Paper: Schlegel, P., Binet, M. T., Havenhand, J. N., Doyle, C. J., & Williamson, J. E. (2015). Ocean acidification impacts on sperm mitochondrial membrane potential bring sperm swimming behaviour near its tipping point. Journal of Experimental Biology, 218(7), 1084–1090. doi:10.1242/jeb.114900
Organisms that can’t move or don’t move much, like corals or sea urchins, can’t “get a room” and mate like many of the animals we’re familiar with. Instead, they use a tactic called broadcast spawning, which is exactly what it sounds like – female animals release eggs into the water column, male animals release sperm, and they rely on water mixing to do the rest of the work for them. This strategy has been successful for marine animals because they all release such large quantities of gametes: some of them are bound to collide. What hasn’t been accounted for, though, is how the changing climate might lead to changes in this time-tested system of reproduction. To close that gap in our knowledge, this study looked at how sea urchin sperm swimming might be affected by future ocean acidification conditions.
Some work has already been done on sperm in broadcast spawners under acidified conditions, but the results so far have been inconclusive – some studies have reported slower swimming, other studies have reported no effect or even an enhanced effect on swimming. Scientists have hypothesized that the low pH of the surrounding water is slowing the sperm’s metabolism, and that stops it from swimming as fast or as long as it normally would. The energy that a sperm needs to swim around to find an egg comes from adenine triphosphate (ATP), which is made in a specific structure in the cell, the mitochondria. The mitochondria have to be at an optimal pH to make ATP, so if the pH is different than what it usually is, there won’t be as much energy being made for cell functions. Actively swimming sperm need a lot of energy, so the mitochondria need to be working at their optimal pH levels to make fertilization happen.
To investigate how effective sperm are under ocean acidification conditions, these researchers induced spawning events in Centrostephanus rodgersii, a sea urchin found in Australia that’s similar to the California sea urchin (Figure 1).
Those spawning events were done in three treatments – control pH (conditions now), medium pH (conditions by 2100), and low pH (conditions by 2300). pH was changed by bubbling the water with CO2 gas. Once the sea urchins spawned in those three conditions, the researchers took some of the sperm and stained it so that they could see how much ATP the sperm was making. The staining changes the color of the sperm to correspond to how much energy the sperm can make, measured as the mitochondrial membrane potential (MMP) – a highly mobile sperm will glow brightly in green, and a lethargic sperm will glow a faint orange color. The rest of the sperm was put under a microscope to see how well it swam around under those three different conditions. Under the microscope, they measured the fraction of sperm that were motile (able to move spontaneously and actively – a process that consumes energy) and swimming speed.
In all lowered pH conditions, the sperm struggled – the percent of sperm that were active, average swimming speed, and energy all dropped in the lowest pH treatment. Interestingly, the percent motility and the average sperm swimming speed increased in the medium pH treatment (the two positive points in figure 2), which the researchers attribute to the sperm source – some males had sperm that were better in acidified conditions.
The researchers also found a positive correlation between sperm motility and swimming speed with MMP, shown in figure 3. This pair of graphs suggests strongly that the reason that less sperm swam at slower speeds because of the reduced MMP – when the sperm don’t produce enough ATP in the mitochondria, they’re unable to move as fast.
Animals can usually tolerate a stressor like low pH up to a certain tipping point – at that point, whatever stressor it is will completely overwhelm the animal and it will be unable to perform it’s normal function. The researchers think that the pH levels expected by 2300 (the high treatment) is beyond the tipping point for these sea urchin sperm.
The reason the researchers saw the decrease in sperm swimming speed, motility, and MMP is because the delicate environmental balance inside the animal was disrupted. All animals, whether an amoeba or a sea urchin or a human, have to maintain an optimal cell environment for biochemical pathways such as ATP production to work. If something changes in the cell environment, such as a lower pH or a decreased temperature or increased salts, the cell machinery won’t function like it should – kind of like your phone not working when it gets too hot. Since these sea urchin sperm are exposed to an acidic environment, their cell machinery and pathways (the MMP) are not working the way they should and the sperm isn’t able to swim as well to find the egg it needs.
Interestingly, the researchers remind us that a decreased energy metabolism (represented by the MMP) means that the sperm can live longer. When the sperm isn’t using or producing as much energy, it saves its energy resources and can survive longer in the water column. That means that there could be an increase in sperm-egg fertilization – kind of like the tortoise and the hare, the slower swimmers might find an egg because they’ve got a longer time to do so. That could be good news for this species under ocean acidification, but there needs to be more work done with fertilization experiments to see if that really is the case.
What do you think? Will the slower, longer lived sperm help the species in the long run, or will the species suffer because their sperm aren’t fast swimmers?