Thalassia testudinum phosphate uptake kinetics at low in situ concentrations using a 33P radioisotope technique

Abstract

A new 33 P-tracer technique was used to define low-level phosphate (Pi) uptake kinetics for a dominant tropical seagrass, Thalassia testudinum. We established seagrass Pi uptake kinetics at high (western Bay) and low (eastern Bay) nutrient sites in a fine-grained carbonate lagoon of south Florida (Florida Bay), where P limitation of seagrass has been documented. Sediment Pi adsorption kinetics were also investigated to test whether carbonate sediments could sequester Pi to the threshold levels we established for T. testudinum, defined as S min or the physiological threshold value where Pi uptake is positive. At Pi levels characteristic of Florida Bay (Յ0.26 mol L Ϫ1), leaf and root Pi uptake was linear with increasing levels of Pi, and similar affinities (␣) were found for leaves and roots (0.12-0.30 and 0.10-0.20 mol g dry weight Ϫ1 h Ϫ1). S min for roots and leaves was extremely low (0.004-0.009 mol L Ϫ1 ) regardless of the P status of the site. While T. testudinum was able to take up Pi at nanomolar levels, the uptake rates were insufficient for plant requirements. Thus, sediment pools or transient fluxes of P to the water column must sustain high seagrass production rates in the Bay. Sediment adsorption-desorption equilibrium for Pi was 10-fold lower in the eastern versus western Bay sites, meeting Ͻ10% and Ͼ87% of the P demand for T. testudinum, respectively, a result that might account for the reported P limitation of seagrass biomass and production in eastern Florida Bay.

Comparison of the role of the foliar sheath in nutrient (ammonium and phosphate) acquisition by the seagrass Thalassia hemprichii (Ehrenb.) Aschers. at two different sites on tropical Hainan Island, China

“…of T. hemprichii in this study was in line with the study by Stapel et al (1996) of T. hemprichii (21-60). The K m for P i of T. hemprichii in this study was almost the same as earlier estimates for T. hemprichii (7.7-15 for blade by Stapel et al, 1996) and for T. testudinum (12 for blade and 4 for root by Gras et al, 2003, 7.89 for blade and 3.34 for root by Nielsen et al, 2006), again suggesting that no major kinetic differences exist between sites or even species in terms of nutrient uptake at saturation. The consistency of our T. hemprichii nutrient kinetic results have important consequences for modeling nutrient uptake in tropical seagrass ecosystems dominated by Thalassia.…”

“…At low substrate availability, a is a critical nutrient uptake property (Nielsen et al, 2006). We found the a in this study, ranging between 0.36 and 1.39 for P i using traditional chemical techniques, pre-starved, and 10-h incubations, to be much higher than those from southeast Asia, where a was calculated to be 0.13-0.19 for T. hemprichii blades in 10-h incubations (Stapel et al, 1996).…”

“…In general, nutrient uptake experiments on seagrass have focused on defining the maximum uptake rates (V max ) and half saturation constants (K m ) for modeling purposes (Pérez-Lloréns & Niell, 1995;Terrados & Williams, 1997;Gras et al, 2003;Rubio et al, 2007;Vonk et al, 2008). Some of the studies also focused on another characteristic, a (or nutrient affinity; Lee & Dunton, 1999;Nielsen et al, 2006).…”

“…It is now well established that seagrasses take up N and P via both leaves and roots in amounts depending on the relative availability of nutrients in the sediment versus amounts in the water column (Stapel et al, 1996;Lee & Dunton, 1999), and nutrient uptake by leaves is regarded as a process which contributes a considerable part of nutrient demands (reviewed by Hemminga et al, 1991). Currently, there is a paucity of literature on nutrient uptake strategy in tropical seagrass ecosystems, with the exception studies focusing on quantification of the kinetics of N and P uptake by leaves and roots (Schroeder & Thorhaug, 1980;Stapel et al, 1996;Lee & Dunton, 1999;Gras et al, 2003;Cornelisen & Thomas, 2004;Nielsen et al, 2006;Vonk et al, 2008).…”

The role of organic acid exudates in liberating phosphorus from seagrass‐vegetated carbonate sediments

“…A number of studies have investigated total sediment dissolution rates and seagrass uptake rates and found that P i concentrations provided by rates of bulk sediment dissolution are not sufficient to relieve P-limitation (Jensen et al 1998;Gras et al 2003;Nielsen et al 2006). Recent evidence suggests that there are increased rates of sediment dissolution in marine rhizosphere sediments, and that high rates of oxidation caused by seagrass O 2 release may provide the required amount of acid to explain enhanced rates of dissolution (Ku et al 1999;Burdige and Zimmerman 2002;Burdige et al 2008).…”