How a newly identified bacterium saps corals of their energy

A mysterious bacterium had been detected living in Caribbean corals, sapping them of energy and organic building blocks needed to grow. The microbe thrives in corals growing in nutrient-rich water. And now scientists know how.

The microbe can sense increased nutrient levels in corals themselves, and may be anticipating when corals and the beneficial algae that live with them will increase their production of amino acids, scientists report August 5 in the ISME Journal. That heads-up may give the microbe more time to siphon off more energy and amino acids from the corals, allowing this typically low-level parasite to reproduce rapidly.

This bacterium is “weakening the corals enough to make them susceptible” to other harmful pathogens (SN: 8/3/19, p. 14), says microbiologist Grace Klinges at Oregon State University in Corvallis. That’s cause for alarm, she says, as nutrient levels, and thus levels of the bacteria, can rise quickly with sewage and agricultural runoff in some coastal waters.

In 2013, researchers studying the effects of such nutrient enrichment on staghorn corals off Florida discovered relatively unknown bacteria proliferating far faster than others. The microbes’ population went from accounting for about 11 percent of the corals’ overall microbial community to as much as 88 percent as nutrient levels rose. “It’s really unusual to see one bacterium become so dominant,” Klinges says.

Her and her colleagues’ analysis of the bacteria’s DNA revealed the microbes to be part of a new genus, Candidatus Aquarickettsia. The coral-dwelling species, which the researchers named Candidatus A. rohweri, was also found to be related to an ancient, energy-producing bacterium that evolved into cellular power sources called mitochondria.

But while mitochondria make an energy-carrying molecule called ATP, short for adenosine triphosphate, that animals and plants use widely, Ca. A. rohweri steals ATP away from corals for its own use, Klinges says.