The effect of a short realistic heat wave on algal-bacterial interaction

Background/Question/Methods

Climate change research has made substantial progress in analysing effects of gradual increases in average temperatures on ecological communities. However, scenarios of climate change also predict more frequently occurring extreme temperatures. Such short-term temperature disturbances – heat waves - can cause non-linear changes in the community structure by altering interactions between species. We hypothesize that an increasing frequency of heat waves may change the function of intertidal areas, by rearranging the structure of diatom-dominated periphyton communities. Specifically, we expect that differences in the physiological responses among autotrophs and heterotrophs may lead to high bacterial abundances which in the long run ensure competitive advantage for nutrient uptake. Thus, a regional temperature disturbance may induce a community state where bacteria dominate over diatoms also after the initial temperature conditions are restored, leading to a long term decrease in the carrying capacity of producer biomass. To test this we studied effects of short term heat stress on model communities of diatoms and their associated bacteria. We performed the experiments in the laboratory using climate chambers to mimic a realistic summer heat-wave under continuous nutrient additions.

Results/Conclusions

We found that a relatively short heat stress event sufficed to shift the dynamics between algae and bacteria, and that this shift did not reverse back to control conditions even after a prolonged time of recovery. Directly after the heat-wave peaking at 28 °C, total diatom biomass decreased by 42%, while bacterial abundances increased with 167%, compared to communities exposed to a control temperature of 16 °C. After recovery periods of 10 to 29 days at 16 °C, the total biomass of the diatoms slowly increased but the carrying capacity never reached the same levels as the control cultures. Bacterial abundances were still significantly elevated in the heated communities throughout the prolonged recovery periods. However, we found no evidence of changed nutrient conditions that could explain the shift in dynamics between algae and bacteria. Instead, the heat stress induced a dominance shift within the bacterial composition that was irreversible over time. This indicates that temperature stress may induce a shift in bacterial composition that has a prolonged negative effect on periphyton primary production.