Living beside a tranquil lake may be idyllic, but living under its waters is anything but calm. Freshwater animals are constantly battling their environment, trying to get enough oxygen to survive. At the same time, these aquatic animals need to stop the salts inside them from leeching into the dilute water as their bodies need them to keep functioning; and doing so requires lots of energy. This really becomes a problem if the oxygen levels in the water are low because the animals can't make enough energy to keep pumping the salts into their bodies. However, very little is known about how oxygen levels interact with the salt balance of freshwater inhabitants. To find out more about how aquatic organisms keep themselves saltier than the water around them when the oxygen levels drop, Jamie Cochran and David Buchwalter of North Carolina State University, USA, turned to the mayfly (Neocloeon triangulifer), whose young – called nymphs – inhabit the shallow edges of streams.
First, the scientists wanted to see whether a drop in the oxygen levels changed the amount of salt that the insects could absorb from the surrounding water. After placing the young mayflies into small vials filled with water containing salts with radiolabelled sodium and sulfate, the researchers started dropping the oxygen levels in the water from normal (21%) to 11.7%. This amount of oxygen shouldn't cause any problems for the mayflies, but to the team's surprise, after spending 3 h at 11.7% oxygen, the insects took up 19% less sodium and 51% less sulfate. Although the insects should have been able to make enough energy to power their salt absorption, they started using energy for other activities even when the oxygen levels were still rather high. Cochran and Buchwalter continued to drop the oxygen levels every 3 h until it reached 1.6%. At that level, the mayflies could only absorb 25% of the sodium and 7% of the sulfate that they could when the oxygen levels were normal.
If the mayflies aren't absorbing as much salt from the water around them, are they still losing the salt that they already have? To answer this, the duo measured the amount of salt in the bodies of the nymphs. The researchers found that when the oxygen levels in the water get very low (5.4% and lower) then insects have at least 25% less salt in them than they would when the oxygen levels are normal. This can cause all sorts of problems for the mayflies, even death. Now Cochran and Buchwalter knew that the lower oxygen was lowering the mayfly's ability to keep in salt, but they wondered whether there might be changes in any of the genes controlling their salt uptake.
The team focused their attention on a few genes that code for proteins which transport salts into the insect's body. Interestingly, the expression of these genes increased until the oxygen levels were 8.5% or 5.4% before drastically decreasing when the levels got even lower. This means that the low oxygen levels in the water could also be changing how these genes are expressed. The researchers also found that expression of the gene atypical guanylate cyclase was higher at 8.5% and 5.4% oxygen. They believe that this could be acting as an oxygen sensor, alerting the nymphs that the oxygen levels are getting low. Whether these changes happen when the animals face longer time periods at low oxygen remains to be seen, but it seems that life in freshwater becomes even harder when keeping yourself salty gets tougher as the oxygen levels drop.
