The Water-Water Cycle in Algae

Photosynthetic Parameters Show Specific Responses to Essential Mineral Deficiencies

In response to decreases in the assimilation efficiency of CO2, plants oxidize the reaction center chlorophyll (P700) of photosystem I (PSI) to suppress reactive oxygen species (ROS) production. In hydro-cultured sunflower leaves experiencing essential mineral deficiencies, we analyzed the following parameters that characterize PSI and PSII: (1) the reduction-oxidation states of P700 [Y(I), Y(NA), and Y(ND)]; (2) the relative electron flux in PSII [Y(II)]; (3) the reduction state of the primary electron acceptor in PSII, QA (1 - qL); and (4) the non-photochemical quenching of chlorophyll fluorescence (NPQ). Deficiency treatments for the minerals N, P, Mn, Mg, S, and Zn decreased Y(II) with an increase in the oxidized P700 [Y(ND)], while deficiencies for the minerals K, Fe, Ca, B, and Mo decreased Y(II) without an increase in Y(ND). During the induction of photosynthesis, the above parameters showed specific responses to each mineral. That is, we could diagnose the mineral deficiency and identify which mineral affected the photosynthesis parameters.

Reduction-Induced Suppression of Electron Flow (RISE) Is Relieved by Non-ATP-Consuming Electron Flow in Synechococcus elongatus PCC 7942

Photosynthetic organisms oxidize P700 to suppress the production of reactive oxygen species (ROS) in photosystem I (PSI) in response to the lower efficiency of photosynthesis under high light and low CO₂ conditions. Previously, we found a positive relationship between reduction of plastoquinone (PQ) pool and oxidation of P700, which we named reduction-induced suppression of electron flow (RISE). In the RISE model, we proposed that the highly reduced state of the PQ pool suppresses Q-cycle turnover to oxidize P700 in PSI. Here, we tested whether RISE was relieved by the oxidation of the PQ pool, but not by the dissipation of the proton gradient (ΔpH) across the thylakoid membrane. Formation of ΔpH can also suppress electron flow to P700, because acidification on the luminal side of the thylakoid membrane lowers oxidation of reduced PQ in the cytochrome b₆/f complex. We drove photosynthetic electron transport using H₂O₂-scavenging peroxidase reactions. Peroxidase reduces H₂O₂ with electron donors regenerated along the photosynthetic electron transport system, thereby promoting the formation of ΔpH. Addition of H₂O₂ to the cyanobacterium Synechococcus elongatus PCC 7942 under low CO₂ conditions induced photochemical quenching of chlorophyll fluorescence, enhanced NADPH fluorescence and reduced P700. Thus, peroxidase reactions relieved the RISE mechanism, indicating that P700 oxidation can be induced only by the reduction of PQ to suppress the production of ROS in PSI. Overall, our data suggest that RISE regulates the redox state of P700 in PSI in cooperation with ΔpH regulation.

Diversity in photosynthetic electron transport under [CO2]-limitation: the cyanobacterium Synechococcus sp. PCC 7002 and green alga Chlamydomonas reinhardtii drive an O2-dependent alternative electron flow and non-photochemical quenching of chlorophyll fluorescence during CO2-limited photosynthesis

Some cyanobacteria, but not all, experience an induction of alternative electron flow (AEF) during CO₂-limited photosynthesis. For example, Synechocystis sp. PCC 6803 (S. 6803) exhibits AEF, but Synechococcus elongatus sp. PCC 7942 does not. This difference is due to the presence of flavodiiron 2 and 4 proteins (FLV2/4) in S. 6803, which catalyze electron donation to O₂. In this study, we observed a low-[CO₂] induced AEF in the marine cyanobacterium Synechococcus sp. PCC 7002 that lacks FLV2/4. The AEF shows high affinity for O₂, compared with AEF mediated by FLV2/4 in S. 6803, and can proceed under extreme low [O₂] (about a few µM O₂). Further, the transition from CO₂-saturated to CO₂-limited photosynthesis leads a preferential excitation of PSI to PSII and increased non-photochemical quenching of chlorophyll fluorescence. We found that the model green alga Chlamydomonas reinhardtii also has an O₂-dependent AEF showing the same affinity for O₂ as that in S. 7002. These data represent the diverse molecular mechanisms to drive AEF in cyanobacteria and green algae. In this paper, we further discuss the diversity, the evolution, and the physiological function of strategy to CO₂-limitation in cyanobacterial and green algal photosynthesis.

