A fluorescence image taken with the new microscope reveals individual microalgae (red dots) living within the tissue of a coral. [Image: Or Ben-Zvi]
Scientists at the University of California, San Diego, USA, have built a diver-operated microscope that can reveal the complex biological processes that enable coral reefs to grow and thrive (Methods Ecol. Evol., doi: 10.1111/2041-210x.70078). The submersible imaging system offers new insights into the health and physiology of these vibrant natural habitats and could inform ongoing efforts to understand the mechanisms that cause coral bleaching.
Photosynthetic efficiency
Corals gain their reef-building energy from photosynthesis, but they lack the biological tools needed to drive this light-activated chemical reaction. Instead, they rely on microalgae living within their tissues to absorb sunlight and convert it into oxygen and energy-rich sugars.
To assess the efficiency of this symbiotic process, the researchers incorporated an imaging technique into the microscope that measures the fluorescence produced by chlorophyll molecules within the algae. Images are captured first under ambient conditions and then again after the microalgae have been fully illuminated with an array of light-emitting diodes. The difference between the two fluorescent intensities provides a measure of the photosynthetic efficiency that can be mapped at the scale of individual algal cells.
Initial tests in the laboratory showed that the microscope can detect variations in the photosynthetic efficiency at much smaller scales than can be achieved with a commercial imaging tool. As an example, the researchers measured elevated conversion efficiencies close to the mouths of coral polyps around 1 mm across, a detail that could not be resolved with the commercial system. Further lab-based experiments revealed how corals growing in deeper waters can achieve higher photosynthetic efficiencies than those living in the sunlit shallows.
Seeing coral in a new light
Researcher diving with new submersible microscope to image coral. [Image: Or Ben-Zvi / Scripps Oceanography, University of California San Diego]
To investigate the microscope’s underwater performance, marine biologist Or Ben-Zvi imaged corals off the coast of Maui, Hawaii, USA, that have two distinct morphologies. One of these variants was found to be more efficient at converting sunlight into energy, which the researchers suggest could result from their structural differences, varying light conditions across the reef, or a change in the genetic identity of the microalgae.
The submersible microscope also captures full-color pictures and videos, as well as more general fluorescence images and movies with submicron resolution. Color-coding the fluorescence images provides a clear view of individual microalgae residing within the coral tissue. Ben-Zvi was also able to record the rapid contraction of a polyp’s tentacles in response to a passing particle.
The researchers hope that the data collected by the microscope could provide early warning signs of damage to these diverse ecosystems, which are under threat from warming seas and poor environmental conditions. “We can get a lot of information about the health of these corals without needing to interrupt nature,” says Ben-Zvi.