Changes in the Rubisco to photosystem ratio dominates photoacclimation across phytoplankton taxa

Functional consequences of modification of the photosystem I/photosystem II ratio in the cyanobacterium Synechocystis sp. PCC 6803

The stoichiometry of photosystem II (PSII) and photosystem I (PSI) varies between photoautotrophic organisms. The cyanobacterium Synechocystis sp. PCC 6803 maintains two- to fivefold more PSI than PSII reaction center complexes, and we sought to modify this stoichiometry by changing the promoter region of the psaAB operon. We thus generated mutants with varied psaAB expression, ranging from ~3% to almost 200% of the wild-type transcript level, but all showing a reduction in PSI levels, relative to wild type, suggesting a role of the psaAB promoter region in translational regulation. Mutants with 25%-70% of wild-type PSI levels were photoautotrophic, with whole-chain oxygen evolution rates on a per-cell basis comparable to that of wild type. In contrast, mutant strains with <10% of the wild-type level of PSI were obligate photoheterotrophs. Variable fluorescence yields of all mutants were much higher than those of wild type, indicating that the PSI content is localized differently than in wild type, with less transfer of PSII-absorbed energy to PSI. Strains with less PSI saturate at a higher light intensity, enhancing productivity at higher light intensities. This is similar to what is found in mutants with reduced antennae. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea present, P700+ re-reduction kinetics in the mutants were slower than in wild type, consistent with the notion that there is less cyclic electron transport if less PSI is present. Overall, strains with a reduction in PSI content displayed surprisingly vigorous growth and linear electron transport.

IMPORTANCE

Consequences of reduction in photosystem I content were investigated in the cyanobacterium Synechocystis sp. PCC 6803 where photosystem I far exceeds the number of photosystem II complexes. Strains with less photosystem I displayed less cyclic electron transport, grew more slowly at lower light intensity and needed more light for saturation but were surprisingly normal in their whole-chain electron transport rates, implying that a significant fraction of photosystem I is dispensable for linear electron transport in cyanobacteria. These strains with reduced photosystem I levels may have biotechnological relevance as they grow well at higher light intensities.

Phytoplankton package effect in oceanic waters: Influence of chlorophyll-a and cell size

In this study, the interaction between the packaging effect (Q_a^⁎) and total chlorophyll-a concentration (C_t) or total size index (SI_t) was investigated to explore the potential bio-optical mechanism in phytoplankton cells in the global oceans. In addition, the long-term spatiotemporal characteristics of these interactions were necessary for grasping their variation. Numerous in situ surface measurements (phytoplankton pigment and absorption coefficients) from the global oceans were analyzed first, and then correlation and causality analyses were performed on the satellite-deduced Q_a^⁎, C_t, and SI_t in the global oceans during 2002-2020. The results show a negative correlation between Q_a^⁎ and C_t or SI_t in the low latitudes (30°S-30°N) and a positive correlation in the middle latitudes (30°S-55°S and 30°N-55°N). The causality analysis reveals a mutual and asymmetric cause-effect relationship between Q_a^⁎ and C_t or SI_t in the low latitudes. The stabilization effect of Q_a^⁎ contributes to a 10%-50% variation in C_t and SI_t, with 40%-60% uncertainty of Q_a^⁎ caused by C_t and SI_t in the low latitudes, which is inverse in the middle latitudes. The remaining contribution to each variable mainly originates from long-term trends and noise. Combining the analysis between Q_a^⁎ and the irradiance, the balancing processes in phytoplankton cells are different in the low (phytoplankton-driving mode) and middle latitudes (irradiance-driving mode), which is related to photoacclimation and photoinhibition. The analyses provide insights into the quantitative interpretation of the relationship between Q_a^⁎ and C_t or SI_t, which contribute knowledge that has not been previously reported.

