Diurnal responses of photosynthesis and fluorescence in Ulva rotundata acclimated to sun and shade in outdoor culture

Abstract

Thalli of the chlorophyte macroalga Ulva rotundata were acclimated to sun (100 % I,) and shade (9% I,) in outdoor tumble tanks under nutrient-sufficient conditions. Diurnal patterns were determined for area1 photosynthetic O2 exchange capacity (P,,) and quantum yleld (@), respiration (Rd), and photosystem (PS) I1 chlorophyll fluorescence using 2 independent techniques. Net P, was maximal at midday in sun-grown thalli and at midmorning in shade-grown thalli exposed to 100 "L I,. A slight afternoon decline in net P, in shade-grown thalli exposed to 100 % I, could be explained entirely by a concurrent rise in Rd. Sun thalli exhibited a larger diurnal fluctuation of net P, which was inconsistent with photoinhibition or end product inhibition by accumulated photosynthate. This leaves endogenous control as the most likely cause, with slight modification by Rd Fluorescence data were consistent with simultaneous damage to PSI1 and photoprotection by thermal deexcitation, with sun thalli showing greater capacity for protection. Both of these processes reduced @ but apparently did not affect P,,. In that U. rotundata thalli were light-saturated in our tumble tanks for most of the day, we conclude that photoinhibition (E damage + protection) had little effect on ~ntegrated diurnal production under these conditions. Our results demonstrate the utility of fluorescence analysis and the importance of precise determination of P,, @ and Rd in ascribing mechanisms to diurnal fluctuations in photosynthetic performance. The validity of using photosynthetic light-response (P-I) curves determined at one time of day to predict integrated diurnal production will depend on the extent to which P,, @ and Rd vary. This in turn depends on species, irradiance history, and possibly other factors such as temperature and nutrition.

Daily course of photosynthesis and photoinhibition in marine macroalgae investigated in the laboratory and field

“…This is consistent with the finding that the subtidal P. nereocystis is faster and more strongly photoinhibited than P. perforata growing in the eulittoral (Herbert & Waaland 1988). Henley et al (1991) even observed different responses of individuals of the same species, Ulva rotundata, if they were adapted to different Light conditions. Sunadapted thalli exhibited a greater capability for protection than shade-adapted ones.…”

“…This is surprising, because a fast recovery during the day should be a prerequisite for a light-adaptation mechanism protecting the algae in their natural environment. However, in fleld experiments Huppertz et al (1990), Henley et al (1991) and showed with brown algae and with the green alga Ulva rotundata that photoinhibition occurred at excessive fluence rates and recovery of photosynthesis commenced during the afternoon. Full recovery was mostly reached in the evening.…”

Laminaria saccharina photosynthesis measured in situ: photoinhibition and xanthophyll cycle during a tidal cycle

“…The diurnal course of the effective quantum yield (Φ PSII ) was directly correlated to underwater irradiance. The rapid recovery of the quantum yield suggested that the responses of Laminaria saccharina to high light intensity are linked to a down-regulation of PSII (now usually termed 'dynamic photoinhibition') rather than to the onset of photosynthetic damage ('chronic photoinhibition'), which reduces photosynthesis for longer periods (Henley et al 1991, Osmond & Grace 1995, Jones & Hoegh-Guldberg 2001, Werner et al 2001. Photoprotection after exposure to solar radiation has been observed in other macroalgae (Hanelt et al 1997, Ensminger et al 2000, Rodrigues et al 2000.…”

Photoinhibition in common atlantic macroalgae measured on site in Gran Canaria

“…Macroalgae growing at greater depth have been found to be more sensitive to photoinhibition than surface algae (Henley et al 1991;Häder and Figueroa 1997;Hanelt 1998), just like shade-adapted higher plants. This was attributed to better photoprotection in surface algae which seems to be genetically fixed (Hanelt 1998).…”