Short- and Medium-Term Exposure to Ocean Acidification Reduces Olfactory Sensitivity in Gilthead Seabream

Abstract

The effects of ocean acidification on fish are only partially understood. Studies on olfaction are mostly limited to behavioral alterations of coral reef fish; studies on temperate species and/or with economic importance are scarce. The current study evaluated the effects of short- and medium-term exposure to ocean acidification on the olfactory system of gilthead seabream (Sparus aurata), and attempted to explain observed differences in sensitivity by changes in the protonation state of amino acid odorants. Short-term exposure to elevated PCO₂ decreased olfactory sensitivity to some odorants, such as L-serine, L-leucine, L-arginine, L-glutamate, and conspecific intestinal fluid, but not to others, such as L-glutamine and conspecific bile fluid. Seabream were unable to compensate for high PCO₂ levels in the medium term; after 4 weeks exposure to high PCO₂, the olfactory sensitivity remained lower in elevated PCO₂ water. The decrease in olfactory sensitivity in high PCO₂ water could be partly attributed to changes in the protonation state of the odorants and/or their receptor(s); we illustrate how protonation due to reduced pH causes changes in the charge distribution of odorant molecules, an essential component for ligand-receptor interaction. However, there are other mechanisms involved. At a histological level, the olfactory epithelium contained higher densities of mucus cells in fish kept in high CO₂ water, and a shift in pH of the mucus they produced to more neutral. These differences suggest a physiological response of the olfactory epithelium to lower pH and/or high CO₂ levels, but an inability to fully counteract the effects of acidification on olfactory sensitivity. Therefore, the current study provides evidence for a direct, medium term, global effect of ocean acidification on olfactory sensitivity in fish, and possibly other marine organisms, and suggests a partial explanatory mechanism.

Keywords: amino acid; carbon dioxide; fish; ocean acidification; olfaction; olfactory epithelium; protonation; receptor.

Figure 1

Normalized olfactory nerve responses of control fish (kept at pH 8.2) to (A) L-serine, (B) L-leucine, (C) L-arginine, (D) L-glutamine, and (E) L-glutamic acid; under control (red: odorant pH 8.2) and elevated PCO₂ conditions (blue: odorant pH 7.7). (F) Effects of acute exposure to elevated PCO₂ in the olfactory detection threshold of L-serine, L-leucine, and L-arginine. Values are shown as mean ± S.E.M. ^*p < 0.05, ^**p < 0.01, ^***p < 0.001; n = 6.

Figure 2

Normalized olfactory nerve responses of control fish (kept at pH 8.2) to conspecific bile fluid (A) and intestinal fluid (B) with odorant pH 8.2 (red) and pH 7.7 (blue). Values are represented as mean ± S.E.M; ^*p < 0.05, n = 6.

Figure 3

Normalized olfactory nerve responses of high CO₂ fish (kept at pH 7.7) to (A) L-serine, (B) L-leucine, (C) L-arginine, (D) L-glutamine, and (E) L-glutamic acid; under odorant pH 8.2 (red) and odorant pH 7.7 (blue) conditions. (F) Effects of exposure to elevated PCO₂ in the olfactory detection threshold of L-serine, L-leucine, and L-arginine. Values are shown as mean ± S.E.M. ^*p < 0.05, ^**p < 0.01, ^***p < 0.001; n = 6.

Figure 4

Normalized olfactory nerve responses of high CO₂ fish (kept at pH 7.7) to conspecific (A) bile fluid and (B) intestinal fluid with odorant pH 8.2 (red) and pH 7.7 (blue). Values are shown as mean ± S.E.M; n = 6.

Figure 5

Normalized olfactory nerve responses of control and high CO₂ fish to L-glutamate (A,B) and intestinal fluid (C,D) with the nostril conditioned with water pH 8.2 (A,C) and pH 7.7 (B,D). Values are shown as mean ± S.E.M. ^*p < 0.05; ^***p < 0.001; n = 6.

Figure 6

Normalized olfactory nerve responses of control fish (red) with the nostril conditioned with water pH 7.7 and expected response (blue), based on the effective odorant concentration at pH 7.7 and the linear regression equation or three-parameter Hill equation fit (A) L-serine, (B) L-leucine, (C) L-arginine, (D) L-glutamine, and (E) L-glutamic acid. Values are shown as mean ± S.E.M.

Figure 7

Active (Arginine^D, left) and protonated (Arginine^Z, right) conformers of L-arginine with their molecular electrostatic potential mapped onto an iso-electron density surface. Negative charge is colored in blue, neutral in green, and positive charge in red. The orange circles highlight differences in the charge distribution between the two conformer models.

Figure 8

Representative histological sections of the olfactory lamellae of the gilthead seabream stained with periodic acid-Schiff/Alcian blue to show the effects of exposure to high PCO₂/low pH on tissue morphology. (A,B) control fish; (C,D) high CO₂ fish. CC, Central core; MC, Mucus cell; NSE, non-sensory epithelium; SE, sensory epithelium.