Thermal and Herbicide Tolerances of Chromerid Algae and Their Ability to Form a Symbiosis With Corals

Abstract

Reef-building corals form an obligate symbiosis with photosynthetic microalgae in the family Symbiodiniaceae that meet most of their energy requirements. This symbiosis is under threat from the unprecedented rate of ocean warming as well as the simultaneous pressure of local stressors such as poor water quality. Only 1°C above mean summer sea surface temperatures (SSTs) on the Great Barrier Reef (GBR) can trigger the loss of Symbiodiniaceae from the host, and very low concentrations of the most common herbicide, diuron, can disrupt the photosynthetic activity of microalgae. In an era of rapid environmental change, investigation into the assisted evolution of the coral holobiont is underway in an effort to enhance the resilience of corals. Apicomplexan-like microalgae were discovered in 2008 and the Phylum Chromerida (chromerids) was created. Chromerids have been isolated from corals and contain a functional photosynthetic plastid. Their discovery therefore opens a new avenue of research into the use of alternative/additional photosymbionts of corals. However, only two studies to-date have investigated the symbiotic nature of Chromera velia with corals and thus little is known about the coral-chromerid relationship. Furthermore, the response of chromerids to environmental stressors has not been examined. Here we tested the performance of four chromerid strains and the common dinoflagellate symbiont Cladocopium goreaui (formerly Symbiodinium goreaui , ITS2 type C1) in response to elevated temperature, diuron and their combined exposure. Three of the four chromerid strains exhibited high thermal tolerances and two strains showed exceptional herbicide tolerances, greater than observed for any photosynthetic microalgae, including C. goreaui . We also investigated the onset of symbiosis between the chromerids and larvae of two common GBR coral species under ambient and stress conditions. Levels of colonization of coral larvae with the chromerid strains were low compared to colonization with C. goreaui . We did not observe any overall negative or positive larval fitness effects of the inoculation with chromerid algae vs. C. goreaui . However, we cannot exclude the possibility that chromerid algae may have more important roles in later coral life stages and recommend this be the focus of future studies.

High temperature inhibits cnidarian-dinoflagellate symbiosis establishment through nitric oxide signaling

“…Coral reefs are facing extinction due to an increasing number of mass bleaching events caused by the warming of the oceans due to climate change [44,45]. However, climate change also has negative consequences for other fundamental aspects of coral biology such as sexual reproduction, symbiosis establishment, calcification and susceptibility to disease [23,[46][47][48][49]. Thus, it is imperative to better understand not only the symbiosis disruption, but also the underlying mechanisms and effects of environmental stress in other essential biological processes of corals to protect these critically important ecosystems.…”

Intracellular Bacterial Symbionts in Corals: Challenges and Future Directions

“…Symbiodiniaceae cells have also been reported to harbor intracellular bacteria [51]. Other microorganisms, including viruses [58], archaea [59], fungi [60], and other eukaryotes [61,62], associate with corals and are not represented in this figure.…”

Inorganic carbon concentrating mechanisms in free‐living and symbiotic dinoflagellates and chromerids

“…It is not clear if chromerids (basal to non‐photosynthetic apicomplexans) are mutualistic symbionts of the corals with which they associate (Chakravarti et al. 2019). Other alveolates associated with corals are non‐photosynthetic apicomplexans that retain four chlorophyll synthesis genes (Kwong et al.…”