Abstract
Hydrogen peroxide is widely recognized as the most stable of the reactive oxygen species (ROS) produced by abiotic, biotic and sediments in natural waters. By mediating redox transformations, hydrogen peroxide may directly or indirectly affect aquatic ecosystems. It might be particularly important in the biology and ecology of harmful freshwater cyanobacteria blooms which are increasingly occurring in nutrient-rich freshwater bodies. Environmental interactions between cyanobacteria and natural hydrogen peroxide could play a large role in cyanobacterial blooms, but little is known about these interactions. To better understand the interactions of hydrogen peroxide and cyanobacteria, we determined the hydrogen peroxide level in the presence and absence of cyanobacterial blooms. Also, we aimed to understand the ecological significance of hydrogen peroxide production by cyanobacteria in the waters of southwest Florida, a subtropical zone which experiences strong sunlight and heavy precipitations in the rainy season. In this study, hydrogen peroxide concentrations were determined using a fast response amperometric hydrogen peroxide microsensor in 24 freshwater bodies. Our results indicated that rainwater contains higher hydrogen peroxide levels (0-92.9 µM) than freshwater bodies under cyanobacterial bloom conditions (0-5.3 µM). To determine the potential biodegradation of hydrogen peroxide during sample transportation in the dark (opaque containers), water samples were passed through 0.2 µm filters immediately after sampling and compared with unfiltered water samples in the laboratory. We found the filtered water samples retained higher concentrations of hydrogen peroxide than non-filtered samples; and we determined the mean biodegradation rate of hydrogen peroxide was 44.7 ±10.6 nmol/h in unfiltered water. We also determined the baseline hydrogen peroxide concentrations and microscale depth profiles (10-60 mm) of hydrogen peroxide using a recently developed hydrogen peroxide microsensor. The hydrogen peroxide concentrations were higher at M. aeruginosa bloom sites compared to control sites and higher in locations exposed to sunlight than in shaded locations. In addition, micro-profiles showed extremely high hydrogen peroxide concentrations (3.3 to 20.9 µM) in the uppermost layer of the lake water in cyanobacterial blooms. All together, we found hydrogen peroxide hotspots in fresh waterbodies in a subtropical region that were generated by both abiotic and biotic factors. In addition, we found extremely high hydrogen peroxide concentrations in cyanobacterial bloom sites compared to reported values from other freshwater systems which could have a large effect on the aquatic microbial community.
