ca. −27‰), confirming our measurements in the mature trees. Among compartments, leaf δ 13C was unexpectedly similar to branch and root δ 13C, and surprisingly, even less negative than stem δ 13C. The highly depleted leaf δ 13C suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of δ 13C enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.

BackgroundHalophila stipulaceaseagrass 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 endophytic 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 studiedH. stipulaceacollected 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 monitorH. stipulaceaecology under the changing climate.

Vachellia (formerly Acacia) trees are native to arid environments in Africa and the Arabian Peninsula, where they often support the local animal and plant communities acting as keystone species. The aim of this study was to examine whether oil pollution affected the central metabolism of the native keystone trees Vachellia tortilis (Forssk.) and V. raddiana (Savi), as either adults or seedlings. The study was conducted in the Evrona Nature Reserve, a desert ecosystem in southern Israel where two major oil spills occurred in 1975 and in 2014. Leaf samples were collected to analyze the central metabolite profiles from oil-polluted and unpolluted adult trees and from Vachellia seedlings growing in oil-polluted and unpolluted soils in an outdoor setup. We found that oil pollution had a stronger effect on one-year-old seedlings than on adult trees, reducing the levels of amino acids, sugars, and organic acids. While adult trees are mildly affected by oil pollution, the effects on young seedlings can cause a long-term reduction in the population of these keystone desert trees, ultimately threatening this entire ecosystem. The research was part of a postcontamination monitoring program in the Evrona Nature Reserve, funded by the Israel Nature and Parks Authority (INPA) and managed by the HaMaaragIsrael National Nature Assessment Program. We are grateful for the ongoing support to the ADSSC (Winters G.) by the Israel Ministry for Science and Technology (MOST). M.F. was the recipient of a postdoctoral fellowship from the Jacob Blaustein Center for Scientific Cooperation. info:eu-repo/semantics/publishedVersion

Seagrasses and associated microbial communities constitute a functional unit (holobiont) which responds as a whole to environmental changes. However, it is still unclear how the microbial colonizers are selected. In this study we compared the epiphytic microbial communities associated with Posidonia oceanica and Halophila stipulacea, Mediterranean native and exotic seagrass species, respectively, growing side by side in monospecific patches within the port of Limassol (Cyprus, Eastern Mediterranean Sea). To evaluate whether the environment rather than the host species and/or its physiological condition play a role in shaping the seagrass epiphytic microbial community, the environmental microbial communities (seawater and sediment) and seagrass associated ones were determined by using 16S rRNA gene amplicon sequencing. Plant ecological status was evaluated by morphological (biometry), structural (density) and biochemical (pigment/phenol content) descriptors. In both species, leaf associated microbial communities are clearly similar to seawater microbes; conversely, microbes associated with H. stipulacea roots/rhizomes differ from the microbial communities in surrounding sediment. In both seagrasses, Pseudomonadaceae was the most abundant family on leaves, but each species harboured unique microbial families. To our best knowledge, this is the first study on these two neighbouring seagrass species, coupling plant ecological status with associated microbial communities. Results demonstrated that each seagrass responded differently to the same environmental conditions and selected different epiphytic microbial communities, supporting their putative use as ecological indicators.

Among living tree species, Acacia raddiana (Savi) and Acacia tortilis (Forssk), species of the legume family, populate some of the hottest and driest places on earth. Our research investigates the physiological processes underlying the unique survival of these trees in their extreme environmental conditions. We measured Acacia trees in their natural habitat once a month for two years to unravel the photosynthesis dynamics and water relations. Leaf gas exchange and leaf water potential were measured, as well as atmospheric and soil parameters. Daily and annual gas-exchange curves showed higher carbon assimilation during noon and in summer, when temperature and radiation were maximal (44°C, 2000 µmol m-2 s-1), and the air was dry (21% RH). Additionally, we found that the maximum rate of carbon assimilation was at PAR (photosynthetic active radiation) of 3000 µmol m-2 s-1. Our results suggest that water did not drive net carbon assimilation but rather light and temperature, which are already close to their maximum in our hyper-arid ecosystem.

