Micro‐computed tomography: Applications for high‐resolution skeletal density determinations: An example using annually banded crustose coralline algae

Abstract

Warming and acidification of the world's oceans are expected to have widespread consequences for marine biodiversity and ecosystem functioning. However, due to the relatively short record of instrumental observations, one has to rely upon geochemical and physical proxy information stored in biomineralized shells and skeletons of calcareous marine organisms as in situ recorders of past environments. Of particular interest is the response of marine calcifiers to ocean acidification through the examination of structural growth characteristics. Here we demonstrate the application of micro‐computed tomography (micro‐CT) for three‐dimensional visualization and analysis of growth, skeletal density, and calcification in a slow‐growing, annually banded crustose coralline alga Clathromorphum nereostratum (increment width ∼380 µm). X‐ray images and time series of skeletal density were generated at 20 µm resolution and rebinned to 40, 60, 80, and 100 µm for comparison in a sensitivity analysis. Calcification rates were subsequently calculated as the product of density and growth (linear extension). While both skeletal density and calcification rates do not significantly differ at varying spatial resolutions (the latter being strongly influenced by growth rates), clear visualization of micron‐scale growth features and the quantification of structural changes on subannual time scales requires higher scanning resolutions. In the present study, imaging at 20 µm resolution reveals seasonal cycles in density that correspond to summer/winter variations in skeletal structure observed using scanning electron microscopy (SEM). Micro‐CT is a fast, nondestructive, and high‐resolution technique for structural and morphometric analyses of temporally banded paleoclimate archives, particularly those that exhibit slow or compressed growth or micron‐scale structures.

Arctic crustose coralline alga resilient to recent environmental change

“…X‐ray micro computed tomography (micro‐CT) imaging was used to measure the skeletal density in the selected specimens, following methods established in Chan et al (2017). A micro‐CT scanner (GE Healthcare, eXplore Locus RS‐9) imaged each C. compactum at a peak X‐ray energy of 80 kVp and tube current of 450 μ A at the Robarts Research Institute at the University of Western Ontario.…”

“…( d ) Diagram showing perithellium cells in C. compactum . Intrafilament crystals form year round but the interfilament crystals only grow in the summer, leading to a summer maximum density, ( c ) modified from Chan et al (2017) and ( d ) modified from Adey et al (2013).…”

“…During the summer, higher daily solar irradiance drives the formation of larger and more numerous interfilament crystals, increasing cell wall thickness making the skeletal structure denser (Adey et al 2013). As daily solar irradiance declines into the fall, the formation of interfilament crystals significantly decreases, reducing the thickness of the cell walls making the skeletal structure less dense (Chan et al 2017). Thus, the higher density skeleton in Maine reflects the extended summer solar irradiance, in comparison to the much lower density skeleton in Nunavut with a much shorter summer season.…”

Calibration of the pH-δ11B and temperature-Mg/Li proxies in the long-lived high-latitude crustose coralline red alga Clathromorphum compactum via controlled laboratory experiments

“…Coralline algae of the genus Clathromorphum grow at a rate of 300-400 μm yr -1 in the warmer fringes of the subarctic ocean and Aleutian Islands, and 100 μm yr -1 or less in the colder reaches of the subarctic and Arctic oceans (Adey et al, 2013). Clathromorphum produce annually resolved layers of high Mg-calcite crystals over their lifetime, delineated by seasonal changes in skeletal density (Chan et al, 2017). Living tissue and photosynthetic epithelial cells cover the meristem, protecting the calcified layers beneath the meristem, the perithallium, from diagenetic alteration (Alexandersson, 1974) and from herbivory (Steneck, 1986).…”

Phymatolithon (Melobesioideae, Hapalidiales) in the Boreal–Subarctic Transition Zone of the North Atlantic

“…As this development must result from thickening of carbonate cell walls, perhaps with a stronger development of IF, this would enhance crust strength to allow the cantilevered growth achieved by these plants. Such changes in density, tied to the production of interfilament carbonate, have been demonstrated for Clathromorphum compactum (Chan et al, 2017), and it would be valuable to perform such studies in the species presented here.…”