Maximum efficiency of charge separation of photosystem II of the phytoplankton community in the Eastern Tropical North Pacific off Mexico: A nutrient stress diagnostic tool?

Abstract

Phytoplankton growth in oligotrophic regions might be constrained by a low photosynthetic capacity related to nutrient stress. A reduced photosynthetic capacity has been documented by low values of the maximum quantum efficiency of charge separation (Fv/Fm) of photosystem II (PSII). In the present work, spatial and temporal variations of Fv/Fm in phytoplankton samples of the Eastern Tropical North Pacific (ETNP) off Mexico were characterized to explore the potential of this parameter as a nutrient stress indicator. Our results show that Fv/Fm was lower than 0.5 in most of the samples and that in surface waters this parameter was highly variable. The variability of Fv/Fm was related to the time of sampling, and Fv/Fm changes followed a day/night cycle with maximum values at dawn and a strong reduction at noon related to down-regulation of PSII activity. Enrichment experiments showed that Fv/Fm did not increase after nitrate addition to surface samples. These results suggest that photosynthesis of the phytoplankton community in the ETNP dominated by picoplankton, of which 60–80% was represented by prokaryote biomass (Prochloroccocus and other cyanobacteria), was not rate-limited under natural conditions. Therefore, low Fv/Fm values associated with PSII down-regulation and inherent of specific phytoplankton species are not indicative of nutrient stress in picoplankton-dominated environments such as the ETNP off Mexico. These sources of variability that are not directly associated with the nutritional status of the phytoplankton community should be taken into account when using Fv/Fm as a diagnostic tool for nutrient limitation.