Halophila stipulacea is a small tropical seagrass species native to the Red Sea. Due to its invasive character, there is growing interest in understanding its ability to thrive in a broad range of ecological niches. We studied temporal (February 2014 and July 2014), depth (5, 9, 18 m) and spatial (NB and SB) related dynamics of H. stipulacea meadows in the northern Gulf of Aqaba. We evaluated changes in density, morphometry, biomass, and biochemical parameters alongside the reproductive effort. In both sites, maximal growth and vegetative performance occurred in the summer with a marked increase of 35% in shoot density and 18% in biomass; PAR reduction with season and depth induced a significant increase of 28% in leaf area. Sexual reproduction efforts were only observed in July, and the density of plants carrying male or female flowers decreased significantly with depth. The favorable growth responses of H. stipulacea plants observed in the N-enriched NB site suggests their capacity to acclimate to human-disturbed nearshore environments.

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70 mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Deviations from the 1:1 sex ratio are common in dioecious plants. The tropical seagrass Halophila stipulacea is among an extremely rare group of dioecious plants that are widely recognized as female-biased. Here we report on differences in sex ratios between native (Eilat, northern Red Sea) and invasive (Cyprus, Mediterranean Sea) populations. While H. stipulacea populations were female-biased in their native region, invasive populations were either male- or female-biased. The existence of both sexes simultaneously in the Mediterranean invasive populations might help its ongoing expansion in the Mediterranean, thereby threatening local seagrasses species.

Along the arid Arava, southern Israel, acacia trees (Acacia raddiana and Acacia tortilis) are considered keystone species. Yet they are threatened by the ongoing aquifer depletion for agriculture, the conversion of natural land to agricultural land, seed infestation by bruchid beetles, and the reduction in precipitation level in the region. In the acacia dominated Evrona reserve (southern Arava), adding to these threats are recurrent oil spills from an underground pipeline. We report here a study of the effects of contaminated soils, from a recent (December 2014) and a much older (1975) oil spills. The effects of local petroleum oil-contaminated soils on germination and early growing stages of the two acacia species were studied by comparisons with uncontaminated (control) soils from the same sites. For both acacia species, germination was significantly reduced in the 2014 oil-contaminated soils, whereas delayed in the 1975 oil-contaminated soil. There was no significant effect of oil volatile compounds on seed germination. At 105 days post transplanting (DPT), height, leaf number, stem diameter, and root growth were significantly smaller in the oil-contaminated soils. While photosynthetic performance (quantum yield of photosystem II) did not differ considerably between treatments, reductions of chlorophylls content and protein content were found in seedlings growing in the contaminated soils. Significant increases in superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities were found in roots of seedlings growing in oil-contaminated soils. These results demonstrate that seed germination and seedling growth of both acacia species were strongly restricted by oil contamination in soils, from both recent (2014) and a 40-year old (1975) oil spills. Such long-term effects of oil spills on local acacia seedlings could shift the structure of local acacia communities. These results should be taken into account by local authorities aiming to clean up and restore such polluted areas.

1. In the Gulf of Aqaba (GoA), coral reefs are considered the dominating ecosystem, while seagrass meadows, recognized worldwide as important ecosystems, have received little attention. Absence of comprehensive seagrass maps limits awareness, evaluations of associated ecosystem services, and implementation of conservation and management tools. 2. Presented here are the first detailed maps of seagrass meadows along the Israeli coast of the northern GoA. Mapping was performed by snorkelling along transects perpendicular to the shore above meadows growing at 15–25 m. Measurements along these transects included position, meadow depth and visual estimations of seagrass cover. Shallow boundaries of meadows, parallel to shore, were recorded by GPS tracking. Supplementary work included drop-camera boat surveys to determine the position of the deeper edge of meadows. In addition, GIS layers were created that indicated shoreline infrastructures, near-shore human activities and potential pollution threats. Ecosystem services of seagrass meadows mapped were valuated using a benefit transfer approach. 3. In total, 9.7 km of the 11 km shoreline were surveyed and 2830 data points collected. Seagrasses were growing along 7.5 km of the shoreline, with shallow (15–25 m) meadows found to cover an area of 707 000 m2 and valued at more than US$ 2 000 000 yr-1 in associated ecosystem services. Pilot drop-camera surveys (additional 283 data points) indicated that meadows can extend down to 50 m in some places. Coastal uses and threats varied in character and location. A municipality runoff point and drainage canal located close to the largest meadow were identified as the main threats to local seagrasses. 4. These low-cost methods enhance our understanding of seagrass distribution in the northern GoA. They demonstrate a GIS-based tool for assessing how environmental changes might affect the cover and state of seagrasses, improving efforts to conserve seagrass, and have particular relevance to seagrass mapping in developing countries and/or island nations. Copyright © 2016 John Wiley & Sons, Ltd.

