Maximum photosynthetic efficiency of size-fractionated phytoplankton assessed by 14C uptake and fast repetition rate fluorometry

Under high nutrient concentrations and sufficient light conditions, large phytoplankton may display higher photosynthetic efficiency than smaller cells. This is unexpected since smaller phytoplankton, because of their higher surface to volume ratio, possess a greater ability to take up nutrients and absorb light. In order to investigate the causes of the increased photosynthetic efficiency in larger phytoplankton, we assessed the maximum photosynthetic efficiency of coastal assemblages in three size classes (,5, 5–20, and .20 mm) by concurrently conducting 14Cbased photosynthesis–irradiance experiments and fast repetition rate fluorescence measurements. The light-saturated, chlorophyll-specific photosynthesis (P ) and the maximum photosystem II (PSII) photochemical efficiency (Fv/ b max Fm) of each size class were determined during winter mixing (March 2003) and summer stratification (June 2003). During winter mixing, size-fractionated P and Fv/Fm were similar in all size classes. In contrast, during summer b max stratification, size-fractionated P and Fv/Fm were significantly higher in the .20-mm size class. In the entire data b max set, size-fractionated P and Fv/Fm were not significantly correlated. However, a significant relationship was found b max between size-fractionated P and Fv/Fm for phytoplankton assemblages acclimated to low light conditions. Under b max high light, an excess PSII capacity may be responsible for the discrepancy between size-fractionated P and Fv/ b max Fm measurements, whereas under low light conditions, photosynthetic electron transport chain and components downstream of PSII become more balanced, which results in a tight covariation between both variables. Higher maximum photosynthetic efficiencies of large-sized phytoplankton might be associated with a higher PSII photochemical efficiency characteristic of certain taxonomic groups such as diatoms. Phytoplankton size structure plays a crucial role in controlling the trophic and biogeochemical functioning of pelagic ecosystems. Numerous field observations indicate that large-sized phytoplankton form the bulk of phytoplankton biomass in highly productive ecosystems, whereas smaller cells tend to dominate in unproductive regions of the ocean (Chisholm 1992). However, despite this well-established pattern, relatively little information is available on the physiological characteristics of different phytoplankton size classes in natural conditions, and thus the underlying causes for their distribution and dynamic behavior remain uncertain. Traditionally, plankton physiologists have thought that 1 Corresponding author ([email protected]). Acknowledgments We thank D. Suggett and F. G. Figueiras for their insightful comments, which significantly improved an earlier version of the manuscript. We also thank S. Laney for his assistance in the processing of FRRF data and C. G. Castro for nutrient analyses. Two anonymous reviewers provided valuable comments. P.C. and P.E.-B. were supported by postgraduate research fellowships from the Spanish Ministerio de Ciencia y Tecnologı́a (MCYT). This research was funded by MCYT and Xunta de Galicia through research grants REN2000-1248 and PXIC30102PN to E.M. small-sized phytoplankton possess energetic advantages due to their high surface to volume ratio, which results in a higher affinity for nutrients and a reduced package effect (Kiørboe 1993; Raven 1998). However, numerous reports indicate that, under favorable conditions for growth, largesized phytoplankton can attain higher chlorophyll a (Chl a)– normalized photosynthetic rates than smaller phytoplankton (Legendre et al. 1993; Tamigneaux et al. 1999; Cermeño et al. 2005). In principle, these results suggest that under certain conditions large-sized phytoplankton may have a physiological advantage over smaller cells. However, Chl a–normalized photosynthetic rates are difficult to interpret because they depend on the cellular carbon to Chl a ratio, which is known to vary as a function of growth irradiance, nutrient status, taxonomic composition, or cell size. Furthermore, recent studies emphasize the need for interpreting photosynthesis–irradiance (P-E) experiments in the context of carbonspecific photosynthetic rates because under light-saturated conditions photosynthesis is not dependent on light absorption processes but on the efficiency of the photosynthetic electron transfer components [i.e., photosynthetic units] and the activity of Calvin-cycle enzymes (MacIntyre et al. 2002). Unfortunately, carbon biomass estimates in natural samples are subjected to different methodological shortcomings as1439 Photosynthetic efficiency of phytoplankton Fig. 1. Map of the study area (Rı́a de Vigo, northwest Iberian Peninsula) showing the sampling site. sociated with cell counting, calculation of biovolume, and the use of volume to carbon conversion factors. In addition, the photosynthetic parameters derived from P-E experiments, light-limited slope at low light intensities (ab), and maximum Chl a–specific photosynthetic rate at saturating irradiances (P ) provide an empirical description of phob max tosynthetic efficiency but do not account for the underlying (physiological) mechanisms responsible for their variability (see Behrenfeld et al. 2004). Fast repetition rate (FRR) fluorometry provides a measure of photosynthetic efficiency that is specific to photosystem II (PSII) photochemistry (Kolber et al. 1998). Specifically, fluorescence is measured following the progressive closure of PSII reaction centers by a series of subsaturating flashlets. Both light absorption and the number of reaction centers available for photochemistry modify the fluorescence that is observed (Falkowski and Raven 1997; Suggett et al. 2004). An initial fluorescence level, termed F0, is obtained from the first flashlet when all reaction centers are open. In contrast, a maximum fluorescence level, termed Fm, is obtained once the majority of reaction centers are transporting electrons and temporarily closed to further excitation. Variable fluorescence (Fv 5 Fm 2 F0) is scaled to the maximum fluorescence to yield the photochemical conversion efficiency of PSII, Fv/Fm (Butler 1978). Previous research has demonstrated that Fv/Fm is related to CO2 fixation or O2 evolution in natural assemblages of phytoplankton (Boyd et al. 1997; Behrenfeld et al. 1998; Suggett et al. 2001). However, these studies did not investigate whether this relationship applies to all phytoplankton size classes or taxa within the photoautotrophic community. The FRR fluorescence technique has been applied to different size fractions in order to determine the effect of iron addition on the PSII photochemistry of phytoplankton in high nutrient–low chlorophyll regions (Kolber et al. 1994; Boyd and Abraham 2001). These researchers, however, did not investigate whether there were any significant differences in the photosynthetic response of each size fraction. Here we report on the maximum photosynthetic efficiency of three phytoplankton size classes (,5, 5–20, and .20 mm) as assessed from concurrent 14C-uptake P-E experiments and FRR fluorescence measurements. We conducted our measurements during two contrasting hydrographic situations in a coastal embayment (Rı́a de Vigo, northwest Iberian Peninsula) characterized by intense horizontal, wind-driven circulation (Álvarez-Salgado et al. 2001) and a highly variable phytoplankton size structure (Tilstone et al. 1999). The concurrent determination of P-E parameters and Fv/Fm in different phytoplankton size classes allowed us to determine whether a noninvasive, fluorescence-based approach confirmed earlier, 14C-based evidence that photosynthetic efficiency in natural phytoplankton assemblages may increase with cell size. (Cermeño et al. 2005). Our observations are discussed in the context of the underlying physiological mechanisms operating under natural, light-limiting, and light-saturating conditions.