Ecophysiological plasticity of shallow and deep populations of the Mediterranean seagrasses Posidonia ocenica and Cymodocea nodosa in response to hypersaline stress

The differential expression of the plant phenotypic plasticity due to inter- and intraspecific divergences can determine the plant physiological tolerance under stress. In this work, we examined the interspecific ecophysiological plasticity that the main Mediterranean seagrass species with distinct marine environmental distribution (Posidonia oceanica and Cymodocea nodosa) can exhibit in response to hypersaline stress. We also tested the potential implication of ecotypic intraspecific divergences in the development of such plasticities. To this end, plants from shallow (5–7 m) and deep (18–20 m) meadows of both were maintained under two salinity treatments (natural salinity level of 37, and hypersaline treatment of 43; Practical Salinity Scale) during a long-term experiment (i.e. 62 days) developed in a highly controlled mesocosm system. Hypersaline stress caused notable plastic physiological alterations in P. oceanica andC. nodosa, with appreciable inter- and intraspecific differences. Although both species were similarly able to osmoregulate by means of organic solute accumulation (proline and sugars) in response to hypersalinity stress, higher carbon balance reductions were detected in P. oceanica plants from the deep meadow and in shallower C. nodosa plants, due to both photosynthetic inhibition and enhancement of respiration. None of these deleterious effects were found in C. nodosa plants form the deeper meadow. Leaf photosynthetic pigments generally increased in P. oceanica from both depths, but light absorbance capacities by leaves and photosynthetic efficiency followed contrasting patterns, increasing and decreasing in plants from the deep and the shallow meadows, respectively, indicating distinct strategies to cope with photosynthetic dysfunctions. Despite the significant reduction of pigments in the shallower C. nodosa plants, their leaves were able to increase their light capture capacities under hypersaline stress, by means of particular leaf optics adjustments (pigment packaging reduction). The metabolic costs as a consequence of the physiological plasticity integration seemed to compromise the vitality of P. oceanica, but not in the case ofC. nodosa. These results confirm that both the inter- and intraspecific divergences play a key role in the responses which both Mediterranean seagrasses could develop under hypersaline stress conditions, and that these were consistent with their distinct ecological strategies and salinity tolerance ranges.En prens

Driving factors of biogeographical variation in seagrass herbivory

Este artículo contiene 10 páginas, 3 figuras, 3 tablas.Despite the crucial role of herbivory in shaping community assembly, our understanding on biogeographical patterns of herbivory on seagrasses is limited compared to that on terrestrial plants. In particular, the drivers of such patterns remain largely unexplored. Here, we used a comparative-experimental approach in Cymodocea nodosa meadows, across all possible climate types within the seagrass distribution, 2000 km and 13° of latitude in two ocean basins, to investigate biogeographical variation in seagrass herbivory intensity and their drivers during July 2014. Particularly, the density and richness of herbivores and their food resources, seagrass size, carbon and nitrogen content, as well as latitude, sea surface temperature, salinity, chlorophyll, and sediment grain size, were tested as potential drivers. We found that shallow meadows can be subjected to intense herbivory, with variation in herbivory largely explained by fish density, seagrass size, and annual sea temperature range. The herbivorous fish density was themost important determinant of such variation,with the dominant seagrass consumer, the fish Sarpa salpa, absent atmeadows from regionswith lowherbivory. In temperate regionswhere herbivorous fish are present, annual temperature ranges drive an intense summer herbivory, which is likelymediated not only by increased herbivore metabolic demands at higher temperatures, but also by higher fish densities. Invertebrate grazing (mainly by sea urchins, isopods, amphipods, and/or gastropods) was the dominant leaf herbivory in some temperate meadows, with grazing variation mainly influenced by seagrass shoot size. At the subtropical region (under reduced annual temperature range), lower shoot densities and seagrass nitrogen contents contributed to explain the almost null herbivory.We evidenced the combined influence of drivers acting at geographic (region) and local (meadow) scales, the understanding of which is critical for a clear prediction of variation in seagrass herbivory intensity across biogeographical regions.The Portuguese Foundation for Science and Technology (FCT) funded BMC in the ambit of the contract program DL57/2016/CP1361/ CT0004 and CCMAR through the project UIDB/04326/2020.Peer reviewe

Consumption by <i>S</i>. <i>salpa</i> of control vs. fertilized leaves of <i>P</i>. <i>oceanica</i> during paired food preference experiments (cm²·leaf^-1·d^-1) deployed at Tabarca and CIMAR.

<p>Mean ± SE.</p

Consumption by <i>S</i>. <i>salpa</i> recorded at CIMAR and Tabarca in the different sites for control vs. fertilized treatments (cm²·shoot^-1·d^-1).

<p>Mean ± SE.</p

Three way ANOVA results for: (a) growth rates; (b) nutrient content (C:N ratio) in leaves; (c) in epiphytes; and (d) consumption rates showing differences due to investigated factors “Location” (fixed and orthogonal factor with 2 level), “Site” (random and nested in “location” with 3 levels) and “Treatment” (fixed and orthogonal factor with 2 levels) considering “plot” as a replicate.

<p>(n = 3 fertilized/control plots per site; 9 fertilized/control plots per location).</p

C:N ratio recorded in leaves (a) and epiphytes (b) of <i>P</i>. <i>oceanica</i> at CIMAR and Tabarca in the different sites for control vs. fertilized treatments.

<p>Mean ± SE.</p

Plant water relations and ion homoeostasis of Mediterranean seagrasses (Posidonia oceanica and Cymodocea nodosa) in response to hypersaline stress

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