Abstract
Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and are commonly associated with high human population density, yet little is known about the disease pathology or calcification behavior. SEM imagery, skeletal trace elements and boron isotopes (δ11B) have been combined as a novel approach to study coral disease. Low Mg/Ca, and high U/Ca, Mo/Ca, and V/Ca potentially suggest a decreased abundance of “centers of calcification” and nitrogen-fixation in GAs. Estimates of carbonate system parameters from δ11B and B/Ca measurements indicate reduced pH (−0.05 units) and [CO32−] within GA calcifying fluid. We theorize GAs re-allocate resources away from internal pH upregulation to sustain elevated tissue growth, resulting in a porous and fragile skeleton. Our findings show that dystrophic calcification processes could explain structural differences seen in GA skeletons and highlight the use of skeletal geochemistry to shed light on disease pathophysiology in corals.
Highlights
Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and are commonly associated with high human population density, yet little is known about the disease pathology or calcification behavior
Δ11B varies according to internal pH27–29, and recent studies suggest that an all-important second carbonate system parameter can be calculated using paired coral δ11B and B/Ca ratios to estimate internal carbonate ion concentration ([CO32−]extracellular calcifying fluid (ECF))[30,31,32]
The respective mean corallite diameters of 1.50 ± 0.05 mm and 1.36 ± 0.05 mm we measure for GA and unaffected samples are remarkably similar to those previously documented in P. compressa (1.48 ± 0.16 mm and 1.32 ± 0.14 mm respectively17)
Summary
Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and are commonly associated with high human population density, yet little is known about the disease pathology or calcification behavior. Rising atmospheric carbon dioxide represents a global threat to coral reefs by increasing ocean temperatures and reducing ocean pH3, respectively causing wide-scale bleaching events[4] and reducing calcification rates on coral reefs[5] Localized stressors such as eutrophication, sedimentation, and chemical pollution decrease health and resilience of coral reef organisms[6]. GAs are generally characterized by rapid growth of less dense skeletal carbonate, with associated tissues having fewer polyps, fewer endosymbiotic dinoflagellates (family Symbiodiniaceae), and reduced reproductive potential[14,15] These abnormal characteristics do not usually result in immediate mortality of an afflicted colony, the reduction in overall organism fitness makes GAs an ecological threat where prevalence is high[16]. The elemental and boron chemistries have the potential to reveal key differences in calcification processes, as well as differences in wider holobiont physiological and biochemical activity, between healthy and diseased corals
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