Microscopic imaging under the sea

Attention all marine microorganisms — are you ready for your close-up?

Many important biological processes in the ocean take place at microscopic scales, but when scientists remove organisms from their native habitats to study them in the lab, much of the information and its context are lost. In a quest to overcome this challenge, researchers from Scripps Institution of Oceanography have built a diver-operated underwater microscope to study millimetre-scale processes as they naturally occur on the sea floor.

The Benthic Underwater Microscope (BUM) is a two-part system — an underwater computer with a diver interface tethered to a microscopic imaging unit — to study marine subjects at nearly micron resolution. The instrument has a high magnification lens, a ring of focused LED lights for fast exposures, fluorescence imaging capabilities and a flexible tuneable lens, similar to the human eye, to change focus for viewing structures in 3D.

“This underwater microscope is the first instrument to image the sea floor at such small scales,” said PhD student Andrew Mullen, co-lead author of the study. “The system is capable of seeing features as small as single cells underwater.”

In situ underwater operation of the BUM by a scientific diver. This image has been cropped from the original and appears courtesy of the authors under CC BY 4.0

The motivation for building the imaging system was to better understand the many ecological processes taking place underwater on a microscopic scale, according to senior author Jules Jaffe. He explained, “To understand the evolution of the dynamic processes taking place in the ocean, we need to observe them at the appropriate scale.”

To test the new technology’s ability to capture small-scale processes taking place underwater, the researchers used the imaging system to view millimetre-sized coral polyps off the coast of Israel in the Red Sea and off Maui, Hawaii. The results were published in the journal Nature Communications.

During experiments in the Red Sea, the researchers set up the BUM to capture the interactions of two corals of different species placed close to each other. The images revealed microscale processes in which corals emit string-like filaments that secrete enzymes from their stomach cavity to wage a chemical turf battle to destroy the tissue of other species in a competition for sea-floor space. Yet when the researchers placed corals of the same species next to each other, they did not eject these gastric fluids.

“They can recognise friend versus foe,” explained Mullen.

The researchers also captured video of neighbouring individual polyps on a single coral colony taking turns embracing one another, an unknown phenomenon the researchers call coral polyp ‘kissing’.

Next, the researchers deployed the instrument off Maui following one of the largest coral-bleaching events on record, which occurs when single-celled algae that live inside the coral polyp eject themselves during high ocean temperature events. Recently bleached corals are still alive, but in their weakened state can be rapidly invaded and overgrown by filamentous turf algae.

Using the microscope, the research team observed a previously unreported honeycomb pattern of initial algal colonisation and growth in areas between the individual coral polyps during coral bleaching. These findings provide insight into a process that marine ecologist Jennifer Smith, a co-author of the study, called the “succession of algae”, where small filamentous algae initially settle on the ridges between coral polyps and eventually smother the living tissue. The images showed that algae are able to actively overgrow living corals during a bleaching event.

Jaffe and Mullen are now preparing the instrument to take pictures of microscopic particles in the water near the coral’s surface to study how the flow of water over corals allows them to exchange the necessary gases to breathe.

Top image caption: Images of a live coral acquired in situ with the BUM using an ETL focal scan. Scale bars: 500 μm. Images appear courtesy of the authors under CC BY 4.0