Ten years ago, Takema Fukatsu, PhD, prime senior researcher and leader, National Institute of Advanced Industrial Science and Technology (AIST) in Japan, was invited to Kyoto University as a symposium speaker of a meeting organized by Kenji Fujisaki, PhD, a researcher in the University's Graduate School of Agriculture, who had been working on the effects of global warming and other environmental fluctuations on insects for decades. Professor Fujisaki's group invented and was operating a special incubator for simulating global warming conditions.
At the meeting, his postdocs and students presented data showing that using the incubator, particularly in the hot summer season, southern green stinkbugs (Nezara viridula) become yellowish in color and die. "They described the phenomena simply as "high-temperature injury", but I was quite confident that the phenotypic syndrome must be due to high-temperature-induced symbiont degeneration," said Dr. Fukatsu, who is also professor at the University of Tokyo and University of Tsukuba, in Japan. In other stinkbugs and similar insects, he repeatedly had seen similar phenotypes upon heat-induced symbiosis collapse.
Dr. Fukatsu approached Dr. Fujisaki with this possibility and a joint research project was born. In the past several years, the collaborators have compiled a large amount of high quality data studying the southern green stinkbug under simulated warming conditions, constant temperature conditions, and symbiont-suppressed conditions without heat stress. Many animals and other organisms harbor microorganisms inside their body, and some insects, such as the southern green stinkbug, depend on specific gut bacterium for normal growth and survival.
The collaborative work is described in a recent study published online in the journal mBio, a journal of the American Society for Microbiology. Experiments showed that when heat-susceptible bacteria living symbiotically in the guts of insects are exposed to increased temperatures, both the bacteria and the insect are negatively impacted. The research illustrates another way global warming will wreak havoc on life as we know it, setting off deleterious chain reactions among organisms living in symbiosis.
Specifically, the investigators demonstrated that when the southern green stinkbug was reared inside an incubator, in which temperature was controlled at 2.5 °C higher than outside, there was a significant reduction in the symbiont bacteria that sparked severe fitness defects in the insect (ie. retarded growth, reduced size). At higher temperatures, no nymphs were able to reach the adult stage. Global warming is predicted to raise the average temperature by 2.5 °C by 2100.
"Considering that many microbial symbionts are vulnerable to high temperature stress, such symbionts can be the Achilles’ heel of symbiont-dependent organisms in this warming world," said Dr. Fukatsu. He said that people should keep in mind that global warming might impact diverse organisms, not necessarily directly, but possibly indirectly via affecting unseen microbial associates. "Coral breaching, in which symbiotic photosynthetic algae of corals are killed by high temperature and coral reefs are severely damaged, is regarded as a serious environmental issue, but plausibly, similar phenomena may be ubiquitously found in the ecosystem," said Dr. Fukatsu.
Many of the experiments were conducted by Yoshitomo Kikuchi, PhD, at AIST, an expert in stinkbug-bacteria gut symbiotic associations, and Ms. Akiko Tada, a graduate student in Fujisaki's laboratory.