Rising nutrient-pulse frequency and high UVR strengthen microbial interactions

Area of Science:

• Aquatic Ecology
• Environmental Science
• Microbial Ecology

Background:

• Solar radiation and nutrient availability are key drivers of ecosystem function.
• Future climate scenarios predict increased ultraviolet radiation (UVR) and altered nutrient pulse frequencies.
• The combined effects of high UVR and frequent nutrient pulses on aquatic producer-decomposer interactions remain poorly understood.

Purpose of the Study:

• To investigate how repeated nutrient pulses under high UVR affect primary production (PP) and bacterial production (BP).
• To elucidate the mechanisms driving changes in phytoplankton communities, specifically the role of mixotrophic nanoflagellates (MNFs).
• To assess the long-term relationship between nutrient pulse frequency and shifts in aquatic microbial communities.

Main Methods:

• Mesocosm experiments simulating pulsed and repeated nutrient additions under controlled UVR levels.
• Analysis of primary and bacterial production rates within the mesocosms.
• A 6-year whole-lake study monitoring microbial community composition and nutrient dynamics.

Main Results:

• Repeated nutrient pulses under UVR led to greater increases in primary production than bacterial production.
• Photoautotrophs were replaced by mixotrophic nanoflagellates (MNFs), which exerted top-down control on bacteria.
• The frequency of nutrient pulses, such as desert dust, was correlated with shifts from photoautotrophs to MNFs in the whole-lake study.

Conclusions:

• Frequent nutrient pulses under high UVR promote primary producers over bacteria, altering aquatic food web dynamics.
• Mixotrophic nanoflagellates play a crucial role in mediating these responses through grazing and carbon excretion.
• Understanding the interplay between chronic and stochastic environmental factors is vital for predicting ecosystem responses to climate change.