Phenotypic plasticity in the red alga Porphyra abbottae: environmental factors influencing light harvesting ability

Abstract

Gametophytes of the red intertidal macroalga Porphyra abbottae were grown in batch cultures in order to examine some of their plastic responses to key environmental factors. Conchospores from a conchocelis stock culture (originated from a single carpospore) were grown under various conditions of photon flux density (PFD) and nutrient supply (nitrogen, phosphorus enrichment and water motion). The research centered on how these factors affect morphology (shape, thickness) and light harvesting by young gametophytic blades, and how changes in the latter influence photosynthesis and growth.Blades grown at low PFD and high water motion were narrower; under nutrient limitation they were thinner.Whole thallus broadband absorptance (400-700 nm) was used as a measure of light harvesting ability. Absorptance increased at PFDs limiting for growth, indicating photoacclimation by light harvesting pigments, but this increase was not sufficient to compensate for low ambient PFD. The resulting decrease in the amount of light harvested at subsaturating PFDs did not account entirely for the decrease in growth rates at these PFDs: quantum yields of growth in plants grown under a range of PFDs showed decreased light use efficiency at low PFD, suggesting increased metabolic maintenance costs at limiting PFD.Nutrient limitation decreased the light harvesting ability, light-limited and light-saturated photosynthetic rates, and growth. Minor shifts in maximal quantum yields of photosynthesis indicated that differences in light-limited rates were primarily determined by light absorption. Quantum yields of growth were increased by nitrogen limitation.Increased absorptance, brought about by low PFD and/or high nitrogen, was accompanied by enrichment of the accessory pigments phycoerythrin and phycocyanin relative to chlorophyll a, and of phycocyanin relative to phycoerythrin. Addition of accessory pigments to photosynthetic membranes may thus involve an increase in phycobilisome density along with a reduction in the phycoerythrin content of phycobilisomes.