Photoacclimation State of Thalassiosira weissflogii is not Affected by Changes in Optical Depth Under A Fluctuating Light Regime Simulating Deep Mixing1

Satellite-based remote sensing allows for global estimates of phytoplankton primary productivity by converting measurements of ocean color or photon absorption into units of carbon fixation. Models which perform this conversion often require an estimate of phytoplankton photoacclimation state such as the carbon to chlorophyll a ratio (C:Chl). Recently, our group developed a new photoacclimation model that can be applied to models of primary production. The model assumes that the phytoplankton photoacclimation state is not affected by periods of darkness during deep mixing beneath the photic zone, due to reduction in the plastoquinone pool in darkness and the subsequent deactivation of the signal for chlorophyll synthesis. In this study, we tested these assumptions by culturing the marine diatom Thalassiosira weissflogii under fluctuating light conditions simulating three different optical depths with progressively increasing deep mixing periods. The photoacclimation state, measured by the ratio of C:Chl, in T. weissflogii was not affected by changes in the length of simulated deep mixing periods. In addition, analysis of photosynthesis vs. irradiance (PE) curves showed that increases in optical depth caused decreases in both the maximum Chl-normalized rate of photosynthesis (P^b_max ) and in the slope of light-limited photosynthesis (α^b ), but had no effect on the half-saturation irradiance (E_k , another metric of photoacclimation). However, measurements of chlorophyll fluorescence during simulated deep mixing did not support the hypothesis that the PQ pool was reduced during dark periods. Thus, our findings support the use of the photoacclimation model for estimating primary production while suggesting the need for further research into the mechanisms controlling photoacclimation in the upper mixed layer environment of the ocean.

Adverse effects of atrazine, DCMU and metolachlor on phytoplankton cultures and communities at environmentally relevant concentrations using Fast Repetition Rate Fluorescence

The widespread and persistent contamination of freshwater environments by low concentrations of pesticides is a growing concern worldwide. In aquatic environments, herbicide pollution is of greatest concern for phytoplankton, due to their similarities to terrestrial plants. Through the use of Fast Repetition Rate Fluorometry (FRRF) during weeklong experiments on 10 phytoplankton cultures from 4 classes and 4 natural communities, we demonstrate that PSII-inhibiting herbicides, notably atrazine that is extensively used in North America, consistently have effects on freshwater phytoplankton photophysiology at concentrations far below concentrations affecting the most sensitive species in previous studies. The parameters specific to FRRF (Ρ, σ, τ₁, τ₂, τ₃) were those most sensitive to PSII inhibitors, compared to the standard fluorescence parameters derived from other fluorescence protocols such as Pulse Amplitude Modulation (PAM) fluorometry (F₀, F_m, F_v/F_m) and extracted chlorophyll a concentrations. Based on these findings, existing national environmental guidelines and standards are insufficient to adequately prevent adverse effects of atrazine and other PSII inhibiting herbicides on algal physiology in aquatic ecosystems.

Diurnal variations of the optical properties of phytoplankton in a laboratory experiment and their implication for using inherent optical properties to measure biomass

Diurnal variations of phytoplankton size distributions, chlorophyll, carbon and nitrogen content, in vivo fluorescence and associated optical absorption and scattering properties were observed in the laboratory to help understand in situ and spatial observations. We grew triplicate semi-continuous cultures of T. pseudonana, D. tertiolecta, P. tricornutum and E. huxleyi under a sinusoidal light regime. We observed diurnal variations in the particulate absorption (a_p), scattering (b_p), attenuation (c_p), and backscattering coefficients (b_bp), which correlate with carbon and Chl concentrations. Relative variations from sunrise of b_bp are slightly lower than those of c_p, suggesting that b_bp diurnal increases observed in nature are partly caused by phytoplankton. Non-concurrent changes of carbon and Chl-specific backscattering and scattering coefficients and optical cross-sections however indicates that using backscattering to infer scattering or biomass must be done with care.