Seagrasses are one of the most valuable marine ecosystems on earth, yet they are declining worldwide at alarming rates. With most of seagrass monitoring based on long term responses to environmental pressures, there is growing interest in developing alternative diagnostic tools that more effectively identify changes in seagrass ecological status at an early stage. Besides morphological indicators, functional and biochemical descriptors may provide a good understanding of plant's responses to environmental changes. Moreover, the epiphytic microbial communities of seagrasses may also shift in response to changes in environmental conditions, although these have been seldom used as a descriptor of environmental change. In this study three Halophila stipulacea (Forsk.) Aschers meadows, found in the Gulf of Aqaba (northern Red Sea), were characterized using an integrated approach to highlight possible differences in the meadows ecological status. Plant descriptors, including leaves morphometrics (leaf size, leaf number/plant, leaves with lost apex), photosynthetic pigments (Chlorophylls, Carotenoids) and total phenols contents, were investigated and coupled with the plants’ epiphytic microbial community structure and composition, studied using pyrosequencing. The entire suite of descriptors highlighted differences among the meadows ecological status based on changes in plants’ morphology and biochemistry, and their associated microbial communities, in response to the different environmental conditions (water column turbidity, seawater and sediment nutrients) and the geomorphological features (bottom slope, granulometry) of the stations. Leaf morphology and photosynthetic pigment content were modulated in H. stipulacea in response to light availability and hydrodynamics in the Gulf of Aqaba. The highest leaf surface area and photosynthetic pigment contents were observed at the lowest irradiance and hydrodynamics/granulometry among stations. Total phenol content showed differences among stations with increasing concentrations from north to south. The microbial communities showed differences among stations and plant compartments, with high incidence of Gammaproteobacteria and Bacteroidetes in light limiting conditions, while Cyanobacteria and Rhodobacteraceae thrived in conditions of high light availability and hydrodynamics. The mutual response of the seagrass plants and the microbial communities provided evidence of their functional relationship, which undoubtedly needs further investigation. To the best of our knowledge, this is the first time that such descriptors have been used in an integrated approach. We provide evidence of their effectiveness in discriminating seagrass ecological status, even at small spatial scales. This work constitutes a new approach to the assessment of seagrasses and a stepping stone in the application of microbial communities as a putative marker in a changing environment.

The tropical seagrass Halophila stipulacea is the most dominant and widespread seagrass species in the Gulf of Aqaba (GoA, northern Red Sea). In light of the ecological services provided by H. stipulacea in its native regions, and its reported colonization and dispersion in non-native areas, there is a growing global interest in understanding its vegetative development and growth strategies under varying environmental gradients. Aiming to compare growth and biomass production rates in native vs. invasive regions in the near future, it is rather surprising that such baseline data hardly exists for H. stipulacea. Focusing on the northern GoA, we investigated morphological characteristics, in situ growth rates, and biomass production at three different sites (NB, TY and SB), and at each site, at two depths (5 and 14 m) during the summer months (June-July 2019). Significantly larger (11 %) rhizome internodes and longer (19 %) and wider (15 %) leaves were observed in deeper plants compared to those inhabiting shallow meadows. On the contrary, shoot and internode formation rates in shallow plants were markedly higher than in deep-adapted plants, with production values of one shoot and one internode every 5.9 ± 0.8 and 6.9 ± 0.6 days, respectively. Rhizome elongation rates did not vary significantly across sites and depth with an average value of 0.2 cm d−1. In addition, larger leaf area, number of leaves, and higher above and below-ground biomass were observed in the NB site, compared with TY and SB, suggesting that more favourable growth conditions for H. stipulacea existed at NB which is exposed to high anthropogenic pressures. Results highlight how the species is highly responsive to in situ conditions, and thus contribute to an improved understanding of its vegetative dynamics and growth strategy as a basis for future comparisons and temporal assessments.

