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- Light is more important than nutrient ratios of fertilization for cymodocea nodosa seedling developmentPublication . Alexandre, Ana; Silva, João; Santos, RuiRestoration of seagrass beds through seedlings is an alternative to the transplantation of adult plants that reduces the impact over donor areas and increases the genetic variability of restored meadows. To improve the use of Cymodocea nodosa seedlings, obtained from seeds germinated in vitro, in restoration programs, we investigated the ammonium and phosphate uptake rates of seedlings, and the synergistic effects of light levels (20 and 200 mu mol quanta m(-2) s(-1)) and different nitrogen to phosphorus molar ratios (40 mu M N:10 mu M P, 25 mu M N:25 mu M P, and 10 mu N:40 mu M P) on the photosynthetic activity and growth of seedlings. The nutrient content of seedlings was also compared to the seed nutrient reserves to assess the relative importance of external nutrient uptake for seedling development. Eighty two percent of the seeds germinated after 48 days at a mean rate of 1.5 seeds per day. All seedlings under all treatments survived and grew during the 4 weeks of the experiment. Seedlings of C. nodosa acquired ammonium and phosphate from the incubation media while still attached to the seed, at rates of about twice of adult plants. The relevance of external nutrient uptake was further highlighted by the observation that seedlings' tissues were richer in nitrogen and phosphorus than non-germinated seeds. The uptake of ammonium followed saturation kinetics with a half saturation constant of 32 mu M whereas the uptake of phosphate increased linearly with nutrient concentration within the range tested (5 - 100 mu M). Light was more important than the nutrient ratio of fertilization for the successful development of the young seedlings. The seedlings' photosynthetic and growth rates were about 20% higher in the high light treatment, whereas different nitrogen to phosphorus ratios did not significantly affect growth. The photosynthetic responses of the seedlings to changes in the light level and their capacity to use external nutrient sources showed that seedlings of C. nodosa have the ability to rapidly acclimate to the surrounding light and nutrient environment while still attached to the seeds. C. nodosa seedlings experiencing fertilization under low light levels showed slightly enhanced growth if nourished with a balanced formulation, whereas a slight increase in growth was also observed with unbalanced formulations under a higher light level. Our results highlight the importance of high light availability at the seedling restoration sites.
- Nitrogen uptake in light versus darkness of the seagrass Zostera noltei: integration with carbon metabolismPublication . Alexandre, Ana; Silva, João; Santos, RuiWe conducted a study that shows that light and dark conditions do not affect the uptake rates of ammonium and nitrate by the seagrass Zostera noltei. This is an important advantage over some seaweed species in which these rates are severely reduced at night. In the light, the ammonium uptake rates were initially higher (15 and 20molg(-1)h(-1)) and stabilized at a rate of 5molg(-1)h(-1) after 1h, whereas in the dark the rates remained constant at a rate of 10molg(-1)h(-1) over the first 180min of incubation. The rates of nitrate uptake in the light were high within the first 120min of incubation(7.2-11.1molg(-1)h(-1)) and decreased afterwards to lower values (0.8-3.9molg(-1)h(-1)), whereas in the dark the rates fluctuated around 0.0-11.1molg(-1)h(-1) throughout the whole incubation time (7h). The soluble sugar content of Z.noltei leaves increased significantly with both ammonium and nitrate incubations in the light, indicating the metabolic outcome of photosynthesis. In the dark, there was no significant variation in either the soluble sugar or in the starch content of leaves, rhizomes or roots in either the ammonium or nitrate incubations. However, the total starch content of plants decreased at night whereas the total soluble sugars increased, suggesting a process of starch catabolism to generate energy with the consequent production of smaller monosaccharide products. The starch content of rhizomes decreased significantly during the light incubations with nitrate but not with ammonium. These results suggest that carbohydrate mobilization is necessary for Z.noltei to account for extra energetic costs needed for the uptake and assimilation of nitrate. Furthermore, our results suggest that nitrate uptake, at least during the day, requires the mobilization of starch whereas the uptake of ammonium does not.
- 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.
- First description of seagrass distribution and abundance in Sao Tome and PrincipePublication . Alexandre, Ana; Silva, João; Ferreira, Rogério; Paulo, Diogo; Serrao, Ester; Santos, RuiSeagrass meadows in Sao Tome and Principe, eastern Atlantic Ocean, are described here for the first time. Specifically, we quantified the biomass and density of seagrasses, characterized the plant morphology and measure their nutrient content as a proxy of the nutrient environmental conditions where the meadows develop. The seagrass Halodule wrightii was found in two locations of the northeastern coast of the island of Sao Tome: 1) developing throughout an estimated area of 1500 ha surrounding Cabras islet, at a depth range of 4-10 m, on sandy bottom; and 2) at Santana bay with an area of 1500 m(2) at 5-10 m depth, on sandy bottom. A highly morphologically different population of Halodule wrightii was found on the northeastern coast of the island of Principe, off Abade beach, covering an area of 135 m2 at 4 m depth. Further research is needed to assess if this is a different species. Shoot biomass and density was 10 and 4-fold higher in Sao Tome than in Principe, respectively. CN ratios of above and belowground tissues of plants collected in Sao Tome were also significantly higher than in Principe. The carbon content of Halodule leaves from Sao Tome and Principe (41%) was much higher than that reported for other Halodule species, suggesting that meadows may have an important ecological role for carbon fixation. The presence of H. wrightii in Sao Tome and Principe raises ecological and evolutionary questions that warrant further research.