A mutant of Chlamydomonas without LHCSR maintains high rates of photosynthesis, but has reduced cell division rates in sinusoidal light conditions

The LHCSR protein belongs to the light harvesting complex family of pigment-binding proteins found in oxygenic photoautotrophs. Previous studies have shown that this complex is required for the rapid induction and relaxation of excess light energy dissipation in a wide range of eukaryotic algae and moss. The ability of cells to rapidly regulate light harvesting between this dissipation state and one favoring photochemistry is believed to be important for reducing oxidative stress and maintaining high photosynthetic efficiency in a rapidly changing light environment. We found that a mutant of Chlamydomonas reinhardtii lacking LHCSR, npq4lhcsr1, displays minimal photoinhibition of photosystem II and minimal inhibition of short term oxygen evolution when grown in constant excess light compared to a wild type strain. We also investigated the impact of no LHCSR during growth in a sinusoidal light regime, which mimics daily changes in photosynthetically active radiation. The absence of LHCSR correlated with a slight reduction in the quantum efficiency of photosystem II and a stimulation of the maximal rates of photosynthesis compared to wild type. However, there was no reduction in carbon accumulation during the day. Another novel finding was that npq4lhcsr1 cultures underwent fewer divisions at night, reducing the overall growth rate compared to the wild type. Our results show that the rapid regulation of light harvesting mediated by LHCSR is required for high growth rates, but it is not required for efficient carbon accumulation during the day in a sinusoidal light environment. This finding has direct implications for engineering strategies directed at increasing photosynthetic productivity in mass cultures.

Mechanisms that increase the growth efficiency of diatoms in low light

Photoacclimation was studied in Thalassiosira pseudonana to help understand mechanisms underlying the success of diatoms in low-light environments, such as coastal and deep mixing ecosystems. Light harvesting and other cell characteristics were combined with oxygen and carbon production measurements to assess the water-splitting reaction at PSII ([Formula: see text]) and intermediate steps leading to net carbon production (NPPC). These measurements revealed that T. pseudonana is remarkably efficient at converting harvested light energy into biomass, with at least 57 % of [Formula: see text] retained as NPPC across all light-limited growth rates examined. Evidence for upregulation of ATP generation pathways that circumvent carbon fixation indicated that high growth efficiency at low light levels was at least partly due to increases in the efficiency of ATP production. Growth rate-dependent demands for ATP and NADPH were reflected in carbon composition and in unexpected shifts in the light-limited slope (α) of photosynthesis-irradiance relationships generated from chlorophyll-specific (14)C-uptake. Overall, these results suggest that pathway gating of carbon and energy flow depends on light availability and is a key factor promoting the efficiency of diatom growth at low light intensities.

The RUBISCO to Photosystem II Ratio Limits the Maximum Photosynthetic Rate in Picocyanobacteria

Marine Synechococcus and Prochlorococcus are picocyanobacteria predominating in subtropical, oligotrophic marine environments, a niche predicted to expand with climate change. When grown under common low light conditions Synechococcus WH 8102 and Prochlorococcus MED 4 show similar Cytochrome b₆f and Photosystem I contents normalized to Photosystem II content, while Prochlorococcus MIT 9313 has twice the Cytochrome b₆f content and four times the Photosystem I content of the other strains. Interestingly, the Prochlorococcus strains contain only one third to one half of the RUBISCO catalytic subunits compared to the marine Synechococcus strain. The maximum Photosystem II electron transport rates were similar for the two Prochlorococcus strains but higher for the marine Synechococcus strain. Photosystem II electron transport capacity is highly correlated to the molar ratio of RUBISCO active sites to Photosystem II but not to the ratio of cytochrome b₆f to Photosystem II, nor to the ratio of Photosystem I: Photosystem II. Thus, the catalytic capacity for the rate-limiting step of carbon fixation, the ultimate electron sink, appears to limit electron transport rates. The high abundance of Cytochrome b₆f and Photosystem I in MIT 9313, combined with the slower flow of electrons away from Photosystem II and the relatively low level of RUBISCO, are consistent with cyclic electron flow around Photosystem I in this strain.