The tropical seagrass Halophila stipulacea is native to the Red Sea, Persian Gulf and the Indian Ocean. Following the opening of the Suez Canal, H. stipulacea became a Lessepsian immigrant, spreading to most of the eastern Mediterranean Sea. Its arrival in the Caribbean, where it has changed the local seagrass landscapes, has led to concerns about its potential effects on Mediterranean seagrass diversity. Surprisingly, morphological, growth, structural and demographic traits have never been quantitively compared between native and invasive populations of H. stipulacea. Aiming to deepen our understanding of the invasiveness potential of H. stipulacea, we recently established permanent monitoring transects at similar depths in both the native (Eilat, Israel) and invasive (Limassol, Cyprus) habitats. Samples were collected at both locations, four times during 2017 and were used for comparing morphological and growth traits (internode distance¸ leaf surface area) alongside structural and demographic traits (above and below ground biomass, percent cover, shoot density). Results show that above-ground biomass, shoot density and percent cover were higher year-round in Eilat than in Limassol, reaching a maximum in July (Eilat) and October (Limassol). Leaves in Eilat were larger than in Limassol but year-round, plants from Limassol had more apical shoots and larger internodes. This study used a standardized methodology to provide the first quantitive comparison between populations of native and invasive Halophila stipulacea and sheds light on the importance of long-term monitoring in both native (Red Sea) and invasive (Mediterranean and Caribbean Seas) regions. Results from our study are important for understanding the current population dynamics of H. stipulacea in both regions and could be used as baseline data for future assessments.

Genetic diversity and structure of populations at the edge of the species' spatial distribution are important for potential adaptation to environmental changes and consequently, for the long-term survival of the species. Here, we combined classical population genetic methods with newly developed network analyses to gain complementary insights into the genetic structure and diversity of Acacia tortilis, a keystone desert tree, at the northern edge of its global distribution, where the population is under threat from climatic, ecological, and anthropogenic changes. We sampled A. tortilis from 14 sites along the Dead Sea region and the Arava Valley in Israel and in Jordan. In addition, we obtained samples from Egypt and Sudan, the hypothesized origin of the species. Samples from all sites were genotyped using six polymorphic microsatellite loci.Our results indicate a significant genetic structure in A. tortilis along the Arava Valley. This was detected at different hierarchical levels-from the basic unit of the subpopulation, corresponding to groups of trees within ephemeral rivers (wadis), to groups of subpopulations (communities) that are genetically more connected relative to others. The latter structure mostly corresponds to the partition of the major drainage basins in the area. Network analyses, combined with classical methods, allowed for the identification of key A. tortilis subpopulations in this region, characterized by their relatively high level of genetic diversity and centrality in maintaining gene flow in the population. Characterizing such key subpopulations may enable conservation managers to focus their efforts on certain subpopulations that might be particularly important for the population's long-term persistence, thus contributing to species conservation within its peripheral range.

Along the arid Arava, southern Israel, acacia trees are considered keystone species. Based on survival of acacias through drought years that have not seen a single rainfall or flood, it is assumed that acacias endure the local harsh conditions by drawing water from deep underground water reservoirs. Studying water use of acacias has so far been done by exposing root systems or by isotopic water measurements, both problematic methods. We present a new application of a two-dimensional direct current electrical resistivity tomography (ERT) to measure the electrical resistivity of the subsurface. We applied the ERT device to two fixed 100 m transects cutting across (north-south) and along (west-east) the super arid Gidron Wadi during March, April and August 2013. An inversion model was used to create a map of electrical resistivity of the layers below ground, an indirect indicator of the underground water content. We identified a conductive layer located at ∼7–10 m below ground in all surveys. Although we could not identify changes to this apparent perched aquifer, such a layer may be a secondary source of water for acacias that can explain their survival in drought years. Further ERT-aided studies are needed to correlate acacia distribution and ecophysiological state with perched aquifers, even if quantitative analysis of their replenishment in such desert environments is not trivial.

