Inhibition of denitrification by ultraviolet radiation

Area of Science:

• Environmental microbiology
• Marine microbial ecology
• Biogeochemical cycles

Background:

• Ultraviolet-A (UV-A) and UV-B radiation are known to inhibit key microbial processes including photosynthesis, nitrogen fixation, and nitrification.
• The impact of UV radiation on denitrification, a crucial process in marine nitrogen cycling, remains less understood, particularly in intertidal microbial communities.

Purpose of the Study:

• To investigate the direct effects of UV radiation on denitrification rates within a microbial mat community from the Pacific marine intertidal zone.
• To determine if UV inhibition of denitrification is a direct effect or mediated by UV-induced inhibition of photosynthesis or nitrification.

Main Methods:

• Denitrification rates were quantified using the acetylene blockage technique in microbial mat samples dominated by Lyngbya.
• Samples were exposed to different UV conditions: UV opaque (OP3), UV-B blocking (mylar), and UV transparent (UVT) screens.
• Control experiments included Pseudomonas fluorescens cultures, killed controls, and samples treated with DCMU (photosynthesis inhibitor) or nitrapyrin (nitrification inhibitor).

Main Results:

• Denitrification rates were significantly higher in samples protected from UV radiation compared to those exposed to UV.
• This protective effect was observed in both the natural microbial mat and the Pseudomonas fluorescens cultures.
• Inhibition of photosynthesis or nitrification did not affect denitrification rates, indicating a direct impact of UV radiation.

Conclusions:

• UV radiation directly inhibits denitrification in the intertidal microbial mat community.
• UV protection is essential for maintaining denitrification activity in these environments.
• The findings highlight the sensitivity of microbial nitrogen cycling to solar UV radiation.