In the grand scheme of scientific research, 10 weeks is the bat of an eye. In the case of my research on the affects of the chemical nonylphenol on coral health, my work is just the first contribution to a hopefully comprehensive and informative investigation on this chemical. The more researchers understand nonylphenol and the ways in which it interacts with complex organisms such as coral, the better informed those developing water quality standards in the future will be.
My work involved exposing the coral Acropora cervicornis to four different concentrations of nonylphenol (1, 10, 100 and 1000 μg/L) over 96 hours, and evaluating its effects on coral physiology, wound healing and the photosynthetic activity of symbiotic algae that live in the coral tissue. These results were compared to a control group that was not exposed to nonylphenol. The results of each of these tests were interesting in their own right. Put together, they will hopefully help direct future studies on nonylphenol.
In as little as 24 hours, there were visible changes in the overall appearance of coral fragments exposed to the highest two concentrations, which was monitored and quantified using a health index that evaluated tissue coverage, coloration and polyp retraction. This trend continued, and after 96 hours, the lowest three concentrations had significant reductions in physical health compared to controls. This indicates that nonylphenol does have a negative effect on coral health, and gives us a general sense of the concentration range at which nonylphenol exposure is toxic. 96-hours is a comparatively short exposure time, so longer experiments that monitor health of coral exposed to lower doses will better mimic natural exposure in the future.
Tissue regeneration was slightly different among the different treatments and the control, but most significant was the highest concentration of nonylphenol, which experienced a 12% tissue loss, compared to the 62% tissue gain of the controls. Below is an image comparing the initial (top row) and final (bottom row) images of one fragment from each concentration. Tissue has been stained purple to increase contrast, while exposed skeleton is white.
We were surprised to see that the photosynthetic activity of the algal symbionts that live in coral tissue (known as zooxenthallae) did not have a reduction in activity during this exposure period. Fragments without an injury remained active at levels comparable to the control.
This visual (left) shows the heat map produced by analyzing photosynthetic ability. The more blue the fragment, the more active the zooxenthallae in its tissue are. Next to each heat map is an image of the same fragment taken the same time point (96 hours of exposure). The top row shows a control fragment (no nonylphenol) while the bottom image shows a fragment exposed to 1000 μg/L nonylphenol (highest concentration). Given the visible loss in tissue and apparent bleaching, it is surprising that the differences between photosynthetic activity are very small.
It is very possible that there is a differential response to nonylphenol between species, and the zooxenthallae may respond positively to the presence of nonylphenol. There have been studies on nonylpheno that have shown another species of the same phylum as zooxenthallae (dinophyceae), to increase biomass when exposed to nonylphenol (Hense et al., 2003).While the coral and algae interact symbiotically, they are completely different organisms, and interact in different ways with the chemical.
This being said, there was a noticeable difference between the photosynthetic activity between the tissue regeneration fragments exposed to no nonylphenol (controls) and the highest concentration group. As I described earlier top centimeter of the tissue regeneration fragments was cut prior to chemical exposure, and then tissue coverage monitored. These fragments appeared to be more susceptible to the chemical because of their laceration than intact fragments.
Coral such as Acropora cervicornis are often injured by natural forces (waves, etc.) as well as boats hitting coral reefs and divers disrupting their natural environments. It is therefore important to acknowledge the greater sensitivity of injured fragments to the chemical. In the natural environment, this could mean that even lower concentrations of nonylphenol will negatively impact tissue growth and may encourage bleaching that will stop the coral from regaining health.
The findings of this study are an exciting first step, and the chemical will need to be studied further to better understand and interpret these results. This will be an ongoing investigation but my hope is that, in the future, any water quality standard for nonylphenol in the marine environment will be guided by an understanding of how the chemical interacts with stony corals. With strong information we will be able to sufficiently protect and prioritize these incredible, valuable and fragile organisms.
Many thanks to Dr. Cheryl Woodley of the Coral Health and Disease program at the Hollings Marine Laboratory for providing me the opportunity to work in her lab this summer. Thank you to NOAA and to the Fort Johnson REU program for this incredible experience, and to the NSF for the funding to make this project possible.
Hense, Ba., Jüttner, I, Welzl, G, Severin, GF, Pfister, G, Behechti, A, Schramm, KW. Effects of 4-nonylphenol on phytoplankton and periphyton in aquatic microcosms. Environ. Toxicol. Chem. 2003; 22(11): 2727- 2732.