Response of foraminifera Ammonia confertitesta (T6) to ocean acidification, warming, and deoxygenation - An experimental approach

Sammanfattning: Ocean acidification, warmer temperatures, and the expansion of hypoxic zones in coastal areas are direct consequences of the increase in anthropogenic activities. However, so far, the combined effects of these stressors on calcium carbonate-secreting marine microorganisms - foraminifera are complex and poorly understood. This study reports the foraminiferal survival behavior, and geochemical trace elements incorporation measured from the shells of living cultured benthic foraminifera from the Gullmar fjord (Sweden) after exposure to warming, acidification, and hypoxic conditions. An experimental set-up was designed with two different temperatures (fjord’s in-situ 9 ˚C and 14 ˚C), two different oxygen concentrations (oxic versus hypoxic), and three different pH (control, medium, and low pH based on the IPCC scenario for the year 2100). Duplicate aquariums, meaning aquariums displaying the same conditions and same number of species, were employed for the controls and the two lower pH conditions at both temperatures. The stability of the aquariums was ensured by regular measurement of the water parameters and confirmed by statistical analysis. The species Ammonia confertitesta’s (T6) survival (CTB-labeled), shell calcification (calcein-labeled), and geochemical analyses (laser-ablation ICP-MS) were investigated at the end of the experimental period (48 days). Investigated trace elements (TE) ratios were Mg/Ca, Mn/Ca, Ba/Ca, and Sr/ Ca. Results show that A. confertitesta (T6) calcified chambers in all the experimental conditions except for the most severe combination of stressors (i.e., warm, hypoxic, low pH). Survival rates varied by up to a factor of two between duplicates for all conditions suggesting that foraminiferal response may not solely be driven by environmental conditions but also by internal or confounding factors (e.g., physiological stress). A large variability of all the TE/Ca values of foraminifera growing at low pH is observed suggesting that A. confertitesta (T6) may struggle to calcify in these conditions. Thus, this study demonstrates the vulnerability of a resilient species to the triple-stressor scenario in terms of survival, calcification, and trace element incorporation. Overall, the experimental set-up yielded coherent results compared to previous studies in terms of ontogeny, trace elements ratios, and partition coefficient making it advantageous for environmental reconstructions.