Morphological responses of a temperate salt marsh foraminifer, Haynesina sp., to coastal acidification
Coastal acidification leads to widespread impacts on calcifying organisms across the worlds oceans, which could result in decreased calcium carbonate deposition and the dissolution of calcium carbonate. As an abundant group of calcifying organisms, some protists within the phylum Foraminifera demonstrate potential success under elevated partial pressure of carbon dioxide (pCO2) due to their ability to modulate intracellular pH. However, little is known about their responses under more extreme acidification conditions that are already seen in certain coastal environments. Here we exposed Haynesina, a foraminiferal genus that is prevalent in temperate coastal salt marshes, to moderate (pCO2 = 2386.05+/-97.14 atm) and high acidification (pCO2 = 4797.64+/-157.82 atm) conditions through the duration of 28 days. We demonstrate that although this species is capable of withstanding moderate levels of coastal acidification with little impact on their overall test thickness, they could experience deposition deficiency and even dissolution of the calcareous test under highly elevated pCO2. Interestingly, such a deficit was primarily seen among live foraminifera, as compared to dead specimens, throughout the four-week experiment. We propose that a combination of environmental stress and the physiological process of test formation (i.e., calcite precipitation) could induce thinning of the test surface. Therefore, with the acceleration of coastal acidification due to anthropogenic production of CO2, benthic foraminifera amongst coastal ecosystems could reach a tipping point that leads to thinning and dissolution of their calcareous tests, which in turn, will impair their ecological function as a carbon sink. ImportanceThe calcareous foraminifera protists are responsible for large proportions of calcium carbonate production across the global ocean. Their responses to ocean and coastal acidification are essential for understanding carbon and mineral cycling in diverse marine ecosystems. However, relatively few studies have examined more extreme conditions related to what is seen in coastal habitats (e.g. pCO2 > 2,500 atm), and the response of individual test chambers have never been inspected. Here, we consider the response of Haynesina sp., a benthic foraminifera obtained from temperate coastal sediments to moderate and high acidification regimes. Comparison of test thicknesses across treatment conditions and among individual chambers of Haynesina sp. revealed potential tolerance under moderate acidification but demonstrated impaired new chamber formation and test dissolution under high acidification. Our results suggest that with growing anthropogenic CO2 production, foraminifera could reach a tipping point that leaves their ecological function as a carbon sink at greater risk.