Metabolically induced pH fluctuations by some coastal calcifiers exceed projected 22nd century ocean acidification: a mechanism for differential susceptibility?

Abstract

Anthropogenically mediated decreases in pH, termed ocean acidification (OA), may be a major threat to marine organisms and communities. Research has focussed mainly on tropical coral reefs, but temperate reefs play a no less important ecological role in colder waters, where OA effects may first be manifest. Herein, we report that trends in pH at the surface of three ecologically important cold-water calcifiers (a primary producer and herbivores), under a range of fluid flows, differ substantially from one another, and for two of the three calcifiers, the pH, during darkness, is lower than the mean projected pH due to OA for the surface waters of the global ocean beyond the year 2100. Using micro-electrodes, we show that each calcifier had a different pH gradient between its surface and mainstream seawater, i.e. within the diffusion boundary layer (DBL) that appears to act as an environmental buffer to mainstream pH. Abalone encountered only mainstream seawater pH, whereas pH at the sea urchins' surface was reduced by~0.35 units. For coralline algae, pH was~0.5 units higher in the light and~0.35 units lower under darkness than in ambient mainstream seawater. This wide range of pH within the DBL of some calcifiers will probably affect their performance under projected future reductions in pH due to OA. Differing exposure to a range of surface pH may result in differential susceptibility of calcifiers to OA. Such fluctuations are no doubt regulated by the interplay of water movement, morphology and metabolic rates (e.g. respiration, calcification and/or photosynthesis). Our study, by considering physics (flow regime), chemistry (pH gradients vs. OA future projections) and biology (trophic level, physiology and morphology), reveals that predicting species-specific responses and subsequent ecosystem restructuring to OA is complex and requires a holistic, eco-mechanical, approach.

Photosynthesis, respiration and calcification in the Mediterranean crustose coralline algaLithophyllum cabiochae(Corallinales, Rhodophyta)

“…Changes in pH related to the metabolic processes of photosynthesis or respiration may also occur in the diffusion boundary layer between the algal surface and external seawater. Microelectrode data show that the pH at the surface of the crustose coralline alga Sporolithon durum can increase by 0.5 units in the light and decrease by 0.35 units in darkness, relative to the external seawater pH (Hurd et al, 2011). Calcification rates may also have been affected by the changes in pH that occurred in the incubation chambers but these were limited to -0.1 to +0.1 pH units in the light and -0.1 to -0.2 pH units in the dark after 2 h incubation.…”

“…Photosynthesis influences calcification through the formation of the fibrous organic matrix of the cell walls of coralline algae, which is required for the deposition of calcite crystals, and through changes in internal pH. Changes in pH that occur in the cell wall at the site of calcification are affected by both photosynthesis and respiration; as a result calcification is largely regulated by these metabolic activities (Smith & Roth, 1979;Gao et al, 1993;Hurd et al, 2011).…”

Consumers mediate the effects of experimental ocean acidification and warming on primary producers

“…Higher CO 2 concentrations can stimulate plant growth, leading to increased bottom-up control (18)(19)(20). At the same time, lower pH can negatively impact plants as well as herbivores (20,21), thereby reducing top-down control. Moreover, warming and acidification have frequently been found to interact to augment the effects of CO 2 enrichment (18,22,23).…”

Free-ocean CO<sub>2</sub> enrichment (FOCE) systems: present status and future developments

“…The seawater flow at the surface of the organism is also important as it sets the thickness of the layer. For example, over a diel cycle, coralline algae encounters pH changes of +0.5 units in the light and −0.35 units in the dark relative to the pH of ambient seawater (Hurd et al, 2011). Sea urchins may also be subjected to very low pH values (down to 7.5 on the NBS pH scale) at their surface in slow flows.…”

“…However, only the pH of seawater surrounding the organism affects its physiology. The pH at the surface of the organism (diffusion boundary layer) may differ from the mainstream seawater due to exchanges related to metabolic activities, which may locally modify the pH microenvironment (Hurd et al, 2011). The seawater flow at the surface of the organism is also important as it sets the thickness of the layer.…”