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

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

doi:10.1038/s41598-019-45167-2

Abstract

Coral reefs, and their associated diverse ecosystems, are of enormous ecological importance. In recent years, coral health has been severely impacted by environmental stressors brought on by human activity and climate change, threatening the extinction of several major reef ecosystems. Reef damage is mediated by a process called ‘coral bleaching’ where corals, sea anemones, and other cnidarians lose their photosynthetic algal symbionts (family Symbiodiniaceae) upon stress induction, resulting in drastically decreased host energy harvest and, ultimately, coral death. The mechanism by which this critical cnidarian-algal symbiosis is lost remains poorly understood. The larvae of the sea anemone, Exaiptasia pallida (commonly referred to as ‘Aiptasia’) are an attractive model organism to study this process, but they are large (∼100 mm in length, ∼75 mm in diameter), deformable, and highly motile, complicating long-term imaging and limiting study of this critical endosymbiotic relationship in live organisms. Here, we report ‘Traptasia’, a simple microfluidic device with multiple traps designed to isolate and image individual, live larvae of Aiptasia and their algal symbionts over extended time courses. Using a trap design parameterized via fluid flow simulations and polymer bead loading tests, we trapped Aiptasia larvae containing algal symbionts and demonstrated stable imaging for >10 hours. We visualized algae within Aiptasia larvae and observed algal expulsion under an environmental stressor. To our knowledge, this device is the first to enable time-lapsed, high-throughput live imaging of cnidarian larvae and their algal symbionts and, in further implementation, could provide important insights into the cellular mechanisms of cnidarian bleaching under different environmental stressors. The ‘Traptasia’ device is simple to use, requires minimal external equipment and no specialized training to operate, and can easily be adapted using the trap optimization data presented here to study a variety of large, motile organisms.

Highlights

Coral reefs are remarkably productive ecosystems, supporting approximately 9% of the ocean fish biomass and 25% of oceanic species diversity[1]
Study of algal symbiosis in Aiptasia previously has relied on adult or larval fixation, which reveals static algal presence and distribution, but prevents dynamic observation of the host and host-symbiont interactions[5]. This limitation represents a central challenge in marine biology; without the capability of live imaging, early symbiosis initiation and maintenance remains poorly understood and difficult to address via new approaches
No published study has demonstrated time-lapsed live imaging of Aiptasia larvae and their associated algal symbionts, likely due to the challenges associated with designing structures to immobilize deformable, highly motile cnidarian larvae that are large (∼90–140 μm in length, ∼75–100 μm in diameter) in size

Summary

Introduction

Coral reefs are remarkably productive ecosystems, supporting approximately 9% of the ocean fish biomass and 25% of oceanic species diversity[1]. Under environmental and anthropogenic stressors such as rising ocean acidity, pollution, and increasing temperature, the coral-algal symbiotic relationship can break down, resulting in a process known as‘coral bleaching’[1,5] In this process, photosynthetic algal symbionts are expelled from the coral gastrodermal tissue where they normally reside, resulting in www.nature.com/scientificreports/. Study of algal symbiosis in Aiptasia (and other cnidarians) previously has relied on adult or larval fixation, which reveals static algal presence and distribution, but prevents dynamic observation of the host and host-symbiont interactions[5] This limitation represents a central challenge in marine biology; without the capability of live imaging, early symbiosis initiation and maintenance remains poorly understood and difficult to address via new approaches (e.g. bioremediation for widespread coral bleaching). We sought to develop an easy-to-use device for live, longitudinal observation of individual Aiptasia larvae and their algal symbionts over long time courses (2–12 hours) and under different environmental stressors as a model system for the study of cnidarian bleaching

Methods

Results

Conclusion