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C3 photosynthesis can be complemented with a C4 carbon concentrating mechanism (CCM) to minimize photorespiratory losses. C4 photosynthesis is often more efficient than C3 under steady‐state conditions. However, the C4 CCM depends on inter‐cellular metabolite concentration gradients, which must increase following increases in light intensity and could decrease rates of C4 photosynthesis under fluctuating light. Additionally, incomplete flux through photorespiration could prove beneficial to C4 assimilation during light induction of the CCM. Here, we compare metabolic profiles in the closely related C3Flaveria robusta and C4Flaveria bidentis during a light transient from low to high light to determine if these non‐steady state accumulation patterns provide insight to the induction of the metabolite gradients needed to drive C4 intermediate transport and if there is incomplete cycling of photorespiratory intermediates. In these C3 and C4 species, metabolite steady‐state pool sizes suggest that C4 transport acids maintain concentration gradients across the bundle sheath and mesophyll cell types under these light fluctuations. However, there was incomplete flux through photorespiration in the C4F. bidentis, which could reduce photorespiratory CO2 loss via glycine decarboxylation and help maintain higher rates of assimilation during following induction periods. [ABSTRACT FROM AUTHOR]

Light fluctuations are ubiquitous, exist across multiple spatial and temporal scales, and directly affect the physiology and ecology of photoautotrophs. However, the indirect effects of light fluctuations on the sensitivity of organisms to other key environmental factors are unclear. Here, we evaluate how photoperiod regime (period of time each day where organisms receive light), a dynamic element of aquatic ecosystems, can influence the interactive effects of temperature and irradiance (intensity of light) on the growth rate of phytoplankton populations. We first completed a literature review and meta‐analysis that suggests photoperiod alters the individual effects of temperature – but not irradiance – on algal growth rates and that highlights how few studies experimentally manipulate photoperiod, temperature and irradiance. To address this empirical gap, we conducted a set of laboratory experiments on three freshwater phytoplankton species (Chlamydomonas reinhardtii, Chlorella vulgaris and Cryptomonas ovata). We measured performance surfaces relating growth rate to irradiance and temperature gradients for each species in constant (24:0 h of light:dark) environments. We then evaluated whether analogous surfaces measured under different photoperiods (6:18, 12:12 and 16:8 h of light:dark) and scaled by the duration of light availability could be inferred from results under constant light. For a majority of the combinations of species and photoperiods examined, photoperiod meaningfully altered the intercept and shape of performance surfaces. These differences were most pronounced under the shortest photoperiod (6:18 h light:dark), where populations underperformed expectations. Alterations to performance surfaces were non‐linear and mostly structured by temperature with higher temperatures yielding higher than anticipated growth rates. Collectively, these experiments and synthesis reveal the potential for photoperiod regime to influence the effects of temperature, irradiance and their interaction on phytoplankton growth. Beyond the environmental variables and organisms presently considered, this research highlights the capacity for dynamic, abiotic variables to exert direct effects while also influencing relationships among other environmental factors. [ABSTRACT FROM AUTHOR]

A vegetative clone of the chlorophyte macroalga Ulva rotundata was maintained in an outdoor continuous flow system under nutrient sufficient conditions and various light regimes. Step changes between 9 and 100% incident irradiance (I[SUB0]) were employed to simulate cloud passage. Temporary (1-4 h) midday I[SUB0] perturbations evoked net changes in growth rate (μ) and chlorophyll (chl) content. Under I[SUB0] alternating at various periodicities from 15 min to 7 h, net μ was the average of the μ under steady state 9 and 100% I[SUB0], regardless of periodicity. However, the μ in alternating light was considerably less than μ under steady state 55% I[SUB0](= 9% + 100% / 2), as expected based on the nonlinear shape of the μ-I relationship. Unlike μ, chl content depended primarily on the total daily irradiance, probably due to the slower response of chl compared to photosynthetic rate. On time scales ≥ one day, chl was linearly correlated with light-regulated daily μ under both steady state and intraday fluctuating irradiance, consistent with photosynthesis feedback regulation of chl concentration. [ABSTRACT FROM AUTHOR]