Impact of natural ultraviolet radiation on rates of photosynthesis and on specific marine phytoplankton species

Abstract

Radiation in the UV-A region (320-400 nm) was responsible for >50% of the total inhibition due to UVR, with <50% due to UV-B (280-320 nm). Wavelengths <305 nm, which is the spectral region most enhanced under conditions of low ozone concentrations in the atmosphere, accounted for only 15-20% of the total inhibition due to UV-B radiation. Under high-light conditions on sunny days, photosynthetic rates were increased 200-300% by screening off all UVR <378 nm. When the average UVR (295-385 nm) during the incubation period was below a threshold value of 5-10 W m-2, there was no significant depression of photosynthetic rates. UVR is more inhibitory to microplankton than to nanoplankton, and induced the formation of resting spores in diatom species of the genus Chaetoceros. Phytoplankton from tropical waters showed marked resistance to UVR compared to Antarctic phytoplankton. Under the highest light conditions an increase in photosynthetic rates of 10-20% was recorded in some experiments by screening off all UVR <378 nm, but when data from all experiments with samples from surface waters in the tropics were analyzed, there was no significant difference between samples with or without natural solar UVR. Phytoplankton samples from below the pycnocline in tropical waters were very sensitive to solar radiation, suggesting that the resistance shown by phytoplankton from surface waters reflects a photoadaptive process. -from Authors