Abstract
In the warm, oligotrophic oceans, phytoplankton frequently experience high light exposure and must compete for a potentially limited nutrient supply. Additionally, the light regimes are dynamic in view of the diel solar cycle, weather conditions, and depth. Dealing with these challenges is critical to their survival. Here, we explored, using active chlorophyll a fluorescence, the photophysiological responses of cyanobacteria-dominated phytoplankton assemblages to intra-day variations in the light regime in the oligotrophic South China Sea (SCS). Experiments were conducted at three stations (SEATS, DC2, and DC6) where phytoplankton communities were dominated by Prochlorococcus and Synechococcus, and environmental conditions were similar except for daytime light conditions. We found that, at each station, the maximum quantum yield of PSII (Fv/Fm) at dawn was as high as ∼0.5, although concentrations of both dissolved inorganic nitrogen and soluble reactive phosphate were below the detection limits. Subsequently, diurnal patterns of Fv/Fm diverged between stations. At stations DC2 and DC6, we observed significant drops (25–48%) of Fv/Fm around midday, coinciding with the incident solar photosynthetically active radiation (PAR) reaching over 2,000 μmol quanta m–2 s–1, but Fv/Fm was nearly stable at SEATS where the daily maximal PAR was less than 1,000 μmol quanta m–2 s–1. Further analysis suggests that the midday drops in Fv/Fm at DC2 and DC6 were a consequence of high light-induced PSII photoinactivation. On the other hand, we found that the patterns of diurnal variation in maximum relative electron transport rate (rETRmax) presented as being unimodal with a peak around midday at all three stations. Furthermore, we found that the diurnal pattern of rETRmax was mainly controlled by the extent of photochemical quenching (qP) which reflects the redox status of electron carriers downstream of PSII. In conclusion, our results indicate that, in the oligotrophic SCS, through increasing the activity of the electron-consuming mechanisms (high qP), the phytoplankton communities are able to maintain their midday photosynthetic potential (high rETRmax) even with a degree of PSII photoinactivation (low Fv/Fm).
Highlights
Marine phytoplankton are responsible for approximately half of global net primary production (Field et al, 1998; Westberry et al, 2008; Zhao and Running, 2010)
We investigated the diurnal patterns of photophysiology of cyanobacteria-dominated phytoplankton at three stations in the northern South China Sea (SCS) at which environmental conditions were similar except for the contrasting light regimes at the time of sampling
The DIN and SPR were depleted (Table 1) and previous studies indicate that the primary limitation to phytoplankton growth in the northern SCS is the availability of nutrients (Lee Chen, 2005; Lee Chen and Chen, 2006), but the values of Fv/Fm at dawn were as high as 0.5 at all stations (Figure 3)
Summary
Marine phytoplankton are responsible for approximately half of global net primary production (Field et al, 1998; Westberry et al, 2008; Zhao and Running, 2010) This production supports most life in the oceans and substantially affects global biogeochemical cycles and climate. The light regimes to which phytoplankton are exposed are primarily determined by solar irradiance, and present as periodically variable, with diel and seasonal cycles. Superimposed on such regularity are chaotic and stochastic variations relating to meteorological conditions such as cloud cover, precipitation, and atmospheric aerosol concentration and composition. The light regime for phytoplankton can be highly variable on different time scales, driven by both predictable and unpredictable elements
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