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Phycobilisomes (PBSs) are pigment-rich super-complexes required for efficient harvest and transfer of light energy to photosynthetic reaction centers of cyanobacteria. The model cyanobacterium Fremyella diplosiphon is able to adjust PBS pigmentation and size in response to the prevailing light spectrum through a process called complementary chromatic acclimation to optimize spectral light absorption, concomitantly optimizing photosynthesis and growth. We explored the fitness costs versus advantages of modulating antennae size and composition under sinusoidal continuous and fluctuating light conditions in F. diplosiphon by comparing growth of wild-type (WT) cells with a mutant strain deficient in PBSs in both monoculture and polyculture conditions. Comparative analyses of WT and the PBS-deficient FdCh1 strain under continuous vs. fluctuating sinusoidal light suggest a potential fitness advantage for maintaining PBSs in WT cells during continuous light and a fitness cost during transitions to and acclimation under fluctuating light. We explored the physiological changes correlated with the observed differential growth to understand the dynamics and biochemical bases of comparative fitness of distinct strains under defined growth conditions. Wild-type F. diplosiphon appears to accumulate longer PBS rods and exhibits higher oxidative stress under fluctuating light conditions than continuous sinusoidal light, which may impact responses and the fitness of cells that do not adapt to rapid changes in external light. [ABSTRACT FROM AUTHOR]

Outdoor microalgae systems are a promising platform for fuels and chemicals, but are currently limited by relatively low productivities. This study investigated the effects of photoacclimation on the productivity of Chlamydomonas reinhardtii grown under fluctuating light regimes which simulate well-mixed cultures in outdoor reactors. Simulations represented cells cycling between high light and dark zones in high-density ( HD Fluc , light fraction (LF) = 0.5) and low-density ( LD Fluc , LF = 0.9) cultures. Each fluctuating treatment was controlled by cultures grown under non-fluctuating light of the same hourly average irradiance ( HD Avg and LD Avg ) to differentiate between light dosage and regime. The large dark fraction of HD Fluc resulted in a low-light acclimated phenotype displaying up-regulation of light harvesting pigments and low NPQ caused by reduced levels of the dissipative protein LHCSR3. All other treatments led to high-light acclimation phenotypes. HD Fluc showed an estimated three-fold lower biomass yield relative to light absorbed and significant reductions in the quantum yield of PSII compared to HD Avg . This suggests that during high light periods of fluctuating cycles, higher absorption and an inability to safely dissipate excess light, resulted in greater photodamage and respiration required for repair. A framework for including these findings in predictive modelling of mass cultures is presented. [ABSTRACT FROM AUTHOR]

Ulva prolifera , a common species of green macroalgae, is often harmful-algal-bloom causative and significantly impacts local marine ecosystems. Previous studies on the physiological characteristics of U. prolifera have been conducted under constant light (CL). However, light in the natural environment continually changes, and little is known about fluctuating light (FL). Ocean acidification (OA) has been proposed to interact with dynamic surrounding environments to affect the physiological performance of macroalgae. Therefore, we investigated the combined effects of FL (80/300, alternating between 80 μmol photons m−2 s−1 for 2.5 h and 300 μmol photons m−2 s−1 for 1.5 h, with an average light intensity of 160 μmol photons m−2 s−1 and OA (1000 ppm CO 2) on U. prolifera. The results clearly showed that FL had no significant effect on the relative growth rate (RGR), whereas OA obviously improved RGR. However, under FL-OA combination conditions, RGR was inhibited significantly, accompanied by a concomitant downgraded photosynthetic performance, while the photoprotective abilities were enhanced. The results would help us accurately predict the primary productivity of macroalgae in coastal waters under future OA conditions with irradiance fluctuations. • High CO 2 concentration improves the growth of Ulva prolifera. • Fluctuating light inhibit the growth of U. prolifera under high CO 2 concentration condition. • Photosynthetic performance of U. prolifera was downgraded by fluctuating light and high CO 2 concentration. [ABSTRACT FROM AUTHOR]