Mass coral bleaching is increasing in frequency and severity, leading to the loss of coral abundance and diversity. However, some corals are less susceptible to bleaching than others and can provide a model for identifying the physiological and biogeochemical traits that underlie coral resilience to thermal stress. Corals from Eilat in the Gulf of Aqaba in the northern Red Sea do not bleach unless seawater temperatures are sustained at +6°C or higher above their average summer maximum. This extreme thermal tolerance qualifies these as super-corals, as most corals bleach when exposed to temperatures that are only +1–2°C above their thermal maximum. Here, we conducted a controlled bleaching experiment (+6°C) for 37 days (equivalent to 32° heating weeks) on three species of corals from Eilat: Stylophora pistillata, Pocillopora damicornis, and Favia favus. To assess the response of the holobiont to thermal stress, the following variables were measured on each coral: endosymbiotic algal cell density, Chlorophyll a, endosymbiotic mitotic cell division, total lipids, protein, carbohydrate, and the stable carbon (δ13C) and oxygen (δ18O) isotopic composition of the skeleton and the δ13C of the animal host tissue and endosymbiotic algae. While all three species appeared visibly bleached, their physiological and biogeochemical responses were species-specific. S. pistillata catabolized lipids but still maintained total energy reserves and biomass. Increases in both skeletal δ13C and δ18O indicates that calcification declined in this species. P. damicornis was the least affected by bleaching. It maintained its total energy reserves and biomass, and isotopic evidence suggests that it maintained calcification and was not dependent on heterotrophy for meeting metabolic demand when bleached. Finally, F. favus catabolized protein and carbohydrates, and suffered losses in total energy reserves and biomass. Nevertheless, isotopic evidence suggest that photosynthesis and calcification were maintained, and that this species has a high baseline heterotrophic capacity. Thus, just like their non-super-coral conspecific counterparts, maintaining energy reserves and biomass, and heterotrophic capacity appear to be traits that underlie the thermal tolerance of these super-corals from Eilat. Given the high thermal tolerance of these super-corals, these populations could provide viable seed stock for repopulating coral losses on other reefs.

Katsanevakis, Stelios ... et al.-- 16 pages, 2 figures, Cumulative human impacts have led to the degradation of marine ecosystems and the decline of biodiversity in the European and contiguous seas. Effective conservation measures are urgently needed to reverse these trends. Conservation must entail societal choices, underpinned by human values and worldviews that differ between the countries bordering these seas. Social, economic and political heterogeneity adds to the challenge of balancing conservation with sustainable use of the seas. Comprehensive macro-regional coordination is needed to ensure effective conservation of marine ecosystems and biodiversity of this region. Under the European Union Horizon 2020 framework programme, the MarCons COST action aims to promote collaborative research to support marine management, conservation planning and policy development. This will be achieved by developing novel methods and tools to close knowledge gaps and advance marine conservation science. This action will provide support for the development of macro-regional and national policies through six key actions: to develop tools to analyse cumulative human impacts; to identify critical scientific and technical gaps in conservation efforts; to improve the resilience of the marine environment to global change and biological invasions; to develop frameworks for integrated conservation planning across terrestrial, freshwater, and marine environments; to coordinate marine conservation policy across national boundaries; and to identify effective governance approaches for marine protected area management. Achieving the objectives of these actions will facilitate the integration of marine conservation policy into macro-regional maritime spatial planning agendas for the European and contiguous seas, thereby offsetting the loss of biodiversity and ecosystem services in this region, This article is based upon work from COST (European Cooperation in Science and Technology) Action 15121 ‘Advancing marine conservation in the European and contiguous seas (MarCons) - supported by the Horizon 2020 framework programme for research and innovation

