Feeding through your gills and turning a toxicant into a resource: how the dogfish shark scavenges ammonia from its environment

Abstract

Nitrogen (N) appears to be a limiting dietary resource for elasmobranchs, required not only for protein growth but also for urea-based osmoregulation. Building on recent evidence that the toxicant ammonia can be taken up actively at the gills of the shark and made into the valuable osmolyte urea, we demonstrate that the uptake exhibits classic Michaelis-Menten saturation kinetics with an affinity constant (K m ) of 379 µmol l −1 , resulting in net N retention at environmentally realistic ammonia concentrations (100-400 µmol l −1 ) and net N loss through stimulated urea-N excretion at higher levels. Ammonia-N uptake rate increased or decreased with alterations in seawater pH, but the changes were much less than predicted by the associated changes in seawater P NH3 , and more closely paralleled changes in seawater NH 4 + concentration. Ammonia-N uptake rate was insensitive to amiloride (0.1 mmol l ), suggesting that the mechanism does not directly involve Na + or K + transporters, but was inhibited by blockade of glutamine synthetase, the enzyme that traps ammonia-N to fuel the ornithine-urea cycle. High seawater ammonia inhibited uptake of the ammonia analogue [ 14 C]methylamine. The results suggest that branchial ammonia-N uptake may significantly supplement dietary N intake, amounting to about 31% of the nitrogen acquired from the diet. They further indicate the involvement of Rh glycoproteins (ammonia channels), which are expressed in dogfish gills, in normal ammonia-N uptake and retention.

Differential Effects of Temperature on Oxygen Consumption and Branchial Fluxes of Urea, Ammonia, and Water in the Dogfish Shark (Squalus acanthias suckleyi)

“…Nawata et al (2015) reported that Squalus acanthias are N limited and, when presented with high concentrations of ammonia in the water, they have the ability to scavenge ammonia-N, convert it into urea, and elevate urea-N excretion rates. This uptake of ammonia exhibits classic Michaelis-Menten saturation kinetics, resulting in a net retention of nitrogen (Wood and Giacomin 2016). Along with other transformations at the gill, it is possible that at high temperatures the ammoniaconserving uptake mechanism could have been impaired, leading to the high temperature sensitivity seen.…”

“…Squalus acanthias was chosen as our experimental model due to the extensive body of literature on the physiology of this species, which is a migratory, long-lived, very important demersal resource that is under threat due to overfishing and global climate change (Taylor and Galluci 2009), like many other marine elasmobranchs (Dulvy et al 2014). The present investigation was part of a larger ongoing study on the osmorespiratory compromise and gill function in this species (Zimmer and Wood 2014;Nawata et al 2015;Wood and Giacomin 2016). The osmorespiratory compromise is the functional trade-off between respiratory gas exchange versus osmolyte and water exchange that has been well documented in freshwater teleosts (Randall et al 1972;Nilsson 1986) but never examined in marine elasmobranchs.…”

“…The elasmobranch gill is also even less impermeable to ammonia than it is to urea, keeping ammonia losses at just a few percent of urea-N losses (Wood et al 1995(Wood et al , 2005. Recently, Nawata et al (2015) and Wood and Giacomin (2016) showed that nitrogen-limited S. acanthias are capable of actively scavenging ammonia-N from the water by a branchial transporter exhibiting saturation kinetics, resulting in increased synthesis of urea. The mechanism of ammonia uptake is unknown but appears to involve Rh proteins.…”

Nitrogen handling in the elasmobranch gut: a role for microbial urease

“…Indeed, Ballantyne (2016) has characterized them as the most nitrogen-limited aquatic vertebrates. Studies using the spiny dogfish shark (Squalus acanthias) as a model system have described the retention and/or scavenging of nitrogen in the gills (Boylan, 1967;Wood et al, 1995;Pärt et al, 1998;Fines et al, 2001;Hill et al, 2004;Wood et al, 2013;Nawata et al, 2015;Wood and Giacomin, 2016), kidney (Kempton, 1953;Forster and Berglund, 1957;Schmidt-Nielsen and Rabinowitz, 1964;Schmidt-Nielsen et al, 1972;Boylan, 1972;Hays et al, 1977;Wood et al, 1995), rectal gland (Burger and Hess, 1960;Zeidel et al, 2005) and gut (Wood et al, 2007b;Anderson et al, 2012;Liew et al, 2013;Anderson et al, 2015). The intestine (also known as the spiral valve in elamobranchs because of its unique structure) appears to be particularly important in this regard.…”

“…Based on only one study (Wood et al, 2009) that measured ammonia concentrations (1-3 mmol l −1 ) in gastric chyme (not intestinal chyme) of Squalus acanthias, ammonia levels are only approximately 1% of urea levels. Nevertheless, the gills are capable of scavenging ammonia from even lower concentrations in the external water (Nawata et al, 2015;Wood and Giacomin, 2016), so it seems likely that the intestine would be able to absorb ammonia from the chyme, especially because evidence for carrier-mediated ammonia uptake has recently been presented for the teleost intestine (Rubino et al, 2014;Rubino et al, 2015).…”