Acclimatization of the Crustose Coralline Alga Porolithon onkodes to Variable pCO2

Abstract

Ocean acidification (OA) has important implications for the persistence of coral reef ecosystems, due to potentially negative effects on biomineralization. Many coral reefs are dynamic with respect to carbonate chemistry, and experience fluctuations in pCO2 that exceed OA projections for the near future. To understand the influence of dynamic pCO2 on an important reef calcifier, we tested the response of the crustose coralline alga Porolithon onkodes to oscillating pCO2. Individuals were exposed to ambient (400 µatm), high (660 µatm), or variable pCO2 (oscillating between 400/660 µatm) treatments for 14 days. To explore the potential for coralline acclimatization, we collected individuals from low and high pCO2 variability sites (upstream and downstream respectively) on a back reef characterized by unidirectional water flow in Moorea, French Polynesia. We quantified the effects of treatment on algal calcification by measuring the change in buoyant weight, and on algal metabolism by conducting sealed incubations to measure rates of photosynthesis and respiration. Net photosynthesis was higher in the ambient treatment than the variable treatment, regardless of habitat origin, and there was no effect on respiration or gross photosynthesis. Exposure to high pCO2 decreased P. onkodes calcification by >70%, regardless of the original habitat. In the variable treatment, corallines from the high variability habitat calcified 42% more than corallines from the low variability habitat. The significance of the original habitat for the coralline calcification response to variable, high pCO2 indicates that individuals existing in dynamic pCO2 habitats may be acclimatized to OA within the scope of in situ variability. These results highlight the importance of accounting for natural pCO2 variability in OA manipulations, and provide insight into the potential for plasticity in habitat and species-specific responses to changing ocean chemistry.

Ocean and Coastal Acidification off New England and Nova Scotia

“…Nonetheless, there are consis tent trends across two major functional groups (i.e., calcareous species vs. "fleshy" algae and marine plants) to CO 2 enrich ment among macrophyte species from the NECAN study area. The vulnerability of calcareous algae and the increased pro ductivity among fleshy algae and marine plants in response to elevated pCO 2 are consistent with several metaanalyses from other ecosystems (Kroeker et al, 2010(Kroeker et al, , 2013Johnson et al, 2014).…”

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

“…Different responses of P. onkodes to high pCO 2 in different locations in this study (as well as among previous studies [12,52]) suggest that in contrast to H. macroloba, massive Porites and P. damicornis, the response of P. onkodes to OA may be influenced by the environmental history prior to collection. At local scale, coralline algae experiencing naturally diel pH variations are acclimatized to variable pH [29], and some temperate species found in habitats exposed to large pH changes (i.e. tidal pools) are more resistant to OA than species living in habitats characterized by more stable pH [28].…”

“…These problems can be reduced by conducting identical OA experiments at multiple locations, but such work remains technically challenging, not least because the environmental conditions to which organisms have been exposed (i.e. saturation states, temperature) on the reef prior to the experiment also are likely to vary among locations, and potentially can confound the contrast of interest [28,29]. In this study, we compared the response of reef calcifiers to OA at multiple locations on a regional scale and used among-location differences in physical conditions to our advantage.…”

Ocean acidification effects on calcifying macroalgae

“…Rather, skeletal mineralogy was likely the more important factor controlling the calcification response of the 3 species investigated. On the other hand, Johnson et al (2014) reported that P. onkodes collected from a habitat with high pCO 2 variability were more resistant to high pCO 2 than individuals collected from a habitat with stable pCO 2 , although both populations decreased calcification rates when exposed to 660 μatm pCO 2 . Because P. onkodes is a dolomite-depositing alga, populations growing in the pCO 2 -variable environment may have accumulated dolomite in their skeletons (which has been shown to occur in this species; Diaz-Pulido et al 2014), making them more resistant to experimental pCO 2 conditions.…”