Underwater world revealed as never before


Rebecca Pool

Thursday, July 14, 2016 - 12:15
Fluorescent image of the coral Pocillopora damicornis; field of view 4.1 X 3.4 mm.
In a breakthrough for oceanographers, US-based researchers have developed a novel underwater microscope to image marine organisms in natural settings at near-micrometre resolution.
Comprising a long-working distance microscopic objective, electrically tunable lens and focused reflectance illumination, the portable instrument can be used by divers to record dynamic natural processes without disturbing organisms.
Andrew Mullen deploys the Benith Underwater Microscope during coral reef studies in Maui. [Kelly/UC San Diego]
"This underwater microscope is the first instrument to image the sea-floor at such small scales," highlights Andrew Mullen from the Scripps Institution of Oceanography at the University of California, San Diego. "The system is capable of seeing features as small as single cells underwater."
Imaging the seafloor at close to micrometre resolution has presented significant challenges, including active instrument positioning, precise focusing and reflectance illumination.
To address these issues, Mullen and colleagues developed the Benthic Underwater Microscope, an imaging system that provides the first in situ, underwater observations of sea floor environments at nearly micrometre resolution.
The instrument comprises three key optical components; first, a long working distance microscope objective lens for non-invasive imaging, and second, a shape changing electrically tunable lens for rapid and precise focusing in unstable environments.
Finally, a custom-designed ring of six LEDs provides the high intensity light for short duration reflectance illumination, necessary for underwater conditions.
The Benith Underwater Microscope has a high magnification lens surrounded by focused LED lights and a companion computer with ceramin buttons [Mullen/UCSD]
"Each LED is focused with a condenser lens and angled such that all light sources converge at the plane being imaged," explains Mullen.
Crucially, all elements are integrated into a compact, submersible imaging system that includes a camera, electronics and user interface.
The system is divided between an imaging unit, containing optical components, and control unit with a computer and live diver interface.
To demonstrate the technology, 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.
a) In situ images of coral Stylophora with X5 objective lens and white illumination. b) Fluorescent image of coral Pocilopora in lab tank, X5 objective. c) In situ image of Rhopalaea idoneta with X5 lens and white illumination. d,e) In situ images of Porites comperssa with bleaching, X3 lens. f) In situ image of fully bleached Porites Iobata, X3 lens.
Images, for example, revealed micro-scale processes in which neighbouring corals emit string-like filaments that secrete enzymes from their stomach cavity to destroy the tissue of other species.
Videos also captured polyps on a single coral colony taking turns 'embracing' one another, a previously unknown phenomenon the researchers now call coral polyp “kissing.”
Pocillopora polyps; a 2.8 X 2.4 mm field of view.[Mullen/UCSD]
In addition, researchers also observed an unreported honeycomb pattern of initial algal colonisation and growth in areas between the individual coral polyps during coral bleaching.
The researchers are now preparing the instrument to take images of microscopic particles in the water near a coral's surface, to study how water flow allows the exchange of gases.
Documenting algae as it colonises on the surface of bleached corals; 2.82 X 2.36 mm field-of-view. [Mullen/UCSD]
"This instrument is part of a new trend in ocean research to bring the lab to the ocean, rather than the ocean to the lab," adds Mullen's colleague Dr Tali Treibitz, University of Haifa's Charney School of Marine Science.
Research is published in Nature Communications.
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