Ciona intestinalis sea squirt. Photo credit: Esko Kemppainen.

Ciona intestinalis sea squirt. Photo credit: Esko Kemppainen.

Throughout the history of life on Earth, there have always been locations where some species thrive while others fail. And, as climate change and environmental degradation continue apace, some habitats are becoming more inhospitable as toxins such as hydrogen sulphide begin accumulating in zones when oxygen becomes scarce. The situation has rarely been so rosy for invasive creatures, such as the sea squirt Ciona intestinalis. But what might be giving these intruders the upper hand? Katharina Bremer from Tampere University, Finland, explains that the sea squirt is equipped with a specially adapted enzyme – known as ‘alternative oxidase’ – which allows them to produce the energy they require to survive in the presence of toxic hydrogen sulphide gas that smells of rotten eggs. The close relative of this critical enzyme – used by most creatures – is fatally poisoned by hydrogen sulphide. Could alternative oxidase prove to be the essential key that allows sea squirt invaders to colonise hydrogen sulphide hotspots where others perish?

However, for pioneering sea squirts to gain a new foothold, they must send in a first wave of pilgrim settlers, and, in the case of the sea squirt, that means newly fertilized eggs and larvae. So, Bremer, Paul Debes (Hólar University, Iceland), Hitoyoshi Yasuo (Laboratoire de Biologie du Développement de Villefranche-sur-mer, France) and Howard Jacobs (Tampere University) needed to find out how toxic the dissolved gas is for freshly fertilized sea squirt eggs. After immersing the embryos in water ranging from 0 to 50 μmol l−1 hydrogen sulphide, which can prove fatal for fish, the team found that ∼80% of the youngsters successfully developed into larvae after 18.5 h in fresh seawater, with their survival rate falling to ∼60% when bathed in 10 μmol l−1 hydrogen sulphide. But then the youngsters began to struggle, with only ∼25% developing into larvae in 20 μmol l−1 hydrogen sulphide and none succeeding at 50 μmol l−1. Even with their specially adapted hydrogen-sulphide-proof alternative oxidise, the youngsters were only able to survive reasonably well in hydrogen sulphide concentrations of up to 15 μmol l−1.

But then the team needed to check whether the embryos’ alternative oxidase provided them with their resistance to the toxin, or whether they were resorting to another strategy to survive the poison. The scientists injected the embryos with a molecule that prevented the youngsters from producing alternative oxidase, forcing the developing embryos to depend on other forms of the enzyme for survival in 15 μmol l−1 hydrogen sulphide. Without the key enzyme, their survival plummeted by 81% to 12%. However, when the team provided the impaired youngsters with an additional source of alternative oxidase, their survival rate returned to 27%; the same as normal sea squirt embryos in fresh seawater.

In addition, the team measured whether the embryos were activating the gene responsible for the essential enzyme – and therefore likely to be producing the alternative oxidase – while developing in 15 μmol l−1 hydrogen sulphide, and it was clear that they had increased expression of the gene to combat the toxin. Their alternative oxidase source of energy is the key to the youngsters’ survival when their water reeks of rotten eggs. Bremer also warns that natural increases in hydrogen sulphide in waters across the globe that are becoming deoxygenated could provide other species that possess a hydrogen-sulphide-proof alternative oxidase with the advantage they require to go forth and colonise locations where other species can barely hang on.

Bremer
,
K.
,
Yasuo
,
H.
,
Debes
,
P. V.
and
Jacobs
,
H. T.
(
2021
).
The alternative oxidase (AOX) increases sulphide tolerance in the highly invasive marine invertebrate Ciona intestinalis
.
J. Exp. Biol.
224
,
jeb242985
.