Effects of Nitrate Enrichment on Respiration, Photosynthesis, and Fatty Acid Composition of Reef Coral Pocillopora damicornis Larvae

Cheng-Yue Liu,You-Fang Sun,Xiao-Lei Yu,Hui Huang,Fang Zhang

doi:10.3389/fmars.2020.00531

Abstract

In the foreseeable future, coastal coral reef ecosystems are likely to face further increases in eutrophication. Lipids and fatty acids (FAs), as important components of corals, are becoming a hot topic to study the effects of eutrophication on corals. This study investigated the effects of nitrate (NO3-) enrichment (0, 5, 10, 20, and 40 μM) on the respiration, photosynthesis, and FA compositions of Pocillopora damicornis larvae. Our results showed that saturated fatty acids (SFAs) were the most abundant in P. damicornis larvae over all treatment groups, followed by polyunsaturated fatty acids (PUFAs). The unsaturated to saturated fatty acid ratio (U/S) and unsaturation index (UI) reduced at low nitrate concentrations (20 μM). The UI and U/S levels were elevated due to the increased PUFAs levels and decreased SFA levels at higher nitrate concentrations. Compared with the N5 and N10 groups, the fluidity of the biomembrane of the larvae did not continue to decrease but instead increased at higher nitrate concentrations, indicating that the biomembrane restructuring in the larvae may have become ineffective. Moreover, respiration increased and the consumption of numerous lipids led to a significant decrease in TFAs. These could adversely affect the dispersal, settlement, and development of larvae. Overall, P. damicornis larvae can adapt to low levels of nitrate (< 10 μM) due to biomembrane restructuring through changes in FA composition. However, negative effects occur in larvae when nitrate exceeds 20 μM.

Highlights

Reef ecosystems are among the world’s most productive and biodiverse marine ecosystems (Hughes et al, 2018; Kubicek et al, 2019)
N5 had the highest PNet and PNet to RD (PNet/RD), which implied that low nitrate enrichment could promote photosynthesis in coral larval zooxanthellae
Our results indicate that the lipids of P. damicornis larvae become progressively saturated when nitrate is less than 10 μM, which can be attributed to decreased polyunsaturated FAs (PUFAs) and a concomitant increase in saturated FAs (SFAs)

Summary

Introduction

Reef ecosystems are among the world’s most productive and biodiverse marine ecosystems (Hughes et al, 2018; Kubicek et al, 2019). Coral reef waters usually contain low levels of inorganic nutrients (Renegar and Riegl, 2005). For the foreseeable future, coastal coral reef ecosystems are likely to face further increases in eutrophication (Humanes et al, 2016). The effect of nutrients on corals and their algal symbionts is still subject to debate. Some researchers have suggested that elevated nutrient levels can significantly decrease the growth and calcification of coral (Ferrierpagès et al, 2000; Renegar and Riegl, 2005). Positive responses in coral to increased nutrient availability have been found, including reduced thermal stress, zooxanthellae supplementation, and moderate bleaching (Atkinson et al, 1995; Bender-Champ et al, 2017)

Methods

Results

Discussion

Conclusion