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- Nutrition of the seagrass Cymodocea nodosa: pulses of ammonium but not of phosphate are crucial to sustain the species growthPublication . Alexandre, Ana; Santos, RuiWe investigated the nutrient acquisition of the seagrass Cymodocea nodosa and the importance of sediment to water column nutrient pulses, which regularly occur in coastal lagoons forced by incoming flood tides. Ammonium was preferentially taken up and accounted for 60-90% of the total daily N acquisition, whereas amino acid acquisition through belowground plant parts represented the second most important source of N, accounting 8-34%. The uptake of ammonium pulses increased dramatically the daily N acquisition from 9.5 mmol m-2 day-1 to 1.33 mmol m-2 day-1 at ambient nutrient concentrations, enabling the species to meet its N demand. Roots accounted for 96% of the total phosphate acquisition, with no evidence for P limitation. The ability of seagrasses to adapt to nutrient pulses may be an overlooked nutrition strategy common to many ecosystems where nutrient availability in the water column is low but nutrient pulses occur.
- The takeover of Thalassia testudinum by Anadyomene sp. at Biscayne Bay, USA, cannot be simply explained by competition for nitrogen and phosphorousPublication . Alexandre, Ana; Collado-Vides, Ligia; Santos, RuiApart from direct light effects, we tested whether the takeover of the seagrass Thalassia testudinum by the seaweed Anadyomene sp. in high nutrient areas of Biscayne Bay, Florida, USA, is related to a faster nutrient surge uptake capacity of the seaweed and/or a negative effect on the seagrass uptake rates. Anadyomene sp. and T. testudinum showed a similar ammonium surge uptake capacity, but the seagrass performed better than the seaweed in mixed incubations at high ammonium concentrations. T. testudinum was faster than Anadyomene sp. at taking up pulses of phosphate, but the uptake rates of the seagrass were significantly decreased in the presence of the seaweed. The takeover of T. testudinum by Anadyomene sp. at Biscayne Bay is likely dominated by light and cannot be simply explained by their single or mixed nutrient surge uptake rates, but the phosphate availability and the seagrass uptake inhibition by the seaweed may also play a key role in the process.
- High nitrogen and phosphorous acquisition by belowground parts of Caulerpa prolifera (Chlorophyta) contribute to the species' rapid spread in Ria Formosa lagoon, Southern PortugalPublication . Alexandre, Ana; Santos, RuiDespite worldwide proliferation of the genus Caulerpa and subsequent effects on benthic communities, little is known about the nutritional physiology of the Caulerpales. Here, we investigated the uptake rates of ammonium, nitrate, amino acids, and phosphate through the fronds and rhizoids + stolon, the internal translocation of nitrogen, and developed a nitrogen budget for the rapidly spreading Caulerpa prolifera in Ria Formosa lagoon, southern Portugal. Caulerpa prolifera acquired nutrients by both aboveground and belowground parts at similar rates, except nitrate, for which fronds showed 2-fold higher uptake rates. Ammonium was the preferential nitrogen source (81% of the total nitrogen acquisition), and amino acids, which accounted for a significant fraction of total N acquisition (19%), were taken up at faster rates than nitrate. Basipetal translocation of 15 N incorporated as ammonium was nearly 3-fold higher than acropetal translocation, whereas 15 N translocation as nitrate and amino acids was smaller but equal in either direction. The estimated total nitrogen acquisition by C. prolifera was 689 μmol · m-2 · h-1 , whereas the total nitrogen requirement for growth was 672 μmol · m-2 · h-1 . The uptake of ammonium and amino acids by belowground parts accounted for the larger fraction of the total nitrogen acquisition of C. prolifera and is sufficient to satisfy the species nitrogen requirements for growth. This may be one reason explaining the fast spreading of the seaweed in the bare sediments of Ria Formosa where it does not have any macrophyte competitors and the concentration of nutrients is high.
- Urea as a key nitrogen source for the invasion of thesouthern coast of Portugal by the brown seaweedRugulopteryx okamurae (Dictyotales, Phaeophyceae)Publication . Jiménez Herrero, Javier; Alexandre, Ana; Silva, João; Santos, RuiThe invasive seaweed Rugulopteryx okamurae, native to East Asia, is spreading rapidly along the western Mediterranean and southern Portugal, severely affecting coastal biodiversity, ecosystem structure, and economic sectors such as fisheries and tourism. This study examined the nutrient up-take kinetics of R. ok amurae, including ammonium, nitrate, urea, amino acids, and phosphate, and their role in nitrogen and phosphorus budgets based on laboratory growth rates. R. ok amurae demonstrated the highest uptake for ammonium (Vmax = 57.95 μmol · g −1 DW · h −1), followed by urea (7.74 μmol · g −1DW · h −1), nitrate (5.37 μmol · g −1 DW · h −1), and amino acids (3.71 μmol · g −1DW · h −1). The species showed higher uptake affinity for urea (α = 1.8), which accounted for 70% of nitrogen uptake. Phosphate uptake was low, and total nitrogen uptake exceeded growth requirements. These findings suggest that R. okamurae relies on organic nitrogen (urea) and may guide toward effective management strategies to mitigate its spread in coastal ecosystems.