The red macroalga Delesseria sanguinea as a UVB-sensitive model organism: selective growth reduction by UVB in outdoor experiments and rapid recording of growth rate during and after UV pulses

The sublittoral. red alga Delesseria sanguinea was pre-cultivated in the la boratory in a predominantly green fight field at 10 mu mol m(-2) s(-1) (10 degreesC), with an approximately 0.5% daily increase in blade area. Such pr e-cultivated blades were exposed horizontally in flow-through chambers for 5-6 days in April and May to solar radiation reduced to 19% and 11%, respec tively, by neutral density screens. In all three experiments, the full sola r spectrum (UVB + UVA +PAR) reduced growth rate significantly to 0-50% of v alues obtained in PAR alone, while this occurred with UVA +PAR in only one experiment. The growth rate of a Delesseria blade may thus be used as a sen sitive and reliable biological indicator of UVB in natural solar radiation at a pre-chosen, neutral reduction level. Rapid measurements of growth rate of apical parts of Delesseria blades during and after UVB + UVA pulses of 2, 3 or 6 h duration were performed in the laboratory by measuring thallus area every 2 min by means of a CCD camera coupled to an on-line, computer-a ided image analysis system. A single pulse of 2 or 3 h duration administere d during the light phase caused a temporary drop in growth rate during and after the pulse, with recovery starting 2.5 h after the end of the pulse an d completed by the end of the light phase. A single 6 h pulse at a biologic ally effective UVB dose (BEDDNA300nm) of only 0.5 kJ m(-2) reduced growth r ate by 55% if administered around noon, but halted growth almost completely if supplied at night, when no photoreactivation was possible. The UVB-sens itive behaviour of Delesseria compares well with the highly sensitive phyto plankton alga Emiliania huxleyi whose growth was reported to be halted at a daily, weighted BEDDNA300nm of 0.4 kJ m(-2) administered during 4 days for 3 h in the middle of the light period.