Diner, R. E., Benner, I., Passow, U., Komada, T., Carpenter, E. J., & Stillman, J. H. (2015). Negative effects of ocean acidification on calcification vary within the coccolithophore genus Calcidiscus. Marine Biology, 1287–1305. doi:10.1007/s00227-015-2669-x
The dissolution of carbon export, one cell at a time…
The Earth breathes, so to speak. Each inhalation relies on terrestrial and aquatic plants, as well as marine phytoplankton, to photosynthesize and fix carbon. But one type of phytoplankton, known as a coccolithophore, does more than photosynthesize. In addition to taking carbon dioxide (CO2) into its cell, a coccolithophore builds and dons tiny armor-like plates (coccoliths) made of calcium carbonate. Coccolithophores ultimately clump together and sink into the deep ocean, thereby removing carbon from the atmosphere’s grasp.
This sequestering of carbon in the ocean is of great importance as it helps mitigate levels of atmospheric carbon that contribute to regulating global temperatures. Recent increases in atmospheric CO2 through greenhouse gas emissions have led to increased dissolved CO2 in the ocean’s surface waters and, subsequently, ocean acidification (OA). Increased acidity makes it challenging for coccolithophores to create their armor and could decrease the amount of carbon sent to the ocean depths. But what if some coccolithophores were more resilient in the face of this adversity?
The response of coccolithophores to acidic conditions has been studied before, specifically in Emiliania huxleyi—a prevalent coccolithophore in today’s oceans. Although E. huxleyi is abundant, it is only responsible for approximately 10% of the total coccolithophore carbonate production and potential removal of carbon from the surface to the deep ocean.
An in-depth laboratory study conducted by Rachel Diner and colleagues focused on some heavily-calcified cousins of Emiliania: species of the genus Calcidiscus. Compared to E. huxleyi, Calcidiscus’ carbonate production accounts for closer to 50% of calcification across all coccolithophores, which makes the genus a relatively heavy hitter for carbon export. Since previous E. huxleyi studies revealed highly variable responses to OA, Calcidiscus was not expected to perform any differently.
Diner’s team cultured one strain of Calcidiscus leptoporus and one strain of the newly described Calcidiscus quadriperforatus in 5 different treatments of acidified seawater. A second strain of C. leptoporus was subjected to only 4 treatments. The team achieved distinct chemistries by dosing seawater with measured amounts of hydrochloric acid and sodium carbonate. These acidity levels reflected the conditions expected for ocean surface waters at various atmospheric concentrations of CO2, from pre-industrial levels (200 ppm) to levels higher than those predicted for the year 2100 (900 ppm). Each coccolithophore strain was acclimated to the altered seawater conditions over seven generations, after which samples were taken to analyze population growth rate, carbon storage both inside and outside the cell, and coccolith formation.
Results and Significance:
The common trend across all strains was the decreasing number of normally constructed coccoliths with increasing acidity. Descriptors like “degraded” and “severely malformed” became more and more prevalent. Interestingly, the strains with the higher rates of calcification (the first strains of C. quadriperforatus and C. leptoporus) showed fewer than 10% of coccoliths formed normally in the 400 ppm treatment, a.k.a. our atmosphere today. Paradoxically, the least calcified strain of C. leptoporus coped remarkably well at ambient levels with over 80% of coccoliths retaining normal shape. At the highest acidity levels, though, no strain was able to produce unblemished, whole coccoliths. What little calcification occurred resulted in crude, misshapen discs.
While these results seem bleak, Diner et al. highlight a positive aspect to this mutable response. The extensive genetic diversity that led to E. huxleyi’s varied sensitivities is also thought to contribute to the species’ prolific success. The varied response by strains of Calcidiscus appears to indicate a similar mechanism could be at work here. The resilience by the lesser-calcified C. leptoporus strain could indicate that strains with smaller ratios of inorganic to organic carbon may be of greater importance. A decrease in this ratio, as seen in the highly calcified strains in this study, could negatively alter the rate of carbon export to the deep ocean on a short time scale. Strains with lower initial carbon ratios might persist through stressful acidified conditions. As the second C. leptoporus strain also showed an increased mean growth rate under moderately high acidity, this could bode well for mitigating the decrease in carbon exported to the deep ocean should highly-calcified strains collapse.
However, because the other Calcidiscus strains tested already showed high coccolith degradation at ambient levels, it’s possible this genus is more vulnerable than initially thought. The genus itself remains largely unexplored and undocumented, though, so further exploration and research is required. The ocean’s water chemistry will change, and no scenario currently suggests coccolithophores will come away unscathed. Even if Calcidiscus isn’t capable of surviving, the variable response elicited by the genus, combined with the known genetic diversity that allowed E. huxleyi to become so prevalent in ocean waters, indicates it could be possible for other coccolithophores to evolve and take up the mantle and continue crafting their armor despite the adverse conditions.