Ocean Acidification

Single-celled protists called foraminifera perform critical ecosystem functions across the world’s oceans, including cycling of biogeochemically relevant compounds, sequestering carbon, and serving as biological monitoring tools of ecosystem health. However, anthropogenic climate change increases risks for these species in the oceans of the future. As ocean conditions change due to increased carbon dioxide in the atmosphere from anthropogenic sources, dire consequences to the world’s oceans are emerging such as oceanic deoxygenation, or the reduction of dissolved oxygen in water due to increased heat; coastal acidification, or decreases in coastal water pH due to increased dissolved carbon dioxide; and sea-level rise, which is cause by rising temperatures and melting icecaps. Currently, the responses of foraminifera to these important climate change risk-factors have not been well-studied. This study examines the responses of this group to the threats of climate change to predict their ecological success and capacity to serve as bioindicators as oceans continue to change.

In Manuscript I, transcriptomes were collected from two species of foraminifera collected from the Santa Barbara Basin off the coast of California: Nonionella stella and Bolivina argentea. These two species thrive in anoxic to euxinic and hypoxic sediments, respectively. However, the metabolic processes that enable these species to achieve high ecological success in extreme conditions are unclear. This study presented detailed metabolic reconstructions and differential gene expression that illustrated the cellular processes localized to the peroxisome and mitochondria. This metabolism enables survival in oxygen-depleted sediments and suggested that these species are likely to experience range expansion as deoxygenated regions get larger with climate change.

Manuscript II investigated responses of a Rhode Island salt marsh foraminifera, Haynesina sp., to coastal acidification. As carbon dioxide concentrations raise in the atmosphere, chemical equilibria dictate that carbon dioxide concentrations increase in the ocean as well. When carbon dioxide dissolves in seawater, several spontaneous chemical reactions occur that lead to decreased pH, which can have detrimental impacts on calcium carbonate-depositing organisms. These processes can be exacerbated in coastal systems, where conditions fluctuate to higher extremes than in the open ocean. Many foraminifera, including Haynesina sp., have calcium carbonate tests that could leave these species at high risk due to ocean acidification. This study detailed the morphological responses of Haynesina sp. to coastal acidification over biologically relevant timescales to determine that, although Haynesina sp. may be resistant to moderate elevated carbon dioxide, exposure to high elevated pCO₂ leads to morphological defects in living cells. Altogether, this study demonstrated that Haynesina are susceptible to extreme coastal acidification and risk dissolution under those conditions.

In Manuscript III, the scope of foraminifera examined was expanded from individual species to whole communities by using DNA metabarcoding to examine how communities may shift in response to sea-level rise mitigation efforts. As global climate change proceeds, temperatures are expected to rise, which will result in increases in sea-level as water stored in ice and glaciers continues to melt. Due to increases in sea-level, it is expected that many coastal regions of the United States could be submerged in the next 100 years. To mitigate increases sea-level rise, conservation efforts are underway to raise the elevation of salt marshes through thin layer placement of sediment. This restoration technique involves adding large amounts of sediment to the surface of salt marshes and has been noted to have beneficial impacts for vegetation; however, impacts on other associated ecosystems, such as the subtidal and intertidal regions, are unknown. This study found that each of the three sites examined across the Rhode Island coast had distinct foraminiferal communities. Additionally, in the two restored marshes, TLP seemed to significantly impact foraminiferal alpha diversity. Across the two restored marshes, variable responses in alpha and beta diversity were observed. The results show that Rhode Island salt marshes have divergent responses to thin layer placement and need to be studied individually to determine the impacts of restoration. Despite this, our results suggest that TLP can have positive impacts on the health of some intertidal ecosystems.

In conclusion, these studies demonstrate that foraminifera have complex and varied responses to risk factors associated with climate change and climate change mitigation efforts. In some scenarios, such as oceanic deoxygenation, some taxa are poised to experience success and range expansion. However, other scenarios, such as ocean acidification, may lead to increased risk for species within this group under extreme scenarios. Further, foraminifera have the capacity to act as bioindicators, as is seen in the face of sea-level rise mitigation efforts, where foraminifera demonstrate a strong potential to act as an indicator species for ecosystem health.