Influences of light and UV-B on growth and sporulation of the green alga Ulva pertusa Kjellman

The cosmopolitan presence of Ulva spp. is partly due to its great reproductive ability, but relatively little information is available for the radiation conditions triggering reproduction. In the present study, we investigated the effect of photon irradiance, photoperiod, and spectral qualities of light on growth and reproduction of Ulva pertusa. During 8-day culture of discs cut from marginal parts of the thallus of U. pertusa, the size of the thallus discs was greatest at 10 mumol m(-2) s(-1), while saturation of reproduction occurred at 30 mumol m(-2) s(-1). The minimum photon irradiance allowing growth and reproduction was 5 and 10 mumol m(-2) s(-1), respectively. Rapid increases in the size and subsequent initiation of sporulation were observed in samples transferred to saturating irradiance from 5 mumol m(-2) s(-1) or darkness for 9 days. When exposed to different photoperiods (8:16-, 12:12-, 16:8-h LD and continuous light regimes) combined with different photon irradiances (10 and 100 mumol m(-2) s(-1)), U. pertusa thallus showed that the thallus size attained at the low irradiance was similar in daylengths longer than 12 h per day, while the surface area increased in parallel with increasing light duration at the high irradiance. The degree of sporulation at 10 mumol m(-2) s(-1) varied, ranging from no sporulation in 8:16-h LD to 80% in 16:8-h LD and continuous light. On the other hand, there was no remarkable difference in the degree of sporulation between the photoperiods except for slightly smaller percentage sporulation in 8:16-h LD at 100 mumol m(-2) s(-1). At 5 mumol m(-2) s(-1), the growth of U. pertusa was markedly lower in green than in blue or red light, but there was no sporulation in any spectral quality. The degree of sporulation at 20 mumol m(-2) s(-1) was almost twice as much in blue or red as in green light. The size of plants irradiated with 1.0 W m(-2) of UV-B for 20-40 min increased by 18-21% relative to control, whereas higher UV irradiance caused inhibition of growth. There was a significantly lower incidence of sporulation in UV-B-irradiated plants with the degree of reduction being greater in those exposed to higher UV doses. The total biologically effective UV-B dose for 50% inhibition of sporulation was 0.085 Dos(eff) kJ m(-2). The time required to achieve 50% inhibition would be longer than 13 h at depths below 1 m in Ahnin coastal waters. The vertical attenuation coefficient of PAR ( lambda = 400-700 nm) and UV-B (lambda = 300-320 nm) in April 1998 at Ahnin on the eastern coast of Korea was 0.21 m(-1) for K-PAR and 0.54 m(-1) for KUV-B. A large reduction of light quantity and rapid disappearance of blue wavelength occurred by shading from overlying thalli. Percentage inhibition of sporulation was only 14-18% at 150-200 mumol m(-2) s(-1) compared with 70% at 10 mumol m(-2) m(-1), when plants were incubated under different irradiances of PAR immediately after UV-B exposures. These different photoadaptive strategies for sporulation may in part account for the great ecological success of U. pertusa. (C) 2003 Elsevier Science B.V All rights reserved.