Nutritional supplementation, which provides corals with additional macronutrients or micronutrients, has been proposed as a method to support coral resilience at multiple biological scales (Grottoli et al., 2006; Rosset et al., 2017). 

The coral holobiont 

The coral holobiont, including its bacterial and viral communities, is integral to coral health and nutrient cycling (Rosenberg et al., 2007). Beneficial bacteria contribute to nitrogen fixation, organic matter decomposition, and pathogen inhibition. Studies have shown that nutrient amendments can support microbial communities that enhance these functions, potentially increasing resilience to environmental stress (Peixoto et al., 2017). Probiotic treatments targeting coral-associated microbiomes may complement nutritional supplementation efforts, further stabilizing coral health (Sweet et al., 2019).

Coral-algae symbiosis

Nutrient availability plays a central role in maintaining the coral-Symbiodiniaceae symbiosis, particularly during thermal stress. The symbionts provide corals with energy via photosynthesis, while corals supply essential nutrients such as nitrogen and phosphorus (Hughes et al., 2018). Environmental stress disrupts this symbiosis, resulting in symbiont expulsion and bleaching. Experimental studies have shown that nutrient supplementation, particularly with dissolved organic carbon and inorganic nitrogen, can enhance symbiont retention and improve photosynthetic efficiency under stress conditions (Ezzat et al., 2015). Additionally, increased nutrient availability supports the upregulation of heat shock proteins and antioxidant systems, reducing cellular damage during thermal anomalies (Rosset et al., 2017).

Coral heterotrophy 

Corals are mixotrophic organisms capable of deriving nutrients from autotrophic (Symbiodiniaceae) and heterotrophic sources (zooplankton and particulate organic matter). During stress events, heterotrophy becomes critical for maintaining energy balance and facilitating recovery (Grottoli et al., 2006). Zooplankton supplementation has been demonstrated to enhance coral energy reserves, enabling improved tissue repair and reproduction (Fox et al., 2018). Additionally, studies suggest that nutrient-rich diets improve coral immune responses, enhancing resilience against pathogens (Bourne et al., 2016).

Ecosystem dynamics 

At the ecosystem level, nutritional supplementation can influence coral reef community dynamics and metabolism, providing increased availability of energy sources for a variety of reef organisms in addition to corals. However, these benefits are contingent on maintaining beneficial nutrient concentrations, as excessive or improper balances of nutrient input can promote algal overgrowth and create otherwise undesirable trophic changes (D’Angelo & Wiedenmann, 2014). 

Strategy 

Nutritional supplementation is a powerful tool for increasing coral reef ecosystem resilience, but given the possibility of algal overgrowth in degraded systems, it must be approached with caution. Evidence supports the use of this tool in environments that are periodically exposed to oceanic sources of nutrients, and it can be most safely applied in areas with short water residence times. Identifying phase shift thresholds in heavily degraded areas that are subjected to land-based sources of nutrients and pollution is important, as it may be possible to apply carefully controlled supplementation in those areas, but this requires further investigation prior to implementation.

References

• Bourne, D. G., Morrow, K. M., & Webster, N. S. (2016). Insights into the coral microbiome: Underpinning the health and resilience of reef ecosystems. Annual Review of Microbiology, 70, 317-340.

• D’Angelo, C., & Wiedenmann, J. (2014). Impacts of nutrient enrichment on coral reefs: New perspectives and implications for coastal management and reef survival. Current Opinion in Environmental Sustainability, 7, 82-93.

• Ezzat, L., Maguer, J. F., Grover, R., & Ferrier-Pagès, C. (2016). New insights into carbon acquisition and exchanges within the coral-dinoflagellate symbiosis under NH4+ and NO3− supply. Proceedings of the Royal Society B, 283(1828), 20161592.

• Fox, M. D., Williams, G. J., Johnson, M. D., et al. (2018). Gradients in primary production predict trophic strategies of mixotrophic corals across spatial scales. Current Biology, 28(21), 3352-3357.

• Grottoli, A. G., Rodrigues, L. J., & Palardy, J. E. (2006). Heterotrophic plasticity and resilience in bleached corals. Nature, 440(7088), 1186-1189.

• Hughes, T. P., Kerry, J. T., Álvarez-Noriega, M., et al. (2017). Global warming and recurrent mass bleaching of corals. Nature, 543(7645), 373-377.

• Peixoto, R. S., Rosado, P. M., Leite, D. C. A., et al. (2017). Beneficial microorganisms for corals (BMC): Proposed mechanisms for coral health and resilience. Frontiers in Microbiology, 8, 341.

• Rosset, S., Carricart-Ganivet, J. P., & Fine, M. (2017). Physiological parameters of stony corals: A meta-analysis and discussion of scaling models. Coral Reefs, 36(1), 59-70.

• Sweet, M. J., Croquer, A., & Bythell, J. C. (2019). Experimental antibiotic treatment identifies potential pathogens of white band disease in the endangered Caribbean coral Acropora cervicornis. Proceedings of the Royal Society B, 276(1669), 2925-2934.