Variations in light-harvesting characteristics of the seagrass, Thalassia testudinum: evidence for photoacclimation

Abstract

In recent years, there has been some question regarding the biological significance of photoacclimation in Thalassia testudinum Banks ex König. However, aside from a few descriptions of adjustments in pigment content and stoichiometry, little is known about how physiological compensation is achieved in this seagrass. The objective of this study was to determine the extent to which T. testudinum is able to compensate for variation in light regime. Adult plants were collected from two sites in South Texas (East Flats: EF; Redfish Bay: RFB) that differed in depth (0.5 and 1.6 m, respectively) and were exposed to different light environments. When measured at ambient seawater temperature, the photosynthetic apparatus exhibited classic photoacclimation responses. Rates of light-saturated photosynthesis ( P max) were slightly higher in high-light (HL), shallow EF plants than in relatively low-light (LL), deeper growing RFB plants (ca. 290 cf. 230 nmol O 2 cm −2 h −1). In contrast, the RFB population exhibited chlorophyll a values that were ca. 1.4 times higher than the EF population (ca. 26 cf. 19 μg cm −2). Adjustments in pigment were associated with changes in the photosynthetic unit (PSU) size of PSI. Low-light plants synthesized larger photosynthetic units, presumably to aid light harvesting, while high-light plants synthesized smaller photosynthetic units, presumably to prevent photodamage. Neither photosystem densities (PSII or PSI) nor the size of PSII differed between populations. Changes in chlorophyll were also correlated with fluorescence measurements and maximum photon yield ( φ max: O 2 evolved per absorbed photon). Fluorescence yields ( F v/ F m), thought to be indicative of φ max, were higher in low-light plants than high-light plants; differences in F v/ F m ratios were due to differences in initial fluorescence emission ( F 0). Since increases in F 0 typically reflect either partial impairment or photodamage at PSII, these observations suggest that T. testudinum may exhibit a trade-off between photosynthetic efficiency and photoprotection; photoprotection was manifested as a down-regulation of photosynthesis. We present clear evidence to indicate this seagrass has the ability to adjust both structure and function of the light-harvesting apparatus in response to changes in irradiance. The ability to photoacclimate is most certainly of benefit to T. testudinum, an organism commonly found to inhabit shallow, potentially damaging, high-light environments.