The response of the scleractinian coral Turbinaria reniformis to thermal stress depends on the nitrogen status of the coral holobiont

In non-stressed healthy corals, it has been shown that nutrient addition (nitrogen alone or in combination with phosphorus) may negatively impact coral metabolic functions, such as calcification. Indeed, symbiont and/or chlorophyll concentrations as well as areal rates of photosynthesis are often increased under eutrophication (Snidvongs and Kinzie, 1994; Marubini and Davies, 1996; Fagoonee et al., 1999). This leads to a limitation in dissolved inorganic carbon for calcification (instead used for symbiont photosynthesis) or to a decrease in the translocation of photosynthetic products to the coral host (Kinsey and Davies, 1979; Stambler et al., 1991; Marubini and Davies, 1996; Ferrier-Pagès et al., 2000; Koop et al., 2001). Coral mortality was also observed in long-term enrichment treatments (Koop et al., 2001). However, in most of the above studies, levels of nitrogen were relatively high (above 5 μmol l−1), especially when nitrate was used as the nitrogen source. Also, the seawater chemistry was not controlled, and a limitation in inorganic carbon might have occurred in the experimental tanks following a limited seawater renewal or mixing.

In a few experiments involving a moderate increase in nitrogen and/or phosphorus levels, an enhancement of coral metabolism was instead observed (Meyer and Schultz, 1985; Tanaka et al., 2007; Godinot et al., 2011). Our results with non-stressed nubbins of T. reniformis (C-26 and N-26 samples) add to the growing body of evidence that a slight increase in nutrient concentration in seawater does not necessarily impact coral health. Indeed, although there was an increase in cellular chlorophyll levels, this increase was counterbalanced by a decrease in symbiont density. Therefore, no main physiological changes in terms of calcification, photosynthesis and respiration were observed in these non-stressed enriched nubbins.

Overall, results obtained in this study have shown that phosphate enrichment mainly affected the coral symbionts, by decreasing their C:P and N:P ratios, while increasing their carbon, nitrogen, and phosphorus contents, as well as their specific growth rate, maximal photosynthetic efficiency of the PSII, and rate of photosynthesis normalized to chlorophyll content. Phosphate enrichment also affected the skeletal compartment, by increasing the skeletal growth and the P/Ca ratio. Conversely, few changes were observed in the animal host tissue.

Among other things, this explains why flow is so important in the mix.  It keeps gas exchange at the coral’s surface and CO2-levels within the coral at maximal availability within the coral.

I think 5 µmol l-1  nitrogen is roughly equivalent about .3 ppm nitrate (NO3).  Not much by typical tank standards.

The Godinot et al., 2011 citation leads to “Tissue and skeletal changes in the scleractinian coral Stylophora pistillata Esper 1797 under phosphate enrichment” is also interesting.

Here are the authors’ own highlights:

• We examined P enrichment’s impact on calcification and tissue composition in corals.
• We assessed a possible phosphorus limitation in symbiotic zooxanthellae.
• Photosynthetic efficiency, CNP contents, and specific growth of symbionts increased.
• Results indicated a phosphorus limitation of zooxanthellae growth in hospite.
• Skeletal growth rates and phosphorus incorporation into the skeleton also increased.

We’re making a new post for that story now.  🙂