GROWTH OF ANTARCTIC CYANOBACTERIA UNDER ULTRAVIOLET-RADIATION - UVA COUNTERACTS UVB INHIBITION

A mat-forming cyanobacterium (Phormidium murrayi West and West) isolat ed from an ice-shelf pond in Antarctica was grown under white light co mbined with a range of WA and WE irradiances. The 4-day growth rate de creased under increasing ultraviolet (UV) radiation, with a ninefold g reater response to WE relative to UVA. In vivo absorbance spectra show ed that UVA and to a greater extent UVB caused a decrease in phycocyan in/chlorophyll a and an increase in carotenoids/chlorophyll a. The phy cocyanin/chlorophyll a ratio was closely and positively correlated to the UVB-inhibited growth rate. Under fixed spectral gradients of UV ra diation, the growth inhibition effect was dominated by UVB. However, a t specific UVB irradiances the inhibition of growth depended on the ra tio of UVB to UVA, and growth rates increased linearly with increasing WA. These results are consistent with the view that WE inhibition rep resents the balance between damage and repair processes that are each controlled by separate wavebands. They also underscore the need to con sider UV spectral balance in laboratory and field assays of UVB toxici ty.