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.
Highlights
A low photosystem II (PSII) efficiency is caused by a reduction in the turnover rate of reactions involved in the electron transport chain and CO2 fixation that is probably caused by nutrient stress (Kolber and Falkowski 1993)
Low fluorescencia variable (Fv)/Fm values do not necessarily reflect a limitation of the photosynthetic rate of the phytoplankton community associated with nitrate stress
The use of Fv/Fm as a nutrient stress indicator in oceanography is based on the observation that this parameter increases from the surface towards the nutricline (Geider et al 1993a)
Summary
IntroducciónActive fluorescence techniques have been widely used as a proxy to assess the physiological status of marine phytoplankton (Kolber et al 1990, Falkowski et al 1991, Geider etLas técnicas de fluorescencia activa han sido utilizadas ampliamente como una aproximación para conocer el estado fisiológico del fitoplancton marino (Kolber et al 1990; Ciencias Marinas, Vol 34, No 1, 2008 al.1993a, Olaizola et al 1996, Greene et al 1994, Behrenfeld et al 2006). Species inhabiting oligotrophic regimes must have the capability to withstand low nutrient availability without being affected in their physiology. These species probably express physiological strategies to optimize their photosynthetic performance in nitrate-impoverished waters. In the present paper we present a series of experiments performed during a cruise in the Eastern Tropical North Pacific (ETNP) off Mexico to test the hypothesis that in an oligotrophic regime with a high-stratified vertical structure, phytoplankton is adapted to this condition and its photosynthetic rate is not limited. A low Fv/Fm will not reflect an impairment of photosynthetic rate in the region
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