Analyses of the integrated seagrass response to depth support the previously documented low plasticity and consistent shade-adapted leaf physiology of a habitat-builder that dominates well-illuminated reef environments. Two structural responses, “canopy-opening” and “below-ground-mass-depletion”, govern the photoacclimatory response and facilitate, respectively, light penetration within the canopy and functional adjustments in whole-plant carbon balances. Conversely, “canopy-closing” may also explain dense canopies formed close to the waterline, as they provide shade and photoprotection to a susceptible leaf physiology under high-light. Canopy light attenuation is primarily regulated by the leaf area index (LAI), which is governed by changes in shoot size and density. Shoot density diminishes non-linearly with depth, while shoot size increases to a maximum followed by a decline. The initial increase in shoot size, which resembles a self-thinning response, increases LAI and meadow production in shallow depths. These seagrass structural adjustments have relevant ecological implications. Canopy-thinning allows macrophyte diversity to increase with depth, while seagrass production and carbon storage diminish exponentially, and are maximal only in a shallow coastal fringe. The results support the universality of plant self-thinning, from phytoplankton to complex canopies, likely the consequence of simple physical laws related to light limitation and pigment self-shading within photosynthetic structures and communities.

No abstract

Here we present the results of a multiple organizational level analysis conceived to identify acclimative/adaptive strategies exhibited by the seagrass Posidonia oceanica to the daily fluctuations in the light environment, at contrasting depths. We assessed changes in photophysiological parameters, leaf respiration, pigments, and protein and mRNA expression levels. The results show that the diel oscillations of P. oceanica photophysiological and respiratory responses were related to transcripts and proteins expression of the genes involved in those processes and that there was a response asynchrony between shallow and deep plants probably caused by the strong differences in the light environment. The photochemical pathway of energy use was more effective in shallow plants due to higher light availability, but these plants needed more investment in photoprotection and photorepair, requiring higher translation and protein synthesis than deep plants. The genetic differentiation between deep and shallow stands suggests the existence of locally adapted genotypes to contrasting light environments. The depth-specific diel rhythms of photosynthetic and respiratory processes, from molecular to physiological levels, must be considered in the management and conservation of these key coastal ecosystems.

A complete understanding of the mechanistic basis of marine ecosystem functioning is only possible through integrative and interdisciplinary research. This enables the prediction of change and possibly the mitigation of the consequences of anthropogenic impacts. One major aim of the European Cooperation in Science and Technology (COST) Action ES0609 “Seagrasses productivity. From genes to ecosystem management,” is the calibration and synthesis of various methods and the development of innovative techniques and protocols for studying seagrass ecosystems. During 10 days, 20 researchers representing a range of disciplines (molecular biology, physiology, botany, ecology, oceanography, and underwater acoustics) gathered at The Station de Recherches Sous-marines et Océanographiques (STARESO, Corsica) to study together the nearby Posidonia oceanica meadow. STARESO is located in an oligotrophic area classified as “pristine site” where environmental disturbances caused by anthropogenic pressure are exceptionally low. The healthy P. oceanica meadow, which grows in front of the research station, colonizes the sea bottom from the surface to 37 m depth. During the study, genomic and proteomic approaches were integrated with ecophysiological and physical approaches with the aim of understanding changes in seagrass productivity and metabolism at different depths and along daily cycles. In this paper we report details on the approaches utilized and we forecast the potential of the data that will come from this synergistic approach not only for P. oceanica but for seagrasses in general.

16 17The North Atlantic is the most important sink for atmospheric CO 2 although there still 18 remain uncertainties about the total amount stored by this region and the contribution of 19 the anthropogenic CO 2 (C ANT ) that is exchanged between the Mediterranean Sea and the 20 Atlantic Ocean. During the P 3 A 2 cruise performed in October 2008 throughout the 21 oceanic area covered by the Gulf of Cádiz and the Strait of Gibraltar, which channelizes 22 the water exchange between the Atlantic and the Mediterranean, extensive 23 measurements of the carbon system parameters (pH, total alkalinity and total inorganic 24 carbon) and others related (dissolved oxygen and nutrients) were carried out to analyse 25 their distribution in the area. In order to study the C ANT spatial variability, three 26 observational methods for C ANT concentration assessment (φC T º, ∆C* and TrOCA) 27 were applied. The three water masses identified in the area, North Atlantic Central 28Water (NACW), North Atlantic Deep Water (NADW) and Mediterranean Outflow 29Water (MOW), were shown to contain different C ANT concentration. NADW exhibited 30 the lowest C ANT levels whereas NACW was the most C ANT enriched. Data also indicate 31 a net import of C ANT from the Atlantic towards the Mediterranean through Gibraltar. 32

