In the cold mountain streams of China, the lenok (Brachymystax tsinlingensis) is nearing extinction. This fascinating cold water trout faces a growing threat as rising temperatures from climate change jeopardise its health and survival. Increased temperatures are particularly challenging because fish cannot regulate their body temperature. Hence, their body temperature closely follows the temperature of the environment. That, in turn, affects several tissues and other processes in the body, including the intestine, which plays a crucial role in digestion, absorption of nutrients and immunity. Zhenlu Wang and colleagues at Guizhou University, China, investigated how lenoks handle the challenges posed by high temperatures, focusing on the changes in their intestines that enable these fish to remain healthy.
To understand how the lenok copes with rising temperatures, the team simulated the temperature change that the fish experience in the streams during summer by gradually raising the water temperature from 17°C to 21°C (moderate heat) and then to 23°C (extreme heat). Then, they measured how expression of specific genes that are involved in energy production, cell repair and immunity in the intestine changed when the fish experienced high temperatures, to determine whether these genes were switched on or off, or if their expression levels changed in response to the heat. To understand the fish's ability to recover after a heatwave, the researchers also returned the trout to cooler water (17°C) and tracked their recovery by examining those key genes and whether their levels returned to normal.
The team discovered that as the water temperature rose to 21°C, the trout increased expression of genes involved in energy production in their intestines. This boost likely aided the fish in meeting their increasing energy demands as their metabolism speeded up at higher temperatures and the increased energy demand associated with the protective changes. For example, the fish increased the expression of genes encoding heat shock proteins, which protect tissues from heat damage. The fish also activated key genes that are involved in preventing the intestine from becoming leaky and increase mucus secretion, which is vital to protect fish against viruses, bacteria and toxins.
Interestingly, these protection mechanisms were activated when the temperature was high and remained active for hours afterwards. Thus, the intestines continued to function as the trout's frontline defence by ramping up energy production, to support the immune system fighting infection and repair damage to the intestine caused by the heat to ensure that fish were not vulnerable to infections. This suggests that the lenok can tolerate temperatures of 21°C for a certain period. However, the fish could not maintain their resilience when temperatures hit 23°C. In fact, at these extreme temperatures, Wang and colleagues found evidence of cell death in the fish intestines and the ability to maintain internal resilience began to falter, marking 23°C as the highest temperature the lenok could potentially survive, but likely only for a brief time.
Wang and colleagues have revealed how the lenok intestine works tirelessly to protect the fish during a heat wave. They demonstrated how the trout can handle moderate warming but struggle when the heat is really on. These findings reveal the gut's resilience to warmer temperatures, but only up to a point, which highlights a sobering reality: as temperatures continue to rise, even the most resilient have their breaking point. How much longer can cold water species keep up with the heat?
