Different strategies of nitrogen acquisition in two tropical seagrasses under nitrogen enrichment

Abstract

Summary Tropical marine seagrasses live in environments with low nutrient concentrations. However, as land development intensifies along tropical coastlines, the marine environment in which these organisms grow is becoming more nutrient‐rich. Nitrogen (N) uptake, assimilation, translocation and storage under a diversity of N sources in enriched conditions were investigated in two tropical seagrass species, Cymodocea serrulata and Thalassia hemprichii, from an oligotrophic marine environment. Both seagrasses were able to take up different inorganic and organic N sources through their above‐ and belowground tissues when enriched with high N concentrations. The uptake rates of T. hemprichii were generally higher than C. serrulata in leaves and rhizome, whereas root uptake was systematically higher in C. serrulata. Acropetal and basipetal translocation was observed in both species. Reduction and assimilation of N, measured in terms of their nitrate reductase and glutamine synthetase activity, were correlated with nitrate and ammonium uptake rates, respectively. Cymodocea serrulata showed a tendency to immediately use the available N, whereas T. hemprichii allocated more N in assimilation and storage investment. The responses of these seagrasses to N‐enrichment demonstrate their ability to adapt to over‐enrichment by varying N sources in the first step of the eutrophication process.

Morphological and Physiological Responses of Enhalus acoroides Seedlings Under Varying Temperature and Nutrient Treatment

“…These adult seagrasses, namely Thalassia hemprichii and Cymodocea serrulata, have greater SA and potentially higher absolute uptake rates than seedlings, therefore differences between them could be expected. Moreover, the former rely mainly on external nutrient concentrations for growth (Viana et al, 2019), contrary to recently germinated seedlings which can use resources stored in the seed (Balestri et al, 2009). Competition for nutrients among microalgae, seagrass adults and seedlings, however, cannot be discarded, but was not directly measured in this study.…”

“…Eutrophication has been identified as one of the most important factors affecting productivity, community carbon dynamics, and seagrass growth, and is one of the major threats confronting coastal ecosystems. Direct effects occur through stability of physiological mechanisms (Burkholder et al, 1992;Bird et al, 1998;Brun et al, 2002;Invers et al, 2004;Touchette and Burkholder, 2007) causing increased nutrient uptake ability (Viana et al, 2019), nutrient imbalance (Li et al, 2019), changes in morphological indices (Mvungi and Pillay, 2019), changes in growth (Terrados et al, 1999a), changes in sexual reproduction (Duarte et al, 1997), or direct ammonium toxicity (Van Katwijk et al, 1997). Indirect effects of nutrient inputs occur through blooming algae which cause light depletion or nutrient competition (Duarte, 1995;Short et al, 1995;Moore and Wetzel, 2000;Nixon et al, 2001;Burkholder et al, 2007), through the ecological role of herbivores due to modifications in palatability and plant defenses against herbivory (Tomás et al, 2015;Jiménez-Ramos et al, 2017;Marco-Méndez et al, 2017;Campbell et al, 2018;Hernán et al, 2019), or through oxygen depletion in sediments (Terrados et al, 1999b).…”

Species-Specific Trait Responses of Three Tropical Seagrasses to Multiple Stressors: The Case of Increasing Temperature and Nutrient Enrichment

“…As seagrasses are the only submerged marine angiosperms, they can access both nutrients from water column and pore water. Thus, below-ground (BG) tissues play a key role in taking up different nutrient sources to meet in situ demands and translocating them along the plant (Viana et al, 2019a). This is especially important in the tropics as nitrogen (N) is constantly available in the pore water at higher concentrations than in the water column, and leaf turnover is constant throughout the year, whereas BG tissues have lower turnover rates (Duarte, 1991).…”

Tolerance to Nitrogen ions enrichment in a Gulf of Mexico's freshwater submerged grass

“…Furthermore, N concentration variation, time exposure, and natural N ionic forms at different ratio are factors that have been related to trophic and environmental dynamics [2,4,5]. Moreover, similar effects of limited growth and productivity on submerged grasses have been observed in experiments of stress by N enrichment, with nitrate (NO 3 ), ammonium (NH 4 ) and both ions [2,6,7,8,9,10,11,12,13], compared with experiments of high salt concentration, temperature increases and low light intensity [4,14,15,16,17,18]. Additionally, the genotype-environment interactions and the concentration of all inorganic ions in the sediment and water, differ signi cantly from one experiment to another; making di cult to identify punctual stressors [19,20].…”

“…N-NH 4 , N-NO 3 and N-NO 2 ) in leaf and root of Vallisneria americana seedlings along with its phenotypes and the total biomass caused by different N inorganic sources (NH 4, NO 3 and NH 4 :NO 3 ) enrichments using in vitro approximation. Under the overall premise that the tolerance of submerged grasses varies ontogenetically [5,13]. The hypothesis being tested is that tolerance by N-centred eutrophication in the juvenile stages of V. americana species is expected to be similar with the three sources of N. This response implies increasing their survival by assimilating N in environments subject to gradual enrichment of N. The study offers knowledge about the local species tolerance due the N enrichment by its implications for the resettlement of freshwater wetlands of the Gulf of Mexico.…”