Halophila stipulacea is a small tropical seagrass species. It is the dominant seagrass species in the Gulf of Aqaba (GoA; northern Red Sea), where it grows in both shallow and deep environments (1–50 m depth). Native to the Red Sea, Persian Gulf, and Indian Ocean, this species has invaded the Mediterranean and has recently established itself in the Caribbean Sea. Due to its invasive nature, there is growing interest to understand this species’ capacity to adapt to new conditions, which might be attributed to its ability to thrive in a broad range of ecological niches. In this study, a multidisciplinary approach was used to depict variations in morphology, biochemistry (pigment and phenol content) and epiphytic bacterial communities along a depth gradient (4–28 m) in the GoA. Along this gradient, H. stipulacea increased leaf area and pigment contents (Chlorophyll a and b, total Carotenoids), while total phenol contents were mostly uniform. H. stipulacea displayed a well conserved core bacteriome, as assessed by 454-pyrosequencing of 16S rRNA gene reads amplified from metagenomic DNA. The core bacteriome aboveground (leaves) and belowground (roots and rhizomes), was composed of more than 100 Operational Taxonomic Units (OTUs) representing 63 and 52% of the total community in each plant compartment, respectively, with a high incidence of the classes Alphaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria across all depths. Above and belowground communities were different and showed higher within-depth variability at the intermediate depths (9 and 18 m) than at the edges. Plant parts showed a clear influence in shaping the communities while depth showed a greater influence on the belowground communities. Overall, results highlighted a different ecological status of H. stipulacea at the edges of the gradient (4–28 m), where plants showed not only marked differences in morphology and biochemistry, but also the most distinct associated bacterial consortium. We demonstrated the pivotal role of morphology, biochemistry (pigment and phenol content), and epiphytic bacterial communities in helping plants to cope with environmental and ecological variations. The plant/holobiont capability to persist and adapt to environmental changes probably has an important role in its ecological resilience and invasiveness.

Halophila stipulacea is a small tropical seagrass species. It is the dominant seagrass species in the Gulf of Aqaba (GoA; northern Red Sea), where it grows in both shallow and deep environments (1–50 m depth). Native to the Red Sea, Persian Gulf, and Indian Ocean, this species has invaded the Mediterranean and has recently established itself in the Caribbean Sea. Due to its invasive nature, there is growing interest to understand this species’ capacity to adapt to new conditions, which might be attributed to its ability to thrive in a broad range of ecological niches. In this study, a multidisciplinary approach was used to depict variations in morphology, biochemistry (pigment and phenol content) and epiphytic bacterial communities along a depth gradient (4–28 m) in the GoA. Along this gradient, H. stipulacea increased leaf area and pigment contents (Chlorophyll a and b, total Carotenoids), while total phenol contents were mostly uniform. H. stipulacea displayed a well conserved core bacteriome, as assessed by 454-pyrosequencing of 16S rRNA gene reads amplified from metagenomic DNA. The core bacteriome aboveground (leaves) and belowground (roots and rhizomes), was composed of more than 100 Operational Taxonomic Units (OTUs) representing 63 and 52% of the total community in each plant compartment, respectively, with a high incidence of the classes Alphaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria across all depths. Above and belowground communities were different and showed higher within-depth variability at the intermediate depths (9 and 18 m) than at the edges. Plant parts showed a clear influence in shaping the communities while depth showed a greater influence on the belowground communities. Overall, results highlighted a different ecological status of H. stipulacea at the edges of the gradient (4–28 m), where plants showed not only marked differences in morphology and biochemistry, but also the most distinct associated bacterial consortium. We demonstrated the pivotal role of morphology, biochemistry (pigment and phenol content), and epiphytic bacterial communities in helping plants to cope with environmental and ecological variations. The plant/holobiont capability to persist and adapt to environmental changes probably has an important role in its ecological resilience and invasiveness.

