Coral reefs provide sustenance and income to an estimated half billion people, attract tourists, protect coastlines and are among the most biodiverse ecosystems on our planet. Despite their importance, more than half of the world’s coral reefs are now under stress, primarily due to climate change and related human activities. Stressed corals ‘bleach’, which describes the disruption of the symbiosis between corals and their photosynthetic partners, heavily pigmented Symbiodiniaceae cells that provide most of the energy to their coral host. To recover from bleaching, corals can repopulate more temperature-tolerant symbiont cells that survived within the tissue, or take up new cells from the environment. The temperature sensitivity of a coral thus depends in part on the temperature sensitivity of its symbionts, which has made the investigation of temperature tolerance among coral symbionts an intense research topic.

To further explore how different species of coral symbionts react to temperature stress we developed a high-throughput and uniquely miniaturised approach. This approach combined a pulse amplitude-modulated fluorometry microscope with a credit card-sized microfluidic chip which can house many hundreds of individual coral symbiont cells (Figure 1). These microfluidic devices can be placed onto a unique temperature control system, specifically fabricated to provide a high degree of temperature control across time and space¹. In combination, this approach enabled us to rapidly assess the photophysiological effects of temperature stress on single cells.

Figure 1. (a) The pulse amplitude-modulated fluorometry microscope. (b) The credit card-sized microfluidic chip mounted onto the temperature regulation system. (c) Individual coral symbiont cells immobilized within microwells of the microfluidic chip.

In the first series of experiments, we used this microfluidic approach on five Symbiodiniaceae species and discovered that individual cells behave quite differently from one another under temperature stress. For one, temperature stress increased the photophysiological heterogeneity among individuals, highlighting that populations of single cells exhibit variable thermal sensitivity not only within- but also across species. In addition, we discovered that cells with high photophysiological activity under no-stress conditions typically coped better with temperature stress than cells that had a lower initial photophysiological activity. This simple association allowed us to ‘predict’ the temperature tolerance of a cell before the stress actually occurred.

While we still have a long way to go, our approach might thus be useful for coral reef monitoring. By collecting a small amount of coral tissue, extracting symbiont cells from it and testing the temperature tolerance of these cells we could conceivably assess the thermal tolerance of a coral without the need to sacrifice it. However, this assumes that the temperature tolerance of a coral is directly related to the thermal tolerance of its extracted symbionts, a hypothesis we plan to experimentally address in the near future. Finally, our method also holds potential to accelerate ongoing coral reef restoration efforts by ‘experimentally evolving’ temperature-tolerant coral symbionts. By identifying more temperature-tolerant cells and isolating them via laser capture microdissection, we aim to determine whether their temperature tolerance persists over several generations and to understand the molecular underpinnings that contribute to this tolerance. Ultimately, by enabling the high-throughput testing of single cells under temperature stress, our approach could powerfully complement multiscale efforts that try to sustain biodiversity in a rapidly changing ocean by enhancing the climate-resilience of coral reefs².

Read the full article here: https://doi.org/10.1038/s41396-022-01243-6

References:

1. Andersson, M. et al., A microscopy-compatible temperature regulation system for single-cell phenotype analysis – demonstrated by thermoresponse mapping of microalgae. Lab Chip 2021, 21, 1694-1705

2. van Oppen M. J. H. et al., Building coral reef resilience through assisted evolution. PNAS 2015, 112, 2307-2313.