Corals and zooxanthellae are intimate companions, or "separated" by global warming

Coral reef ecosystems are among the most biodiverse and productive ecosystems in the oceans and are known as "rainforests in the ocean". As the "framework organism" of coral reefs, reef-building stony corals and zooxanthellae are mutually beneficial and symbiotic, and the coral host provides shelter for intracellular zooxanthellae and inorganic nutrients such as nitrogen, phosphorus and carbon dioxide, in return, zooxanthellae photosynthesis and "share" most of the products to the coral host, in order to meet the energy needs of corals and promote the calcification of corals. The number of zooxanthellae in healthy reef-building corals can generally reach millions per square centimeter.

Yet their relationship is facing serious threats from global climate change, with the warming and acidification of seawaters in the most concerned. Abnormal warming of seawater will lead to the collapse of the symbiotic relationship between corals and zooxanthellae, which will lead to coral bleaching and death, while acidification will reduce the rate of coral calcification, which will also affect the normal growth and development of reef-building stony corals, and then affect the stability of symbiotic relationships.

There is not only one species of zooxanthellae

For a long time, it was widely believed that there was only one species of zooxanthellae, until the mid-1970s, studies based on behavior, physiology, and microstructure showed the presence of multiple symbiotic zooxanthellae. Existing studies have confirmed the rich genetic diversity of zooxanthellae, based on ribosomal DNA sequences can be divided into 9 fauna (A-I), each phylogenous group has been divided into genera orders, and each phylum population has been divided into more subtypes according to the transcriptional interval zone ITS2 sequence. Different types of zooxanthellae have obvious physiological characteristics, which in turn affect the physiological performance of symbiotic systems, including photosynthetic physiology and tolerance to high temperature and acidification. Among them, the heat tolerance of D-type zooxanthellae is the best, and the reef-building corals that coexist with D-type zooxanthellae tend to have good resistance to high temperatures and mostly live in extreme environments such as turbidity and high temperatures.

In the adult stage, there are several types of zooxanthellae in corals, but there is usually only one dominant type, and this symbiotic relationship has some plasticity to help corals adapt to environmental changes. For example, after experiencing high temperature stress, corals can adapt to high temperatures by recombining the relative abundance of existing zooxanthellae in vivo or obtaining more adaptable types from the outside world. Of all reef-building stony corals, most corals are ovulatory corals, and the male and female gametes they produce do not contain zooxanthellae, and the gametes develop into non-co-ecological floating wave larvae after in vitro fertilization, and the floating wave larvae are attached to the metamorphosis as larvae and gradually grow and develop until the completion of the entire life history. Early symbiosis can occur at the larval stage or in the larval stage after attachment metamorphosis, but true symbiosis occurs only in the larval stage, where the larvae use the tentacles and cilia at the end of the mouth to capture the symbiotic algae in the water body and transport them to the body cavity, which is recognized by endodermal cells and engulfed to form a stable symbiosis, and then the zooxanthellae continue to multiply and concentrate in the mouthpal and tentacle position.

The symbiotic relationship between corals and zooxanthellae in the early stages of corals is more flexible and dynamic than adult corals. For example, staghorn coral larvae can establish a symbiotic relationship with a variety of zooxanthellae after one month in the wild environment, and multiple types do not appear in adults, and the dominant zooxanthellae types contained in the larvae change as they grow and develop.

The diversity of free-form zooxanthellae in the seawater of coral reef environments is high, and the initial acquisition of symbiotic algae by coral larvae is likely to be random. However, specific types of zooxanthellae can shape the early growth and development of coral symbiotic systems and responses and adaptations to environmental stresses. For example, staghorn coral larvae symbiotic with cordycete cymbidium grow and develop significantly faster than larvae symbiotic with type D, while larvae symbiotic with type D xanthoxate are more tolerant to high temperatures than larvae symbiotic with type C.

Symbiosis is a milestone event in the early life phase of corals, since which zooxanthellae begin to photosynthesize and provide additional nutrients and energy supplies to coral hosts, which is particularly critical for larval survival and environmental adaptability. Corals in the early stages have the potential to screen suitable zooxanthellae types for optimal conditions based on physiological needs and environmental changes during development.

Different corals have different sensitivities to stress

How will the early symbiotic establishment of corals affected by climate change, especially warming and acidification? During this process, can coral larvae regulate the physiological state of the symbiont to cope with and adapt to warming and acidification by selecting certain specific zooxanthellae? What are the similarities and differences in the adaptation potential and strategies of different species of corals?

With the above questions in mind, Huang Hui's research team at the South China Sea Institute of Oceanography of the Chinese Academy of Sciences studied two widely distributed reef-building stony corals in the Indo-Pacific region, the middle staghorn coral and the delicate flat-brained coral, the former being a sensitive branch coral and the latter being a tolerant clump coral. The researchers collected fertilized eggs from two corals and bred them to the larval stage, and then reared them in a flow-through experimental culture system, and studied the early symbiotic establishment process and growth and development of the two coral larvae under the influence of rising temperature and pH by controlling sea temperature and pH.

The study found that warming delays the early symbiotic establishment of the two coral species, while acidification has little effect, but different coral species have different sensitivities to stress. The warming completely inhibited the successful establishment of early symbiosis of intermediate staghorn corals, and at the same time significantly inhibited the asexual budding reproduction of larvae, which showed that early symbiotic establishment played an important role in supporting the growth and development of larvae; for delicate flat-brain corals, the warming only reduced the proportion of coral larvae that successfully established symbiotic corals by 50%.

In addition, the heating will affect the selection preference of intermediate staghorn coral larvae for zooxanthellae, under normal temperature conditions, staghorn coral larvae mainly establish symbiosis with zooxanthellae C group, and larvae prefer to choose heat-tolerant zooxanthellae D group under high temperature conditions; in contrast, delicate flat-brain coral larvae prefer to establish symbiosis with zooxanthellae D group under acidification conditions.

In addition, it was found that under high temperature conditions, the gene expression of zooxanthellae involved in ribosomal synthesis, photosynthesis and basal metabolic processes in the intermediate staghorn coral larvae was downregulated, indicating that although zooxanthellae could enter the intermediate staghorn coral at this time, its photosynthesis and metabolism were inhibited, and it was impossible to multiply in large quantities to establish a stable mutually beneficial symbiotic relationship. Jiang Lei

(According to the WeChat public account "Voice of the Chinese Academy of Sciences")

Source: Science and Technology Daily