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Live imaging of Aiptasia larvae, a model system for studying coral bleaching, using a simple microfluidic device

Will Van Treuren, Kara Brower, Louai Labanieh, Daniel Hunt, Sarah Lensch, Bianca Cruz, Heather N Cartwright, Cawa Tran, Polly M Fordyce

Preprint posted on July 19, 2018 https://www.biorxiv.org/content/early/2018/07/19/370478

How can one image motile sea anemone larvae? A recent preprint by Van Treuren and Brower et al. introduces ‘Traptasia’, a microfluidic device capable of trapping and imaging live Aiptasia larvae, a model for coral symbiosis.

Selected by Samantha Seah

Categories: ecology

Background

Coral reefs are vital ecosystems in our oceans that are home to a tremendous diversity of oceanic species. Due to environmental stressors, along with stress caused by humans, corals are increasingly undergoing ‘bleaching’ – a process in which photosynthetic algal symbionts are ejected from the corals. Over the long term, this could lead to coral death. Even as the stress factors contributing to coral bleaching are known, much work remains to be done to thoroughly elucidate the molecular and cellular mechanisms involved.

Coral symbiosis can be modelled by studying the motile larvae of sea anemone, Aiptasia, together with symbiotic algae from the genus Symbiodinium. A recent preprint by Van Treuren, Brower and their colleagues outline the trapping and imaging of motile Aiptasia larvae, via a microfluidic device, ‘Traptasia’, enabling dynamic observations under different conditions.

 

Key Findings

The authors designed a single layer polydimethylsiloxane (PDMS) trapping device capable of capturing Aiptasia larvae. These resemble previously described cell traps, but as Aiptasia larvae (at 40-100µm) are much larger than typical mammalian cells (around 10µm), the authors optimised and assessed trap-loading efficiencies for traps with varying trap apertures and channel heights. The optimised traps (with heights of 90µm and trap apertures of 20µm) were able to efficiently capture Aiptasia larvae and hold the larvae in traps by the provision of a constant flow of fluid.

The authors then demonstrated that trapped larvae can be imaged by spinning disk confocal microscopy for over 10 hours. Larvae co-infected with algal symbionts could also be imaged with transmitted light alone, and individual algae tracked within the larvae. Larval revolutions could also be followed, enabling the further study of subtle stress responses. The authors also noted multiple potentially interesting larval-death mechanisms which had been previously observed in culture, but have yet to be studied in cellular detail, illustrating how the ‘Traptasia’ could reveal mechanistic details of Aiptasia physiology.

To model coral bleaching upon stress, the authors treated algae-larvae symbionts with DCMU (3-(3,4-dichlorophyli)-1,1-dimethylurea), which has been proposed to stress corals. Under this treatment, 5/33 trapped larvae ‘swam’ upstream of flow or through the trap aperture, suggesting that stress may introduce changes in motility and physiology. In addition, they captured an algal expulsion event from a live Aiptasia larva (Video S6). As algal expulsion is thought to be involved in coral bleaching, the use of ‘Traptasia’ to study Aiptasia larvae under different stress conditions could reveal biological mechanisms involved in coral bleaching, potentially shedding light on the damage accumulating in our coral reefs.

Video S6 from the preprint: Algal expulsion event from live Aiptasia larva. Video reproduced under a CC-BY-NC-ND 4.0 International License.

 

What I like about this work

As a biologist working in the field of microfluidics, I am a great fan of simple microfluidic devices that can be used by biologists to complement the study of otherwise complex biological problems. ‘Traptasia’ falls neatly into this category – a single layer microfluidic device is used to trap and image large, deformable and motile organisms, without the need for excessively complex equipment. As individual algae can be imaged with transmitted light, this technology can be made accessible to many locations, including schools and field research stations.

I see so much potential in this new technology – combining it with genomics and transcriptomics (as mentioned by the authors) could add dimensionality to the data obtained. It could be also adapted for use with other marine creatures and motile organisms, and I am keen to see how others will adapt this technology to investigate other biological problems.

 

Open Questions

  • Do you see the traps working for heterogenous populations, and if not, would it be possible to design such traps?
  • Could the devices be utilised for the short-term culture and study of primary aquatic samples?

 

Further reading:

Microfluidic cell trapping: Dura, B., Servos, M. M., Barry, R. M., Ploegh, H. L., Dougan, S. K., & Voldman, J. (2016). Longitudinal multiparameter assay of lymphocyte interactions from onset by microfluidic cell pairing and culture. Proceedings of the National Academy of Sciences, 113(26), E3599–E3608. https://doi.org/10.1073/pnas.1515364113

Coral-on-a-chip: Shapiro, O. H., Kramarsky-Winter, E., Gavish, A. R., Stocker, R., & Vardi, A. (2016). A coral-on-a-chip microfluidic platform enabling live-imaging microscopy of reef-building corals. Nature Communications, 7, 10860. https://doi.org/10.1038/ncomms10860

Tags: marine biology, microfluidics

Posted on: 9th August 2018 , updated on: 12th August 2018

Read preprint (1 votes)




  • Author's response

    Will Van Treuren, Kara Brower, Polly M Fordyce shared

    Thanks for your interest in the preprint! We are excited about our Traptasia device both because of the ease of use and because of the process that brought it about. As you alluded to in your comments, a lot of microfluidic devices require a substantial amount of expertise and technology to work with. Our goal from the outset was to avoid the complexity of a lot of the microfluidic solutions we read about, and the ultimate requirement of tubing and a syringe pump really makes this device accessible to a wide range of biologists. We are hoping to collaborate with researchers here at Stanford to understand some of the mechanisms that cause coral to bleach. Ultimately, we hope the device could help reveal chemical stressors that would be amenable to simple interventions from a governmental or industrial perspective.

    Another component we are proud of is how the Traptasia project was created. As Dr. Fordyce has observed, there is quite a gulf between microfluidic engineers and biologists who would benefit from the devices they design. Even if a microfluidic research device ends up being simple to use, the design process is not something that most biologists are trained to do. To overcome this barrier, Dr. Fordyce created a class that paired engineers interested in microfluidic design principles with projects pitched by Stanford biologists in need of research devices. The need to image live aiptasia was presented by a post-doc in the Pringle lab (Cawa Tran) and the Traptasia team was able to collaborate closely with her to create a usable device.

    As to the open questions you posed.

    (1) Heterogenous populations (both in size/behavior and in species) would be very interesting to study. The complexity of coral ecosystems is tremendous, and being able to study the effects of environmental changes for all members of the community as once would be powerful. Our current devices will definitely trap a range of sizes of organisms, but we haven’t intentionally studied multi-species populations. We did get some parasites that fed on the aiptasia in one experiment which form a cool movie in S8.

    (2) It would be cool to look at short term cultures, but the real power in the device lies in the sustained imaging of a single entity. That level of tracking is hard to do right now for really motile things, so we are hoping it’s most useful in that regard.

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