Assessment of viability in coral oocytes: a biochemical approach to achieve reliable assays

Given the current threatening scenario faced by coral reefs, reproductive biotechnologies can be a helping hand in the fight against extinction. Therefore, safe methods to assess viability of coral gametes are essential. In this study, we aimed to develop a reliable method to assess oocyte viability for scleractinian corals using Mussismilia harttii as model. We chose trypan blue (TB) test and the thiazolyl blue tetrazolium bromide (MTT) assay. TB tests were performed in two conditions. First, oocytes were exposed to 1, 3, 5, 7 and 9 mol(-1) dimethyl sulfoxide (DMSO) for 20 min, then incubated in 0.4% TB and had their membrane integrity assessed. Second, to identify if TB was able to stain efficiently, dead oocytes were incubated in TB diluted in filtered seawater or sodium dodecyl sulfate (SDS). For MTT experiments, a serial dilution assay was performed using DMSO as cytotoxic compound. TB was able to cross the oocyte membrane, but because of their high lipid content, the observation of blue color was not possible, making this test unfeasible for oocytes of M. harttii. We were able to standardize the MTT assay for determining mitochondrial activity in mature coral oocytes, and consequently assessing their viability. Equally, the positive linear correlation between MTT concentration and DMSO dilution allowed us to set a safe DMSO concentration (1.4 M) to work with coral oocytes. These results provide a new technique to be applied for assessing coral oocyte viability, which can add efforts in reef monitoring programs and conservation strategies.

Automated summary

Given the threat of extinction to coral reefs, reproductive biotechnologies can help fight against extinction. This study aimed to develop a reliable method to assess oocyte viability for scleractinian corals. The trypan blue (TB) test was found to be unfeasible for assessing oocyte viability due to the high lipid content, while the thiazolyl blue tetrazolium bromide (MTT) assay provided a new technique for assessing coral oocyte viability and mitochondrial activity, which can contribute to reef monitoring and conservation strategies.