Photoprotection by photoinhibitory and PSII-reaction centre quenching controls growth of Ulva rigida (Chlorophyta) and is a pre-requisite for green tide formation

Abstract

The combined photoinhibitory and PSII-reaction centre quenching against light stress is an important mechanism that allows the green macroalga Ulva rigida to proliferate and form green tides in coastal ecosystems. Eutrophication of coastal ecosystems often stimulates massive and uncontrolled growth of green macroalgae, causing serious ecological problems. These green tides are frequently exposed to light intensities that can reduce their growth via the production of reactive oxygen species (ROS). To understand the physiological and biochemical mechanisms leading to the formation and maintenance of green tides, the interaction between inorganic nitrogen (N_i) and light was studied. In a bi-factorial physiological experiment simulating eutrophication under different light levels, the bloom-forming green macroalga Ulva rigida was exposed to a combination of ecologically relevant nitrate concentrations (3.8-44.7 µM) and light intensities (50-1100 µmol photons m^-2 s^-1) over three days. Although artificial eutrophication (≥ 21.7 µM) stimulated nitrate reductase activity, which regulated both nitrate uptake and vacuolar storage by a feedback mechanism, nitrogen assimilation remained constant. Growth was solely controlled by the light intensity because U. rigida was N_i-replete under oligotrophic conditions (3.8 µM), which requires an effective photoprotective mechanism. Fast declining Fv/Fm and non-photochemical quenching (NPQ) under excess light indicate that the combined photoinhibitory and PSII-reaction centre quenching avoided ROS production effectively. Thus, these mechanisms seem to be key to maintaining high photosynthetic activities and growth rates without producing ROS. Nevertheless, these photoprotective mechanisms allowed U. rigida to thrive under the contrasting experimental conditions with high daily growth rates (12-20%). This study helps understand the physiological mechanisms facilitating the formation and persistence of ecologically problematic green tides in coastal areas.

Keywords: Ulva; Eutrophication; Macroalgal blooms; Nitrate reductase; Photoprotection; Photosynthesis.

Fig. 1

Relative growth rates (RGR) of Ulva rigida after three days of exposure to different growth light intensities and nitrate concentrations in seawater. Treatments: LL: 52 µmol photons m⁻² s⁻¹; SL: 612 µmol photons m⁻² s⁻¹; HL: 1,111 µmol photons m⁻² s⁻¹; nitrate concentration: 3.8 µM nitrate (white columns), 21.7 µM nitrate (light grey columns) and 44.7 µM nitrate (dark grey columns). Error bars represent SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences of means between treatments (P < 0.001, 2-way ANOVA, Tukey HSD)

Fig. 2

Average nitrate uptake rates of Ulva rigida over 480 min at different seawater nitrate concentrations and growth light intensities (A) on the first and (B) the third day of the experiment. Error bars denote SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences between treatments (P < 0.05, 2-way ANOVA with repeated measures). For details of experimental treatments see Fig. 1 and Table 1

Fig. 3

Nitrate reductase (NR) activities of Ulva rigida after three days of an exposure to different nitrate concentrations in seawater and growth light intensities. Error bars denote SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences of means between treatments (P < 0.05, 2-way ANOVA, Tukey HSD). For details of experimental treatments, see Fig. 1 and Table 1

Fig. 4

Changes in Fv/Fm of Ulva rigida exposed to A limiting (LL: 52 µmol photon m⁻² s⁻¹), B saturating (SL: 612 µmol photon m⁻² s⁻¹) and C high light intensities (HL: 1,111 µmol photon m⁻² s⁻¹) at three seawater nitrate concentrations (3.8, 21.7 and 44.7 µM) over three days. Error bars represent SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences of means between treatments (P < 0.0001, 2-way ANOVA, Tukey HSD). For details of experimental treatments, see Fig. 1 and Table 1

Fig. 5

Photosynthetic parameters and chlorophyll a/b ratio of Ulva rigida after three days of exposure to three different light conditions and three nitrate concentrations in seawater. A Maximum electron (e^–) transport rates (ETR_max). B Light saturation points (E_k) of electron transport rates. C Initial slopes of electron transport rate vs. irradiance (ETR-E) curves. D Chlorophyll a/b ratios. Error bars denote SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences of means between treatments (P < 0.05, 2-way ANOVA, Tukey HSD)

Fig. 6

Non-photochemical quenching (NPQ) of Ulva rigida exposed to three different light conditions and seawater nitrate concentrations on the first (A) and the third day (B) of the experiment. Error bars denote SDs of three replicates per treatment (n = 3). Different lower case letters above the columns indicate statistically significant differences of means between treatments (P < 0.05, 2-way ANOVA, Tukey HSD). For details of experimental treatments, see Fig. 1 and Table 1