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- The effects of epiphytes on light harvesting and antioxidant responses in the seagrass posidonia oceanicaPublication . Costa, Monya; Silva, João; Olivé, Irene; Barrote, Isabel; Alexandre, Ana; Albano, Sílvia; Santos, RuiPosidonia oceanica (L.) Delile is a subtidal seagrass whose leaves are commonly colonized by epiphytes. Epiphytes pose physical barriers to light penetration within the leaves, with possible significant impacts on photosynthesis. Furthermore, epiphytes can indirectly be responsible for leaf chlorosis, necrosis and senescence which are known to be related with the increase of oxygen reactive species (ROS) levels, potentially leading to oxidative stress. The aim of this work was to investigate in situ (i) the effect of epiphytes on the composition and balance of light harvesting pigments in leaves of the naturally growing seagrass P. oceanica, and (ii) evaluate differences in antioxidant responses. Epiphytized and non-epiphytized plants were analyzed to establish potential photosynthetic pigment roleshift between light harvesting and photoprotection functions. The experiments were carried out in Cabo de Gata Natural Park, southern Spain, where epiphytized and non-epiphytized plants can be found at identical depths and light exposure. The results showed that both O2 evolution rate along the day and chlorophyll a/b ratio were higher in non-epiphytized plants, indicating a negative effect of epiphytes on photosynthesis and light harvesting. Although under high irradiance (at solar noon) the xanthophyll cycle was activated in both epiphytized and non-epiphytized leaves, the de-epoxidation-ratio (AZ/VAZ) was lower in epiphytized leaves, due to light attenuation by epiphytes. The antioxidant capacity (TEAC and ORAC essays) and the activity of the antioxidant enzymes ascorbate peroxidase and dehydroascorbate were higher in epiphytized plants, showing that epiphytes can also be a potential source of oxidative stress to P. oceanica. Our results show that despite the light attenuation effect, leaf colonization by epiphytes can also be potentially stressful and reduces plant productivity.
- Posidonia oceanica photosynthesis along a depth gradientPublication . Costa, Monya; Silva, João; Olivé, Irene; Barrote, Isabel; Santos, RuiSeagrass photosynthetic rates depend largely on light availability, along with other environmental factors and the physiological condition of the plants. Along a vertical gradient, seagrasses are permanently exposed to distinct light environments, to which the photosynthetic apparatus must adapt. In this study, the response of Posidonia oceanica photosynthesis to light was investigated in plants collected at three different depths (3, 20 and 30m) in the Bay of Revellata, Corsica, France, in the the marine research station Stareso (42º34’4’’N, 8º43’2’’E) in October, 2011. Photosynthesis-irradiance curves (P-I) were measured in an oxygen electrode system (DW3/CB1, Hansatech). Photosynthetic rates were determined over an increasing range of irradiance values, from darkness to 850 µmol quanta m-2 s-1. The Jassby and Platt hyperbolic model (1957) equation was fitted to the obtained data, to calculate the relevant photosynthetic parameters. For each depth, maximum photosynthetic rate (Pmax), inicial slope (), light saturation point (Ik), light compensation point (Ic) and dark respiration (Rd) were determined. Photosynthetic parameters showed significant variations with depth, particularly between the shallowest plants (3m) and the two other depths. As expected, P. oceanica from 3m depth revealed a “sun-adapted” photosynthetic behavior in contrast with plants from deeper waters.
- Seagrass photo-physiological responses in natural high-CO2 environmentPublication . Costa, Monya; Olivé, Irene; Barrote, Isabel; Procaccini, G.; Mazzuca, S.; Vizzini, Salvatrice; Santos, RuiThe atmospheric concentration of CO2 has been steeply increasing over the last 200 years, with an associated increase of total dissolved inorganic carbon (Ci) and a decrease of the oceans’ pH. Seagrasses are among the most productive marine ecosystems, but yet little is known on the effects of high-CO2/low pH on their photosynthetic physiology and the ecological consequences. Marine CO2 seepage areas have been used as natural laboratories to investigate the performance of marine organisms under long-term exposure to high-CO2 levels that mimic the future ocean. In this work we conducted a series of experiments comparing the photophysiology of the seagrasses Posidonia oceanica and Cymodocea nodosa, growing in the vicinity of submarine CO2 vents around the islands of Vulcano and Panarea (Aeolian Archipelago, Southern Tyrrhenian Sea, Italy). Plants growing close to CO2-seepage sites were compared with plants from control sites. Automated chlorophyll fluorometers were deployed for 24-hour periods to examine the changes in photosynthetic efficiency and energy quenching mechanisms. Samples were collected at predawn and noon and analyzed for pigment composition, antioxidant capacity, and soluble carbohydrates. Differences in gene and protein expression were evaluated as a function of Ci levels. Stable carbon isotopes (δ13C) were also analysed to investigate the contribution of volcanic CO2 to seagrass productivity. Both P. oceanica and C. nodosa plants growing in CO2-seepage sites showed lower allocation of PSII-absorbed energy to photochemistry (φII), while presenting higher proportions of energy dissipation by non-photochemical pathways (down-regulation, φNPQ and other energy losses, φNO). As well, diel photosynthesis-irradiance curves (ETRI), built with data acquired over the 24-hour deployments, showed lower photosynthetic rates in plants from CO2 seepage sites. This unexpected pattern of photosynthetic activity will be discussed in light of the complementary data.