PATTERNS OF BENTHIC FORAMINIFERAL GROWTH IN THE CALIFORNIA BORDERLAND RECONSTRUCTED BY MICRO-CT ANALYSIS (Invited Presentation)

Low-oxygen marine waters will grow to encompass a greater area of the continental shelf in the future due to increasing global carbon dioxide levels and warming. Thus, it is essential to better understand the metabolic and growth strategies of organisms inhabiting hypoxic environments to forecast their potential responses to oceanographic change. Foraminifera are the only eukaryotes with representative species known to respire aerobically and anaerobically, providing a unique opportunity to test the growth strategies of both aerobic and facultative anaerobic species. Foraminifera grow by sequentially adding chambers to their tests that follow a genetically fixed morphological program. However, it is unknown whether patterns of chamber addition remain constant between populations of a species that live under different oxygenation levels. Here, I examine patterns of growth and chamber addition in both microspheric and megalospheric forms of Bolivina argentea collected from the Santa Monica Basin, a modern oxygen minimum zone in the California Borderland. B. argentea spans this basin’s dissolved oxygen gradient and is known to denitrify in the absence of oxygen. To examine internal test morphology, modern foraminiferal specimens were scanned using micro-computed tomography (micro-CT). Rendered micro-CT images were analyzed to quantify proloculus and individual chamber volumes to reconstruct within-species growth patterns. Although the proloculus volumes of megalospheric forms tend to be two orders of magnitude larger than microspheric forms, both forms grow to obtain similar adult sizes. However, the rate of volume increase with each chamber addition is greater in microspheric forms. Newly added chambers in microspheric forms are approximately 30% larger than the previous chamber, whereas megalospheric chambers increase in volume by ~21%. Across the Santa Monica Basin’s dissolved oxygen gradient, the proloculus-to-total test volume ratios of both microspheric and megalospheric forms remain constant. Furthermore, their rates of chamber volume increase do not vary with oxygen concentrations. These results suggest that B. argentea can maintain stable growth patterns across dissolved oxygen levels, possibly due to their ability to denitrify in the absence of oxygen.