Abstract. Submarine volcanic vents are being used as natural laboratories to assess the effects of increased ocean acidity and carbon dioxide (CO 2 ) concentration on marine organisms and communities. However, in the vicinity of volcanic vents other factors in addition to CO 2 , which is the main gaseous component of the emissions, may directly or indirectly confound the biota responses to high CO 2 . Here we used for the first time the expression of antioxidant and stress-related genes of the seagrass Posidonia oceanica to assess the stress levels of the species. Our hypothesis is that unknown factors are causing metabolic stress that may confound the putative effects attributed to CO 2 enrichment only. We analyzed the expression of 35 antioxidant and stressrelated genes of P. oceanica in the vicinity of submerged volcanic vents located in the islands of Ischia and Panarea, Italy, and compared them with those from control sites away from the influence of vents. Reverse-transcription quantitative polymerase chain reaction (RT-qPCR) was used to characterize gene expression patterns.Fifty-one percent of genes analyzed showed significant expression changes. Metal detoxification genes were mostly down-regulated in relation to controls at both Ischia and Panarea, indicating that P. oceanica does not increase the synthesis of heavy metal detoxification proteins in response to the environmental conditions present at the two vents. The up-regulation of genes involved in the free radical detoxification response (e.g., CAPX, SODCP and GR) indicates that, in contrast with Ischia, P. oceanica at the Panarea site faces stressors that result in the production of reactive oxygen species, triggering antioxidant responses. In addition, heat shock proteins were also activated at Panarea and not at Ischia. These proteins are activated to adjust stressaccumulated misfolded proteins and prevent their aggregation as a response to some stressors, not necessarily high temperature. This is the first study analyzing the expression of target genes in marine plants living near natural CO 2 vents. Our results call for contention to the general claim of seagrasses as "winners" in a high-CO 2 world, based on observations near volcanic vents. Careful consideration of factors that are at play in natural vents sites other than CO 2 and acidification is required. This study also constitutes a first step for using stress-related genes as indicators of environmental pressures in a changing ocean.

The effects of light and ammonium levels on net production, fluorescence parameters and non-structural carbohydrates of the seagrass Zostera noltii under different phosphate conditions were studied. A fully factorial design was used with light (low/high levels), ammonium supply and phosphate preculture conditions of the plants as the experimental variables. Both ammonium supply and low light caused negative and synergistic effects on net production, while ammonium toxicity was more severe at high light levels; in this case, it was independent of the non-structural carbohydrate (sucrose and starch) content. Preculturing of plant with added phosphate alleviated the ammonium toxicity, and also attenuated the negative production balance of plants grown at low light levels. The results indicated that phosphate preculture ameliorated the plant's short-term response against the assayed stressors (low light, high ammonium) significantly. An overall consumption of non-structural carbohydrates in response to environmental stressors was recorded throughout the experiment, indicating the importance of carbon and phosphorus reserves to cope with adverse conditions. In addition, phosphate deficiency increased the vulnerability of plants, which could have negative ecological consequences for seagrass species thriving under phosphate deficiency conditions, or in developing seagrass transplantation programs.KEY WORDS: Ammonium toxicity · Carbohydrates · Eutrophication · Fluorescence · Net production · Phosphate limitation · Seagrass Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 365: [67][68][69][70][71][72][73][74][75] 2008 seagrasses has been demonstrated (Burkholder et al. 1992(Burkholder et al. , 1994van Katwijk et al. 1997 and Brun et al. 2002 for ammonium) at concentrations as low as 16 µm ammonium. Seagrass leaves take up ammonium in direct proportion to the concentration in the surrounding water, but are not capable of controlling it (Thursby & Harlin 1982, Iizumi & Hattory 1982, van Katwijk et al. 1997, Touchette & Burkholder 2000. Intracellular ammonium has to be rapidly assimilated into organic compounds to prevent its accumulation to potentially harmful levels, as was shown in an experiment with the seagrass Posidonia oceanica, which maintained low intracellular ammonium levels regardless of the nitrogen enrichment levels at which plants were grown (Invers et al. 2004).Ammonium toxicity mechanisms are complex and not fully understood (Britto & Kronzucher 2002), but the physiological mechanisms are thought to be a combination (among others) of: (1) the uncoupling of ATP production during the photosynthetic electron transport that is triggered by ammonia (Goyal et al. 1982, Marschner 1995, (2) the intracellular depletion of essential cations (such as potassium and magnesium; Kirkby 1968, van Katwijk et al. 1997) that is accompanied by the increase of intracellular inorganic anions (e.g. phosphate; Cruz et al. 1993, van Katwijk et al. 1997, (3) an increa...

Here, we report the first use of massive-scale RNA-sequencing to explore seagrass response to CO -driven ocean acidification (OA). Large-scale gene expression changes in the seagrass Cymodocea nodosa occurred at CO levels projected by the end of the century. C. nodosa transcriptome was obtained using Illumina RNA-Seq technology and de novo assembly, and differential gene expression was explored in plants exposed to short-term high CO /low pH conditions. At high pCO , there was a significant increased expression of transcripts associated with photosynthesis, including light reaction functions and CO fixation, and also to respiratory pathways, specifically for enzymes involved in glycolysis, in the tricarboxylic acid cycle and in the energy metabolism of the mitochondrial electron transport. The upregulation of respiratory metabolism is probably supported by the increased availability of photosynthates and increased energy demand for biosynthesis and stress-related processes under elevated CO and low pH. The upregulation of several chaperones resembling heat stress-induced changes in gene expression highlighted the positive role these proteins play in tolerance to intracellular acid stress in seagrasses. OA further modifies C. nodosa secondary metabolism inducing the transcription of enzymes related to biosynthesis of carbon-based secondary compounds, in particular the synthesis of polyphenols and isoprenoid compounds that have a variety of biological functions including plant defence. By demonstrating which physiological processes are most sensitive to OA, this research provides a major advance in the understanding of seagrass metabolism in the context of altered seawater chemistry from global climate change.