Symbiotic algae known as zooxanthellae live inside the coral tissues and are essential for the survival of reefs, which have been declining as a result of warming oceans, in a phenomenon known as coral bleaching. Coral reefs support more than 25% of the oceans’ global biodiversity. Studies show that oxidative stress plays an important role in the impairment of symbiont thylakoid membranes and in symbiosis breakdown, which leads to coral bleaching.

Lipids, the main components of cell membranes, are susceptible to thermal and oxidative stress. To understand the mechanisms by which temperature increase affects chloroplast membranes – the powerhouse of the plants as well as mitochondria in animals, USP scientists performed lipidomics and oxy-lipidomics analyses. They found that coral reef symbionts subjected to heat shock remodel their lipidome composition as a strategy against the oxidative stress caused by temperature rise. They also identified oxidized lipids as possible biomarkers of heat stress.

“These symbiotic algae are dinoflagellates and have very abundant relatives in global phytoplankton. So this oxidation process that we saw can happen in other phytoplankton species. Understanding how the redox state of cells and membranes can change with the increase in global temperature opens doors for us to understand the ecological consequences of this and seek more effective solutions”, said Marina Tonetti Botana, first author and, together with Marcos Yukio Yoshinaga, corresponding author of the article published in the journal Limnology and Oceanography.

The research was carried out during Botana’s master’s degree at the Instituto Oceanográfico at Universidade de São Paulo (USP), under the supervision of Professor Paulo Y. G. Sumida and co-supervision of postdoctoral fellow Yoshinaga from the Laboratory of Modified Lipids and Redox Biochemistry, at the Instituto de Química at USP, under the supervision of Professor Sayuri Miyamoto, a member of the RIDC Redoxoma.

Zooxanthellae microalgae live within the tissues of reef-building corals in a symbiotic association: they photosynthesize and release nutritious organic compounds to the corals, which in turn provide the algae with shelter and inorganic food. The algae also give the corals their color.

Coral bleaching occurs when this symbiosis breaks, with the expulsion of the microalgae or the destruction of their pigments, which makes the coral tissue translucent, making it possible to observe its calcium carbonate skeleton. A consequence of coral bleaching is the death of these animals. Corals that do not die can take 15 to 100 years to recover.

Thermal shock

To investigate acute changes in the microalgae membranes, the researchers used in vitro cultures of three species of symbionts, which were subjected to a sudden temperature increase of 12 °C, for four hours. Breviolum minutum microalgae were more sensitive to heat and did not survive heat shock, while Cladocopium goreaui and Symbiodinium microadriaticum were relatively more heat-tolerant. “We did an extremely acute heat stress experiment. When we think of a coral reef ecosystem, we have large temperature increases, up to 4 °C to 6 °C, but this will not happen in a period of four hours”, explains Botana.

The lipidomic analysis of the samples showed that the heat-sensitive symbiont displayed a quantitative decrease in glycolipids linked to polyunsaturated fatty acids, followed by enrichment in oxidized lipids and sphingolipids. Despite showing distinct adaptations, the two heat-tolerant symbionts were characterized by the preservation of membrane lipids after heat shock, particularly glycolipids. That is, variations in glycolipid concentrations determine the sensitivity or tolerance of the symbionts to heat stress.

Oxidized lipids, which are increased in heat-sensitive symbionts, may be biomarkers associated with physiological acclimatization strategies in response to heat stress. Although more studies are still needed, these lipids can function as markers of coral health and be used for large-scale monitoring.

The lipid composition of energy-transducing membranes, such as bacterial plasma membranes, thylakoid membranes of chloroplasts, and inner mitochondrial membranes, is critical for the survival of organisms. “The chloroplast thylakoid membrane is formed mostly by glycolipids, which are highly unsaturated and very susceptible to free radicals. The temperature change causes a general change in the permeability profile of the membrane, where the transport of electrons occurs, mainly high-energy electrons generated in the breakdown of water molecules by light. By destabilizing this system, you destabilize the transport of electrons and cause the generation of free radicals”, explains Yoshinaga.

According to the researcher, it is interesting to observe that “the redox imbalance caused by warming oceans causes oxidative stress, lipid oxidation and an increase in free radicals. And all this intertwines the redox processes with global climate change”. Furthermore, the same process occurs in some neurodegenerative diseases, in which the generation of radicals affects mitochondrial membranes.

Lipidomics and oxy-lipidomics

Sayuri Miyamoto highlighted that this work was only possible thanks to the infrastructure provided by RIDC Redoxoma, with analysis instruments such as the TripleTOF mass spectrometer, which allows monitoring and quantifying thousands of lipid molecules simultaneously. “Lipidomics and oxy-lipidomics show the importance of applying these global analyses, both of intact and oxidized lipids, in different topics, ranging from corals to neurodegenerative diseases. Our focus is on neurodegenerative diseases, but we see very similar phenomena happening both in algal chloroplasts and the Central Nervous System. The changes are very similar in terms of responses.”

According to the researcher, oxy-lipidomics - analysis of oxidized lipids - revealed compounds that, in addition to being markers for monitoring algae health, are also signaling agents. “You open doors to study the functional aspect of these molecules, about which very little is known. This is an interesting side that we are trying to explore in our laboratory, the functional side of the molecules produced in this process.”

The association and dissociation triggers of symbiosis, for example, still need to be investigated, because, according to Yoshinaga, “no one knows when the coral will expel the algae.” Many of the compounds modulated by heat shock are lipid mediators that can be used to establish an association between algae and host coral or serve as triggers for their dissociation. The characterization of the diversity of these compounds will allow us to understand these processes more deeply.

The article Thermal plasticity of coral reef symbionts is linked to major alterations in their lipidome composition, by Marina T. Botana, Adriano B. Chaves-Filho, Alex Inague, A. Z. Güth, Flavia Saldanha-Corrêa, Marius N. Müller, Paulo Y. G. Sumida, Sayuri Miyamoto, Matthias Y. Kellermann, Raymond C. Valentine and Marcos Y. Yoshinaga, can be accessed here.