Reduced calcification and lack of acclimatization by coral colonies growing in areas of persistent natural acidification

Abstract

As the surface ocean equilibrates with rising atmospheric CO 2 , the pH of surface seawater is decreasing with potentially negative impacts on coral calcification. A critical question is whether corals will be able to adapt or acclimate to these changes in seawater chemistry. We use high precision CT scanning of skeletal cores of Porites astreoides, an important Caribbean reef-building coral, to show that calcification rates decrease significantly along a natural gradient in pH and aragonite saturation (Ω arag ). This decrease is accompanied by an increase in skeletal erosion and predation by boring organisms. The degree of sensitivity to reduced Ω arag measured on our field corals is consistent with that exhibited by the same species in laboratory CO 2 manipulation experiments. We conclude that the Porites corals at our field site were not able to acclimatize enough to prevent the impacts of local ocean acidification on their skeletal growth and development, despite spending their entire lifespan in low pH, low Ω arag seawater.reef framework | caribbean corals acidic springs S cleractinian corals, whose calcium carbonate (CaCO 3 ) skeletons provide the structural framework of coral reef ecosystems, are subject to numerous direct and indirect stressors and are facing steep global decline (1-3). As the ocean absorbs anthropogenic CO 2 , surface ocean pH and the availability of carbonate ions to corals and other reef calcifiers are decreasing (1-5). Global climate models predict a drop of 0.3 pH units, from 8.1 to 7.8 by the end of the 21st century (6-8), resulting in a 50% reduction in carbonate ion concentration (9). Consequently, it is predicted that ocean acidification will result in a widespread reduction in coral calcification by the year 2065 (10), causing large-scale reef degradation and loss (11).The predicted response of coral reef calcification to decreasing aragonite-saturation (Ω arag ) state is based primarily on model calculations of future Ω arag (6,7,9,12) and the observed response of coral calcification to low Ω arag in short-term laboratory-based or mesocosm carbonate chemistry manipulation experiments (11, 13-16). Additionally, field-based observations of net coral reef ecosystem calcification responses to changes in Ω arag state in situ also suggest declines in calcification (17-21). However, key questions remain regarding the acclimation and adaptation potential of coral calcification to ocean acidification. Acclimatization, or the potential for an organism to adjust to changes in an environment via physical modifications, is distinguished from adaptation, or permanent evolutionary modifications made by an organism in response to repeated stressors. Specifically, an outstanding question is whether corals will be able to acclimate or adapt to maintain sufficient rates of calcification to sustain the reef structure (17,22,23). To address these questions, field-based studies where corals have been naturally exposed to chronic low pH conditions for extended periods could provide important new ins...

Community calcification in Lizard Island, Great Barrier Reef: A 33 year perspective

“…In the Andersson et al study the nighttime dissolution increased by a factor of ~3 on average for a decrease of 1 unit of Ω arag from 2.1 to 1.1. For 3.5 < Ω arag < 4.5, ratios of volume bored to volume of coral measured in coral cores sampled along a naturally occurring acidification gradient in the Eastern Pacific appear to be independent of Ω arag (Derse-Crook et al, 2013). However, this ratio increases exponentially below Ω arag~2 , where in the range 2< Ω arag <3.5 there is no data.…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…40%) and calcification (ca. 30%) when arag was below 2.0, relative to an adjacent control site where arag was >3.5 (Crook et al, 2013). The ojos sites can experience changes in temperature, salinity and light that may in part explain the reduced calcification rates (Iglesias-Prieto et al, 2014), although the collective assessment from the laboratory and field experimentation suggests low pH is largely responsible for the reductions in calcification measured (Paytan et al, 2014).…”

“…The ojos sites can experience changes in temperature, salinity and light that may in part explain the reduced calcification rates (Iglesias-Prieto et al, 2014), although the collective assessment from the laboratory and field experimentation suggests low pH is largely responsible for the reductions in calcification measured (Paytan et al, 2014). To date for this site, no molecular data or information on other lifehistory traits (e.g., metabolic properties) have been published, making it difficult to assess whether this reduction in density does indeed demonstrate a lack of acclimation ability for P. astreoides (Crook et al, 2013) or rather highlights a potential survival process to persist in adverse conditions (e.g., phenotypic or metabolic plasticity, see Pigliucci et al, 2006). Either way, these findings corroborate results from the vent sites that coral calcification will likely be compromised under low pH.…”

“…For example, corals around ojos experienced persistently low pH below 7.7 (Crook et al, 2013), while corals of the PAG in summer experienced temperatures exceeding 35 • C (Sheppard, 2009); these are conditions that are not predicted for the tropical open-ocean to well beyond 2100 (IPCC, 2014). The predictability and duration of the extreme environmental conditions experienced by corals are important considerations when assessing the role these environments can play in coral reef research and management.…”

“…For example, corals across high temperature, low pH, and multistressor sites were able to maintain high calcification rates (e.g., Dandan et al, 2015;Wall et al, 2016), enabled via mechanisms such as unimpaired pH-homeostasis at the site of calcification (Wall et al, 2016) or increased photosynthesis rates to supply the required energy demand (Strahl et al, 2015a). Although this is sometimes associated with trade-offs such as lower skeletal density and increased bioerosion Crook et al, 2013), other corals are able to do so without indication of compromised energy budgets (Strahl et al, 2015a). Similarly, corals in mangrove habitats appear to enhance respiration rates indicative of increased heterotrophy, to facilitate survival in this challenging habitat (Camp et al, 2017).…”