In vitro Symbiosis of Reef-Building Coral Cells With Photosynthetic Dinoflagellates

Abstract

Coral reefs are the biodiversity hot spots of the oceans, but they have suffered from increasing environmental stresses caused principally by anthropogenic global warming. The keystone species of coral reefs are scleractinian corals, which maintain obligatory symbiotic relationships with photosynthetic dinoflagellates. Understanding cellular and molecular mechanisms of symbiosis is therefore essential for future preservation of coral reefs. To date, however, almost no in vitro experimental systems have been devised to illuminate such mechanisms. To this end, our previous study established stable in vitro cell culture lines, including IVB5, originating from planula larvae of the scleractinian coral, Acropora tenuis. Here, we show that soon after mixture with the dinoflagellate, Breviolum minutum, flattened amorphous coral cells with endodermal properties exhibited elevated locomotor activity using filopodia and lamellipodia and interacted with dinoflagellates. Several minutes thereafter, coral cells began to incorporate B. minutum, and in vitro symbiosis appeared to have been accomplished within 30 min. Nearly a half of the coral cells had incorporated algal cells within 24 h in a reproducible manner. Coral cells that harbored algal cells gradually became round and less mobile, and the algal cells sometimes settled in vacuole-like structures in coral cell cytoplasm. This symbiosis state was maintained for at least a month. The IVB5 line of A. tenuis therefore provides an experimental system to explore cellular and molecular mechanisms involved in coral-dinoflagellate symbiosis at the single-cell level, results of which may be useful for future preservation of coral reefs.

Lipid stores reveal the state of the coral-algae symbiosis at the single-cell level

“…Such subtle effect is easily overlooked in bulk tissue analyses and indeed could not be detected in mean colony responses, highlighting the strength and importance of single-cell work for revealing important physiological mechanisms of coral-algal symbiosis. Together with the recent advances in tissue culture techniques for growing and maintaining individual symbiotic and non-symbiotic coral cells [ 7 ], this new marker provides an exciting opportunity for the investigation of mechanisms underlying the maintenance and breakdown of the coral symbiosis.…”

Comparative genomics highlight the importance of lineage-specific gene families in evolutionary divergence of the coral genus, Montipora

“…Although genetic tools for manipulating corals have been established [ 53 , 54 ], development of more efficient methods to deliver gene-knockdown or -knockout reagents into large numbers of zygotes will facilitate rapid screening for relevant phenotypes of candidate genes. In addition, coral cell lines which have the capacity to incorporate algal symbionts has been developed [ 55 ], allowing us to observe ongoing symbiosis at the single cell level. Together, these advances will facilitate a deeper understanding of cellular and molecular mechanisms of coral-algal symbiosis.…”

Larval transcriptomic responses of a stony coral, Acropora tenuis, during initial contact with the native symbiont, Symbiodinium microadriaticum

“…Although RNA-seq techniques have become more feasible than in the previous decade, it is still difficult to capture minute gene expression changes with bulk RNA-seq, because only a small percentage of the volume of a coral larva contains cells with algae. Tissue-specific RNA-seq 19 , 60 , single cell RNA-seq 61 , or coral cell lines 62 may reveal more comprehensive molecular responses of coral symbioses in the future.…”