Time course of physiological response of Ulva rotundata to growth irradiance transitions

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Abstract
A vegetative clone of the macroalga Ulva rotundata was subjected to changes in irradiance (9, 29 and 100% Io) in an outdoor continuous flow system under nutrient-sufficient conditions. Growth rate, chlorophyll and starch content, respiration and photosynthesis were determined for 5 d following changes in irradiance. Starch reserves were highly dynamic, covering the complete range of observed values (about 0.3 to 3.5 mg g-1 dry weight) within 1 d following 9 to 100% or 100 to 9% Io transfers. Respiration decreased to the new level within 1 d following downward irradiance transfers (DT), but accelerated over several days after upward irradiance transfers (UT). Photosynthetic capacity (Pm) exhibited little change for 5d after DT, but increased steadily over the same period following UT. Chlorophyll changes were more gradual, requiring 4 to 5 d for complete recovery following the 100 to 9% and 9 to 100% Io transfers. Specific growth rate (.mu.) expressed on fresh (FW) and dry weight (DW) bases gradually increased over several days following UT, in agreement with Pm data. Downward irradiance transfer resulted in a 1d uncoupling of .mu.FW and .mu.DW; .mu.FW remained at a high level (perhaps cells division was fueled by mobilized starch reserves) while .mu.DW dropped off sharply, presumably due to low photosynthesis at the reduced irradiance. Results indicated that U. rotundata makes efficient use of variable irradiance by positioning at an intermediate state of acclimation due to appropriate rates of physiological response.