Climate models predict increases in frequency of summer heat waves. In Europe, such events have already caused declines in seagrass meadows, highlighting the importance of short-term responses of local communities to climate stress. Understanding the variability among populations along the European thermal gradient in response to heat waves is crucial for seagrass conservation and management. Using a mesocosm we compared effects of a simulated heat wave on the photophysiology of Zostera marina populations coming from low (43° N, Adriatic Sea) and high latitudes (56° N, North and Baltic Seas). Measurements before, during and up to 4 wk after the heat wave included photophysiological parameters derived from light response curves generated by PAM fluorometry and gene expression using qRT-PCR. In all 3 populations, initial exposures to thermal stress were characterized by increases in dark adapted effective quantum yield (Y0), maximum electron transfer rate of PSII (ETRmax) and slope of the light response curve (α), coinciding with upregulations of the gene superoxidase dismutase [Mn]. With continuation of the heat wave these initial effects disappeared, demonstrated by declines in Y0, ETRmax and α relative to controls. Z. marina from the Adriatic suffered from the simulated heat wave as much as its high-latitude counterparts. However, we also demonstrate slight photophysiological differences between the populations during the recovery phase, where performance of high-latitude populations continued declining even after water temperatures returned to control levels, while photochemical activity fully recovered in the Adriatic population. These results might draw the attention of future studies and seagrass conservation efforts.

article Assessing environmental impacts on coral reef communities has become a growing discipline. As most corals grow relatively slowly, the common method of monitoring changes in coral communities may limit our ability to identify stressors and stress responses. Since chlorophyll density (amount of chlorophyll a+c2 per unit of coral surface area) in corals may correlate with coral color, the latter has been suggested as an indicator of the natural seasonal changes in undisturbed ("healthy") corals, as well as an indicator of environmentally-induced stresses in corals, including those related to bleaching. However, the color of underwater objects as perceived through the naked eye or a camera is affected by attenuation of irradiance with depth, changes in spectral properties of underwater light, turbidity and dissolved materials in the water, among other factors. Thus, methodological issues have so far held back the use of color as a quantitative indicator for chlorophyll content. Presented here are two accurate and easy to use methods for quantitative measurements of chlorophyll density from digital photographs of corals. In these methods, the intensities of the red, green and blue channels in digital photographs were compared to measured chlorophyll densities in Stylophora pistillata. Variations in external light, known to bias the true color in underwater images, were eliminated either by photographing corals through a specially built funnel with an internal light source or by mathematically normalizing color channels to a gray reference scale. In both methods, chlorophyll density was highly correlated with the intensity of the red channel, despite large variations in lighting conditions during the photography. These photographic methods enabled the estimation of natural spatial and temporal changes of the chlorophyll density of corals. By predicting chlorophyll density in corals at very low costs, both methods presented here could facilitate the study of large-scale physiological changes in corals.

Photoacclimation of the symbiotic hermatypic coral Stylophora pistillata was studied both for colonies growing at different depths (1 to 20 m) sampled within the same season ('spatial photoacclimation') and for colonies growing at the same depth (5 m) but sampled throughout the year ('temporal photoacclimation'). Specimens were darker and had increased chlorophyll densities (chlorophyll per coral surface area) under low-light conditions caused both by increased depth and, in shallow-growing specimens, by the winter season. However, while increased depth involved increased zooxanthella chlorophyll content, acclimation to low irradiances by shallow-growing corals during the winter mostly involved an increase in algal density. In shallow-growing corals, both zoox- anthella chlorophyll content and coral chlorophyll density fluctuated seasonally in correlation with light more than with nitrogen or temperature. This could be related to the fact that reefs in Eilat, Israel, are exposed to more solar radiation than many other reefs in the world. Histological analyses of high-light corals showed that zooxanthellae were located deeper inside the host tissue and were smaller in diameter compared to zooxanthellae from low-light corals. Additionally, while deep-water colonies hosted clade C zooxanthellae, colonies growing in shallow waters, both in high- and very low-light environments, hosted clade A. These clade differences were found to affect coral physiol- ogy, as examined in a short-term thermal stress experiment, with clade-A colonies having higher thermal resilience than clade-C colonies. Taken together, our results show evidence of different mechanisms that help adjust the light-capturing abilities of S. pistillata zooxanthellae to spatial and temporal changes in irradiance.

