Sadly for the scientists involved, understanding coral health doesn’t always require a wetsuit, air tank or coral reef access. My work with the coral species Acropora cervicornis – or staghorn coral – usually requires a mundane terrestrial dress-code and a green headlamp as my only light source. Working with coral in the lab is more akin to spelunking than scuba diving. This environment, however, allows researchers to control the conditions surrounding the coral in order to more accurately understand the effects of chemical exposure on coral health.
To refresh, I am looking at the effects of the common chemical nonylphenol, which is used in laundry and dish detergents, is a stabilizer in plastic food packaging, and is one of the chemicals that makes up nonylphenol ethyl oxalates, which are found in pesticides, paints, and personal care products. After looking at the development of sea urchins exposed to nonylphenol, I found that the environmentally relevant values (0.1 ug/L – 50 ug/L) did not have a noticeable effect on embryo health. While this is a good sign in terms of the toxicity of the chemical to marine organisms, I was surprised, as most papers that have investigated the toxicity of nonylphenol have found it to be harmful at these levels to a variety of different organisms. Either my urchins were more robust than I had expected, or the chemical was not dissolving in water evenly when I made up my solutions. I decided, because of this, to expose my coral to a wider range of nonylphenol concentrations to get a better sense of whether or not my chemical was reacting. The coral were exposed to nonylphenol at levels ranging from 1 ug/L to 1000 ug/L, and over 96 hours I measured the effects. Let me outline the ways in which researchers monitor coral health in the lab.
During this experiment I consistently measured three things. The first was how well coral tissue regrows when exposed to a chemical, in this case nonylphenol. To do this, I cut the top of half of my coral fragments off at the beginning of the experiment, and took a picture after staining the tissue left on the top of the coral. Over time, a healthy coral fragment will regrow tissue over this wound. An unhealthy coral will take longer to regenerate the tissue, and so at the end of the 96 hour period I took a second picture of the stained tissue to see whether nonylphenol affected the speed of regeneration over this time.
Control fragment regeneration: time 0 (left) to 96 hours (right)
Another way to measure the health of coral is to measure Pulse Amplitude Fluorometery (PAM). Coral have symbiotic algae called zooenthallae, which provide food and oxygen through photosynthesis to the coral polyps. In turn, the coral release carbon dioxide, and provide shelter for the zooenthallae. PAM exposes the coral to a flash of ultraviolet light, which then causes the zooenthalae living in the coral tissue to emit fluorescence.
By measuring the intensity of this florescence, we can better understand the concentration of zooenthallae in the tissue as well as how well they are photosynthesizing. Each day during the exposure I run PAM when the coral are most sensitive to light exposure, right before the light-cycle of their day begins. If you want to see more of the amazing fluorescent world of corals, watch this video created by the reef conservation group Coral Guardians.
Finally, each day I run a basic physioscore to assess how healthy the coral looks and to document changes over time. This involves three different indicators: the polyps; the coloration; and the tissue of coral. I measure these features on a scale of five, with five being completely healthy and zero being nonexistent. Coral extend their anemone-like polyps when they are healthy. The A. cervocornis is a rich brown when its zooenthallae are still present in the tissue, and its tissue should cover the entire fragment of the coral. Below are two images from my study, the first a healthy control after 96 hours and the second a less healthy fragment exposed to nonylpyhenol.
Having finished my 96-hour spelunking experiment with the coral, I will now gather my data and try to draw conclusions about the effects that nonylphenol has on photosynthesis, regeneration and overall health of the coral. With luck, these data will help solidify our understanding about nonylphenol in the marine environment and its effects on coral and similar invertebrates.