Meandering through southern Spain, the Rio Tinto's blood red colour warns that this river is not a pleasant home; it's extremely acidic and brimming with heavy metals. Yet single-celled Chlamydomonas algae thrive in this toxic brew, leading Mark Messerli and his colleagues at the Marine Biological Laboratory in Woods Hole to wonder how these microalgae prosper in acid (p. 2569).
Messerli assumed that the acid-loving algal cells are just like other organisms' cells, on the inside at least. To make sure that their internal biochemistry works as it does in other organisms, the algae must maintain a relatively neutral pH in their cells. But this is not a trivial matter if you live in an acidic river teeming with protons that are constantly storming your cell membrane.
Messerli and his Woods Hole colleagues decided to see if the microalgae maintain a neutral pH inside their cells. When they loaded a fluorescent pH-sensitive dye into the algal cells and monitored the cells' fluorescent intensity, they found that the pH inside the algal cells is indeed close to neutral. And when they moved the algal cells to different pH levels, their internal pH didn't change; the cells clearly control their neutral internal pH. `So there is a huge proton gradient between the neutral algal cell and the acidic river,' Messerli says.
To find out how the microalgae deal with the onslaught of protons from the Rio Tinto, the team measured the microalgae's transmembrane electric potential, which is the electrical difference between the inside of the cell and the river. When they impaled the cells with an intracellular electrode,they were astonished to find that the membrane potential was practically zero. Nearly all plant and animal cells maintain a negative membrane potential,which drives positive ions into the cell. Maintaining zero membrane potential is a smart move in an acidic river, because this eliminates the electrical gradient that would otherwise drive protons from the river into the cell. `But a membrane potential of zero usually means the cell is dead!' Messerli says. So were they really recording inside living algal cells? To prove that they were inside the cells, the team probed the ion channels in the algal cell membranes using a voltage clamp. Sure enough, the channels still worked while the algae were impaled, so they were recording inside the cells. And they knew that the algal cells were alive because `their flagella were still wriggling after the cells were impaled,' Messerli says.
But do the microalgae actively maintain their neutral internal pH? If they do, the team expected the cells to consume more ATP at acidic than neutral pH. To test this, they monitored cellular ATP levels in microalgae kept at pH 2 and 7 using firefly luciferin/luciferase, which emits light when ATP is present. They found that the algae burn 7% more ATP at pH 2 than the same cells burn at pH 7. In other words, living in an acidic river is energetically costly.
So why do the microalgae flourish in the Rio Tinto? Messerli suspects that the answer lies in the fact that no multicellular predators can survive the river's acidity. If you're a microalgal cell, it seems that pumping out protons is a small price to pay to ensure that you remain unmolested.