Marine Biology Section¹, Department of Biology, University of Copenhagen, Denmark
Coral Ecophysiology Laboratory², Centre Scientifique de Monaco, Monaco.
Involved Scientists: W. Dellisanti¹, Q. Zhang¹, C. Ferrier-Pagsès², M. Kühl¹

Background

In the face of continuous climate challenges, the resilience of Mediterranean corals is threatened under current conditions. Recent heatwaves have highlighted the vulnerability of marine life, particularly corals which are susceptible to bleaching and mass mortality events. Essential micronutrients, such as iron (Fe), link coral health to cellular metabolism and growth. Despite the typically low iron concentrations in seawater, runoff discharge from land introduces substantial nutrient loads, including iron, to Mediterranean coastal waters. This innovative project employs advanced sensing technologies, including the Firesting-PRO setup from PyroScience, to assess oxygen dynamics as indicators of coral metabolic health. By studying the impact of elevated temperatures and iron enrichment on corals' ecophysiology, this research aims to unravel the complex relationship between these factors. Ultimately, this study provides insights into Mediterranean coral well-being and physiology, offering a tool to observe and anticipate the repercussions of environmental shifts on the Mediterranean Sea ecological health.

Experimental setup and methodology

We selected two Mediterranean coral species as models for our study: the stony coral Cladocora caespitosa and the gorgonian Eunicella singularis (Fig.1). These species are commonly found in the Mediterranean Sea, although wide mortality events were recorded in association with recent heatwaves. They both live in symbiosis with dinoflagellate algae which perform photosynthesis.

Custom-made chambers were created from transparent polycarbonate to suit the distinct coral morphologies and align with our experimental arrangement (Fig 2). These chambers, with a volume of approximately 50 mL, were equipped with a stirring magnet at the base, facilitating water circulation within. The placement of the oxygen sensor spot on the internal surface, along with the connection of the fibre optic through 3D-printed connectors, ensured a continuous measurement of oxygen concentrations.

Several laboratory conditions were used to replicate distinct seawater scenarios: natural conditions at 18°C (control), simulated daily iron enrichment of 20 nM iron (Fe), elevated temperature at 24°C (Temp), and combined conditions (TempFe). These conditions were maintained over a continuous period of 5 weeks, during which we conducted weekly assessments of the corals' metabolic state through respirometry. This approach enabled us to systematically monitor and assess the health status of the corals.

Preliminary results

Both corals were able to modulate the oxygen availability based on different conditions of seawater temperature and dissolved iron availability. Increasing temperatures from 18°C (control condition) to 24°C (warming condition) had a significant effect on relative oxygen consumption / production through the increased respiration and photosynthesis rates. Nevertheless, the input of dissolved iron allowed corals to recover and mitigate the stress from elevated temperatures, by reducing the respiration and supporting the photosynthetic rate. This result is confirmed by the increased P:R ratio as standardized information of corals energetic productivity (Fig. 3).

Conclusion from data and application

Setting up the Firesting PRO channels proved straightforward, enabling swift calibration of the oxygen sensor spots. Employing 3D printed accessories was essential to tailor the Firesting for optimal experimental utility. With data recorded at 1-second intervals, we achieved exceptional resolution and quality, essential for capturing dissolve oxygen fluctuations in the chambers, within a concise period of 60 minutes (this study). The presented data originated from a singular study concerning coral metabolic performance; however, the Firesting PRO setup accommodates extended measurements, such as overnight or up to 24 hours, affording insights into longer temporal oxygen consumption trends. In a laboratory context, this setup streamlined configuration and facilitated concurrent employment of multiple channels, exemplified here by the simultaneous use of 8 sensors to ensure experimental replication. We recommend this configuration for investigating oxygen dynamics in coral holobionts under diverse environmental exposures.