A miniature platform for studying corals at microscale resolution sheds new light on their biology.
Coral polyps live in symbiosis with photosynthetic, unicellular algae that provide energy in return for nutrients and protection. Some coral species interconnect into massive colonies that deposit calcium carbonate, building large skeletons known as coral reefs. Ocean warming and acidification brought on by climate change are forcing the algae to abandon the corals, leaving the reefs colorless ('bleached') and structurally weak. These global environmental changes also promote a decrease in calcium carbonate deposition and render polyps more susceptible to bacterial infection. Together, these effects cause coral reef demise and have far-reaching consequences for oceanic ecosystems.
Scientists know little about the ways in which reef corals respond to changing environmental conditions at the cellular and molecular levels. One reason for this knowledge gap has been the scarcity of strategies that allow for high-resolution studies of intact coral colonies under conditions similar to those of their natural habitats. To address this, a team from the Weizmann Institute in Israel led by Assaf Vardi has generated a system called coral-on-a-chip. The setup consists of a small microfluidic device with coral microcolonies settled on the bottom of microwells and allows for controlled light, temperature, water quality and flow conditions as well as continuous microscopic observation for extended periods of time.
The first challenge that Vardi's team faced was to gently separate the polyps from the skeleton—a process known as polyp bail-out. They achieved this by gradually increasing the water salinity until the detached polyps, or micropropagates, successfully resettled on glass microscope slides placed in microfluidic wells. They imaged these small and transparent micropropagates using both light and fluorescent microscopy, as the coral produces green fluorescent protein and the algae make chlorophyll that autofluoresces red.
To test whether growing micropropagates in microfluidic devices alters the coral's biology, the researchers recorded the deposition of the coral's skeleton in real time by using the fluorescent dye calcein, which is incorporated into growing calcium carbonate crystals. Their results confirm that the micropropagates retain the basic calcification and skeleton-morphology properties of the mother colony.
The team next focused on two burning issues in coral research: the dissolution of the polyp–algae partnership under environmental stress, and coral disease. They induced coral bleaching by shining an intense light on the micropropagates, and they then tracked and measured the loss of algal symbionts in real time at the level of whole polyps and of individual cells. This indicated the existence of specific cross-talk between the algae and the host. They also introduced fluorescently tagged pathogens into coral microchambers through the inflow channel and monitored disease onset; this led them to identify an unexplored route of coral infection.
These results show that the coral-on-a-chip device mimics coral colony structure and function in a format applicable to the study of many aspects of reef-building coral biology that may include development, physiology and disease. Theoretically, the system also allows for genetic studies of reef-building corals, which could provide a deeper understanding of their physiology and function and reward us with knowledge that might prevent their demise.
Shapiro, O.H. et al. A coral-on-a-chip microfluidic platform enabling live-imaging microscopy of reef-building corals. Nat. Commun. 7, 10860 (2016).
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Todorovic, V. Coral-on-a-chip. Nat Methods 13, 392 (2016). https://doi.org/10.1038/nmeth.3856