The chlorophyll a fluorescence induction curve in the green microalga Haematococcus pluvialis: further insight into the nature of the P-S-M fluctuation and its relationship with the "low-wave" phenomenon at steady-state

Chlorophyll fluorescence is an information-rich signal which provides an access to the management of light absorbed by PSII. A good example of this is the succession of fast fluorescence fluctuations during light-induced photosynthetic induction after dark-adaptation. During this period, the fluorescence trace exhibits several inflexion points: O-J-I-P-S-M-T. Whereas the OJIP part of this kinetics has been the subject of many studies, the processes that underly the PSMT transient are less understood. Here, we report an analysis of the PSMT phase in the green microalga Haematococcus pluvialis in terms of electron acceptors and light use by photochemistry, fluorescence and non-photochemical quenching (NPQ). We identify additional sub-phases between P and S delimited by an inflexion point, that we name Q, found in the second time scale. The P-Q phase expresses a transient photochemical quenching specifically due to alternative electron transport to oxygen. During the transition from Q to S, the NPQ increases and then relaxes during the S-M phase in about 1 min. It is suggested that this transient NPQ observed during induction is a high energy state quenching (qE) dependent on the alternative electron transport to molecular oxygen. We further show that this NPQ is of the same nature than the NPQ, known as the low-wave phenomenon, which is transiently observed after a saturating light pulse given at steady-state. In both cases, the NPQ is oxygen-dependent. This NPQ is observed at external pH 6.0, but not at pH 7.5, which seems correlated with faster saturation of the PQ pool at pH 6.0.

Phytoplankton strategies for photosynthetic energy allocation

Phytoplankton physiology is dynamic and highly responsive to the environment. Phytoplankton acclimate to changing environmental conditions by a complex reallocation of carbon and energy through metabolic pathways to optimize growth. Considering the tremendous diversity of phytoplankton, it is not surprising that different phytoplankton taxa use different strategies to partition carbon and energy resources. It has therefore been satisfying to discover that general principles of energetic stoichiometry appear to govern these complex processes and can be broadly applied to interpret phytoplankton distributions, productivity, and food web dynamics. The expectation of future changes in aquatic environments brought on by climate change warrants gathering knowledge about underlying patterns of photosynthetic energy allocation and their impacts on community structure and ecosystem productivity.

Evolved physiological responses of phytoplankton to their integrated growth environment

Phytoplankton growth and productivity relies on light, multiple nutrients and temperature. These combined factors constitute the 'integrated growth environment'. Since their emergence in the Archaean ocean, phytoplankton have experienced dramatic shifts in their integrated growth environment and, in response, evolved diverse mechanisms to maximize growth by optimizing the allocation of photosynthetic resources (ATP and NADPH) among all cellular processes. Consequently, co-limitation has become an omnipresent condition in the global ocean. Here we focus on evolved phytoplankton populations of the contemporary ocean and the varied energetic pathways they employ to solve the optimization problem of resource supply and demand. Central to this discussion is the allocation of reductant formed through photosynthesis, which we propose has the following three primary fates: carbon fixation, direct use and ATP generation. Investment of reductant among these three sinks is tied to cell cycle events, differentially influenced by specific forms of nutrient stress, and a strong determinant of relationships between light-harvesting (pigment), photosynthetic electron transport and carbon fixation. Global implications of optimization are illustrated by deconvolving trends in the 10-year global satellite chlorophyll record into contributions from biomass and physiology, thereby providing a unique perspective on the dynamic nature of surface phytoplankton populations and their link to climate.