Coral reefs, built over thousands of years out of the exoskeletons of tiny sessile animals called coral polyps, host some of the most diverse ecosystems on Earth—and the most threatened. Reef health depends on the symbiosis between the millimeter-sized polyps and single-celled algae called zooxanthellae; that relationship can be thrown out of balance by climate change, pollution, and a host of other factors.
Coral up close. In this image of a single coral polyp under fluorescent illumination, individual zooxanthellae can be seen emitting red fluorescence from their chlorophyll. (Courtesy of Jaffe Lab, Scripps Institution.)
Coral up close. In this image of a single coral polyp under fluorescent illumination, individual zooxanthellae can be seen emitting red fluorescence from their chlorophyll. (Courtesy of Jaffe Lab, Scripps Institution.)
Perhaps surprisingly, much remains unknown about the behavior of corals and zooxanthellae in the natural environment, due to the difficulty of imaging the minute organisms in situ. For an optical microscope to resolve features as small as a few microns—less than an order of magnitude larger than optical wavelengths—its numerical aperture, which describes the range of angles from which it collects light, must be large. And a microscope with a large numerical aperture can necessarily keep only a thin depth of field in focus at a time. In the dynamic ocean environment, in which currents and organisms are constantly moving, keeping an instrument in focus by mechanically repositioning lenses is infeasible.
Jules Jaffe and his team at the Scripps Institution of Oceanography at the University of California, San Diego, have extensive experience with microscopically imaging free-floating single-celled ocean plankton. That’s an easier task in a few ways than imaging coral. First, rather than actively focusing on specific microorganisms, the microscope can just image those that happen to float across its field of view. Second, plankton imaging can use transmission illumination, just as typical laboratory light microscopes do; reflective illumination of opaque structures such as coral is much less efficient. And third, there’s little worry that plankton will be damaged or disrupted by the microscope’s presence, whereas a microscope that gets too close to a stationary coral reef risks disturbing the polyps’ behavior or even colliding with the reef itself.
Jaffe and his team have now developed an underwater microscope for imaging corals and other organisms on the seafloor.1 The device is based on a deformable, electrically tunable lens similar in principle to the one in the human eye; by changing the lens’s shape, the microsope can automatically refocus in less than 2.5 ms. The lens—like the microscope objectives, CCD camera, and other elements of the device—was purchased commercially. Lead authors Andrew Mullen and Tali Treibitz (the latter now on the faculty of the University of Haifa in Israel) combined the components into a novel optical design that met the other requirements of seafloor imaging.
Kissing corals
Unlike lab microscopes, whose objectives can be brought into close proximity with the specimen, the new instrument was designed to maintain a working distance of 65 mm or more from the imaged subject. Reflectance illumination was provided by a ring of bright LEDs around the objective. The LEDs could be rapidly switched on and off for exposures—and flashes—as brief as 1 ms. Such short light pulses cut down on both motion blur and any behavioral response to artificial illumination.
Mullen and Treibitz, both experienced divers, lent their expertise to making sure that the instrument was practical to use underwater. Most notably, they housed the microscope’s optics separately from the onboard computer and user controls, so a diver could operate the tripod-mounted optics without risk of jostling the tripod or knocking it over.
With their microscope, the researchers studied corals in a lab tank, in the Red Sea, and off the coast of the Hawaiian island of Maui. Two of their lab images are shown here and on the cover of this issue. In the Red Sea, they observed a previously unrecorded behavior of coral polyps: leaning over to “kiss” their neighbors by bringing their digestive openings into contact. The behavior tends to occur after the polyps capture plankton from the surrounding seawater, so it may be a means of sharing organic materials.
In Hawaii, the microscope provided new insights into coral bleaching, in which corals expel their zooxanthellae en masse, are overgrown by harmful algae, and ultimately die. Bleaching can occur when a change in ocean temperature causes the zooxanthellae to produce chemicals harmful to the coral. Mullen and colleagues found that in the event they observed, the algae began to colonize the reef while the corals were still alive: The invaders first attach to the surfaces between polyps and then overgrow the polyps themselves.
Coral reefs are not the only seafloor ecosystems that could benefit from microscopic imaging, and the researchers are keen to build more copies of their microscope—they currently have two—and share them throughout the marine science community. They also hope to work on other imaging techniques such as particle image velocimetry: By seeding the seawater with benign tracer microparticles and snapping two pictures in rapid succession, they could image the flow field around a coral polyp or other organism. Lab experiments suggest that such small-scale water movements are important for marine life; among other reasons, they carry the dissolved gases that the animals breathe. But they have yet to be studied at the microscale in the natural environment.