Corallimorpharians may dominate some habitats on coral reefs and compete with stony corals for access to light, yet little is known concerning their photosynthetic traits. At Eilat in the northern Red Sea, we observed that the abundance of individuals of the corallimorpharian Rhodactis rhodostoma decreased significantly with depth on the reef slope. Field and laboratory experiments revealed that they employ several mechanisms of photoadaptation to high irradiance on the shallow reef flat. Their endosymbiotic microalgae (zooxanthellae) varied significantly in both abundance and chlorophyll content with level of irradiance. Use of a diving pulse amplitude modulated fluorometer revealed that the zooxanthellae of R. rhodostoma effectively disperse excess light energy by expressing significantly higher values of non-photochemical quenching and maximum excitation pressure on photosystem II when experimentally exposed to high light (HL) versus low light (LL). Host corallimorpharian tissues mediated this response by shielding the algal symbionts from high irradiance. The endoderm of host tentacles thickened significantly and microalgal cells were located further from the mesoglea in HL than in LL. The clades of zooxanthellae hosted by the corallimorpharians also varied with depth. In shallow water, all sampled individuals hosted clade C zooxanthellae, while in deep water the majority hosted clade D. The photosynthetic output of individuals of R. rhodostoma was less affected by HL than was that of a stony coral examined. When exposed to both high temperature (HT) and HL, individuals of R. rhodostoma reduced their maximum quantum yield, but not when exposed to HL at low temperature (LT). In contrast, colonies of the scleractinian coral Favia favus reduced their photosynthetic output when exposed to HL in both temperature regimes. After 2 weeks of HT stress, R. rhodostoma polyps appeared to bleach completely but re-established their zooxanthella populations upon return to ambient temperature. We conclude that mechanisms of photoadaptation to high irradiance employed by both the endosymbiotic zooxanthellae and host corallimorpharians may explain in part the abundance of R. rhodostoma on some shallow reef flats. The ability to survive for weeks at HT while bleached also may allow corallimorpharians to repopulate shallow reef areas where scleractinians have been killed by thermal stress.

Within the Gulf of Aqaba, coral reefs have been considered the dominating ecosystem, attracting not only scientific attention, but also restoration efforts, aquarium exhibits and outreach educational programs. Seagrasses, which are known worldwide as a highly important ecosystem, have not received much attention by the public, scientists or authorities involved in conservation and management in this region. This is surprising considering the value of ecosystem services associated with seagarsses and the fact that many of these services are important to adjacent coral reefs. The absence of comprehensive seagrass maps for this region, prohibits other downstream activities such as evaluations of the associated ecosystem services and implementation of potential conservation and management tools. Here we present efforts to map seagrass meadows along the Israeli coast of the northern Gulf of Aqaba. Mapping was done by snorkeling with a GPS and a handheld echo-sounder. Transects perpendicular to the shore were conducted with measurements taken every 10-20 m that included water depth, GPS position and percent of seagrass cover estimated visually up to 13-23 m depending on visibility. In addition to these transects, we also tracked the shallow boundary of the meadows parallel to shore, usually at 3-5 m depth. Both data sets were then fed into Arc-GIS to create an interpolated GIS layer. Out of 11 km available shoreline, we swam along 9.7 km and collected a total of 2830 data points. Seagrasses were found growing along 7.5 km of these shores, with seagrass meadows covering 707,000 sqm2.estimated to be worth = US$ 2,000,000 year-1 in associated ecosystem services. In addition to mapping the seagrass meadows themselvs, we also mapped the potential dangers to seagrasses in the region. Disturbances were ranked as low, medium or high according to severity and frequency. We expect that these maps (GIS layers) will allow us to not only understand the current distribution of seagrasses in the area, but also to develop a GIS-based tool that will improve our understanding of how changes in the Gulf could affect the cover and state of seagrasses, and thus improve conservation efforts in the region.