Contrasting responses of photosynthesis and photochemical efficiency to ocean acidification under different light environments in a calcifying alga

Abstract

Ocean acidification (OA) is predicted to enhance photosynthesis in many marine taxa. However, photophysiology has multiple components that OA may affect differently, especially under different light environments, with potentially contrasting consequences for photosynthetic performance. Furthermore, because photosynthesis affects energetic budgets and internal acid-base dynamics, changes in it due to OA or light could mediate the sensitivity of other biological processes to OA (e.g. respiration and calcification). To better understand these effects, we conducted experiments on Porolithon onkodes, a common crustose coralline alga in Pacific coral reefs, crossing pCO₂ and light treatments. Results indicate OA inhibited some aspects of photophysiology (maximum photochemical efficiency), facilitated others (α, the responsiveness of photosynthesis to sub-saturating light), and had no effect on others (maximum gross photosynthesis), with the first two effects depending on treatment light level. Light also exacerbated the increase in dark-adapted respiration under OA, but did not alter the decline in calcification. Light-adapted respiration did not respond to OA, potentially due to indirect effects of photosynthesis. Combined, results indicate OA will interact with light to alter energetic budgets and potentially resource allocation among photosynthetic processes in P. onkodes, likely shifting its light tolerance, and constraining it to a narrower range of light environments.

Figure 1

Photophysiological responses of CCA to experimental OA and light treatments. Points represent group means ± SEM. The x-axis indicates experimental light treatments, with the mean photon flux density (PFD; µmol photons m^-2 s^-1) for each treatment in parentheses. (a) Area-normalized gross photosynthesis rates, measured at approximately the same PFD that the CCA were kept under in their experimental light treatments (n = 16). Gross photosynthesis for each CCA sample was calculated as the sum of its rate of O₂ evolution through net photosynthesis, plus its rate of O₂ consumption through light-adapted respiration. Photosynthesis increased significantly with treatment light level (p = 0.016), but did not respond to pCO₂. There was no significant light x pCO₂ interaction. (b) Maximum photochemical efficiency (F_v/F_m) of dark-adapted samples (n = 32). There was a significant interaction between light and pCO₂ (p = 0.016).

Figure 2

Photosynthesis-light response (P-E) curves, measured using the change in O₂ concentrations at six photon flux densities (PFDs, µmol photons m⁻² s⁻¹). Points represent mean gross photosynthesis rates (net photosynthesis + dark-adapted respiration) for each pCO₂ and light treatment group (n = 8) at a given PFD. Error bars represent SEM, with data fitted by a hyperbolic tangent function. Shapes represent the light treatments that samples were kept under in their experimental tanks, with the mean PFD for each treatment indicated in parentheses in the figure legend.

Figure 3

Light vs. dark-adapted respiration. Points represent treatment group means ± SEM. Rates are in terms of O₂ consumed, normalized to the surface area of the algal samples (n = 16 for light-adapted respiration, and n = 8 for dark-adapted respiration). The x-axis indicates experimental light treatments, with the mean photon flux density (µmol photons m⁻² s⁻¹) for each treatment in parentheses. Light-adapted respiration increased with treatment light level (p = 0.007). pCO₂ did not affect light-adapted respiration. Dark-adapted respiration had a significant light x pCO₂ interaction (p = 0.039).

Figure 4

Area-normalized net calcification rates of P. onkodes. Points represent treatment group means ± SEM (n = 32). The x-axis indicates experimental light treatments, with the mean photon flux density (µmol photons m⁻² s⁻¹) for each treatment in parentheses. Calcification declined significantly under elevated pCO₂ (p = 0.046), and increased with light (p = 0.001), but there was no light x pCO₂ interaction.