Importance of ultraviolet radiation in photoinhibition of microalgal growth1

Algae vary greatly in their long termability to adapt to levels of natural solar ultraviolet radiation encountered near the surface of the sea. Photosynthetically active radiation (where PAR is defined as quantum flux between 400 and 700 nm) produced little or no growth inhibition even at full surface intensity. In some species the capacity to tolerate intense PAR did not carry into the range of UV‐A (315–400 nm) or of UV‐B radiation (280–315 nm) or both. Photoinhibition became increasingly severe above 20% of surface intensity among the species sensitive to UV radiation. PAR flux in excess of that required for maximum growth was not inhibitory, but rather the higher levels of associated UV radiation reduced growth rate. These results support a previous suggestion that other molecules besides chlorophyll sensitize or shield algae in the UV portion of the solar spectrum.