See algae run

New evidence support that, during coral bleaching,

algae are not getting kicked out but leaving on their own

A colony of the soft coral known as the “bent sea rod” stands bleached on a reef off of Islamorada. Image courtesy of Kelsey Roberts, U.S. Geological Survey, a Wikimedia Commons user 

Corals do not handle change well: They turn ghostly white at a slight shift in water conditions because the algae residing in their cells, the source of their color, disappear. Although this phenomenon, coral bleaching, is well-documented and unfortunately occurring more often now, scientists are still not clear why the algae disappear. An international team of researchers recently reported in Molecular & Cellular Proteomics that the algae may run their own exit strategy when stressful conditions set in.

A coral colony provides a protective environment for algae by allowing the algae to live in membrane-enclosed bubbles, or vesicles, inside its cells. In return for a home, the algae convert the coral’s metabolic waste into oxygen and nutrients that the coral uses to sustain itself. Changes in the environmental conditions, like water temperature or light exposure, break this relationship and cause the algae to disappear. The coral is left bare, and if the algae are not re-established, the coral colony eventually dies.

Scientists have several theories on why the algae disappear. Some believe that the coral cells are breaking off or dying. Others argue that the effects are on the algae themselves, that the algae are being destroyed or getting thrown out by the coral cells. Paul F. Long, who led the team that authored the recent MCP paper, supports the idea that the algae are leaving the coral but that they are leaving on their own accord.

For the contents of a vesicle to be released out of the cell, the membrane of the vesicle has to merge with the cell’s membrane. Membrane fusing involves soluble N-ethylmaleimide-sensitive factor activating protein receptor, or SNARE, proteins. SNARE proteins on the vesicle’s membrane, v-SNAREs, latch onto SNARE proteins on the cell’s membrane, t-SNAREs, bridging the two membranes together and allowing them to become one. During this exocytosis process, the vesicle’s interior becomes exposed to the outside, and the vesicle’s contents are flipped outwards.

An earlier study by Long’s group found SNARE proteins in algae-harboring coral samples. Other studies had shown that intracellular microbes can mediate their own entry into and exit from host cells by encoding their own SNARE proteins. Long’s team proposed that in a similar fashion, algae could have their own SNARE proteins so that they can enter and leave the coral at will. In this new study, Long and his team sought to determine if changes in temperature and light conditions altered the expression of the coral’s and algae’s exocytosis proteins.

The group collected corals from the Great Barrier Reef and, in tanks on a research ship, subjected their samples to temperature and light conditions that cause coral bleaching in nature: high light exposure in normal water temperature, high light exposure in high water temperature and low light exposure in high water temperature. The researchers then used proteomics analyses to measure the changes in protein expression under each condition. They took advantage of having both the coral’s and algae’s gene sequences, which only became available recently, to distinguish which proteins were the algae’s and which were the coral’s.

All three conditions resulted in bleaching of the coral samples and reduced photosynthetic activity of the algae, confirming that these treatments did stress the coral. The investigators observed a high level of t-SNARE and a low level of v-SNARE proteins in the coral, which suggests that the coral cells have the ability to exocytose their constituents. The investigators also saw no evidence of t-SNARE proteins in the algae, as they had expected, although they were surprised that no v-SNARE proteins were detected either.

Based on these data, the researchers propose that the algae express their own v-SNARE proteins when they first sense changes in their environment, initiating their mass flight out of the coral. Those algae that remain in the coral, the ones that ended up in the coral samples, would not express the v-SNARE proteins, because they are the ones that stayed.

The investigators write that the data support their notion that algae taking flight, not algae being destroyed or the coral breaking apart, causes coral bleaching. Moreover, they continue, the study provides more evidence that algae can control their comings and goings. The researchers intend to investigate next how this timing unfolds to, as they put it, better “predict the impact of environmental change on the future resilience of tropical coral reef ecosystems.”

Maggie Kuo Maggie Kuo was an intern at ASBMB Today when she wrote this story. Today she is a writer at the American Physiological Society. She earned her Ph.D. in biomedical engineering at Johns Hopkins University.