Differential resistance and acclimation of two coral species to chronic nutrient enrichment reflect life‐history traits

Abstract

The effects of nutrient pollution on coral reef ecosystems are multifaceted. Numerous experiments have sought to identify the physiological effects of nutrient enrichment on reef‐building corals, but the results have been variable and sensitive to choices of nutrient quantity, chemical composition and exposure duration. To test the effects of chronic, ecologically relevant nutrient enrichment on coral growth and photophysiology, we conducted a 5‐week continuous dosing experiment on two Hawaiian coral species, Porites compressa and Pocillopora acuta. We acclimated coral fragments to five nutrient concentrations (0.1–7 µM NO3‐ and 0.06–2.24 µM PO43‐) with constant stoichiometry 2.5:1 nitrate to phosphate) bracketing in situ observations from reefs throughout the Pacific. Nutrient enrichment linearly increased photophysiological performance of both species within 3 weeks. The effect of nutrients on P. acuta photochemical efficiency increased through time while a consistent response in P. compressa indicated acclimation to elevated nutrients within 5 weeks. Endosymbiont densities and total chlorophyll concentrations also increased proportionally with nutrient enrichment in P. acuta, but not in P. compressa, revealing contrasting patterns of host–symbiont acclimatization. The two species also exhibited contrasting effects of nutrient enrichment on skeletal growth. Calcification was enhanced at low nutrient enrichment (1 µM NO3‐) in P. acuta, but comparable to the control at higher concentrations, whereas calcification was reduced in P. compressa (30%–35%) above 3 µM NO3‐. Stable isotope analysis revealed species‐specific nitrogen uptake dynamics in the coral–algal symbiosis. The endosymbionts of P. acuta exhibited increased nitrogen uptake (decreased δ15N) and incorporation (19%–31% decrease in C:N ratios) across treatments. In contrast, P. compressa endosymbionts maintained constant δ15N values and low levels of nitrogen incorporation (9%–11% decrease in C:N ratios). The inability of P. acuta to regulate endosymbiont nutrient uptake may indicate an emerging destabilization in the coral–algal symbiosis under nutrient enrichment that could compromise resistance to additional environmental stressors. Our results highlight species‐specific differences in the coral–algal symbiosis, which influence responses to chronic nutrient enrichment. These findings showcase how symbioses can vary among closely related taxa and underscore the importance of considering how life‐history traits modify species response to environmental change. A free Plain Language Summary can be found within the Supporting Information of this article.

Differential susceptibility of reef-building corals to deoxygenation reveals remarkable hypoxia tolerance

“…The presence of species-specific variability in deoxygenation responses was not unexpected, considering that corals frequently show species level variation in bleaching susceptibility 32 , 39 and tolerance to other environmental conditions including pH 25 , 40 , nutrient enrichment 41 , and warming 42 , 43 . However, the magnitude of differential effects and temporal scale of response between A. cervicornis and O. faveolata was unexpected.…”

Seabird-Derived Nutrients Supply Modulates the Trophic Strategies of Mixotrophic Corals

“…It suggests that P. damicornis increases its reliance on dissolved inorganic N-sources (mostly NOx) when this resource dominates in the ecosystem (Grover et al, 2003). In Fox et al (2021) P. acuta colonies were cultivated under increasing nitrate concentration and symbionts continuously assimilated nitrate independently of the coral, increasing their density. If nitrate supplementation alone has been shown to be mostly detrimental to corals, Ezzat et al (2015) demonstrated that a combined enrichment of phosphate and ammonium at an appropriate stoichiometry (i.e., close to that of the coral tissues) was beneficial to the symbiosis.…”

“…Some species of Pocillopora have shown an exceptional trophic flexibility, being able to acquire nutrients through various degrees of autotrophy and heterotrophy (e.g., P. meandrina, Fox et al, 2019) which suggests that this genus is well suited to cope with variable N-supply. Paradoxically, this genus is not capable of regulating the nutrient uptake of its symbionts (Pocillopora sp., Fox et al, 2021). This lack of internal regulation makes this genus more prone to a nutritional imbalance, leading to a shift of the symbiosis toward parasitism, exacerbating bleaching susceptibility (Baker et al, 2018;Burkepile et al, 2020).…”

Low-level nutrient enrichment during thermal stress delays bleaching and ameliorates calcification in three Hawaiian reef coral species

“…Coral responses to nutrient elevation are differential, varying by species, local nutrient history, and nutrient type, concentration, and ratio ( Bongiorni et al, 2003 ; Burkepile et al, 2020 ; Fabricius, 2005 ; Fox et al, 2021 ; Tanaka et al, 2017 ). A meta-analysis of historical nutrient research described how nitrogen enrichment oftentimes reduced calcification and enhanced photosynthesis in corals while, in contrast, phosphorous enrichment increased calcification with minimal effects on coral photosynthesis ( Shantz & Burkepile, 2014 ).…”