The amount of light reaching the leaves in seagrass (Zostera marina) meadows.

Seagrass meadows, and other submerged vegetated habitats, support a wide range of essential ecological services, but the true extents of these services are in many ways still not quantified. One important tool needed to assess and model many of these services is accurate estimations of the systems´ primary productivity. Such productivity estimations require an understanding of the underwater light field, especially regarding the amount of light that actually reaches the plants' photosynthetic tissue. In this study, we tested a simple practical approach to estimate leaf light exposure, relative to incoming light at the canopy, by attaching light sensitive film at different positions on leaves of Zostera marina, eelgrass, in four seagrass meadows composed of different shoot density and at two different depths. We found that the light reaching the leaves decreased linearly down through the canopy. While the upper parts of the leaves received approximately the same level of light (photosynthetic photon flux density, PPFD) as recorded with a PAR meter at the canopy top, the average light that the seagrass leaves were exposed to varied between 40 and 60% of the light on top of the canopy, with an overall average of 48%. We recommend that actual light interception is measured when assessing or modelling light depending processes in submerged vegetation, but if this is not achievable a rough estimation for vegetation similar to Z. marina would be to use a correction factor of 0.5 to compensate for the reduced light due to leaf orientation and internal shading.

Chlororespiration Serves as Photoprotection for the Photo-Inactivated Oxygen-Evolving Complex in Zostera marina, a Marine Angiosperm.

As an alternative electron sink, chlororespiration, comprising the NAD(P)H dehydrogenase complex and plastid terminal plastoquinone oxidase, may play a significant role in sustaining the redox equilibrium between stroma and thylakoid membrane. This study identified a distinct role for chlororespiration in the marine angiosperm Zostera marina, whose oxygen-evolving complex (OEC) is prone to photo-inactivation as a result of its inherent susceptibility to excess irradiation. The strong connectivity between OEC peripheral proteins and key chlororespiratory enzymes, as demonstrated in the interaction network of differentially expressed genes, suggested that the recovery of photo-inactivated OEC was connected with chlororespiration. Chlorophyll fluorescence, transcriptome and Western blot data verified a new physiological role for chlororespiration to function as photoprotection and generate a proton gradient across the thylakoid membrane for the recovery of photo-inactivated OEC. Chlororespiration was only activated in darkness following excess irradiation exposure, which might be related to electron deficiency in the electron transport chain because of the continuous impairment of the OEC. The activation of chlororespiration in Z. marina was prone to proactivity, which was also supported by the further activation of the oxidative pentose-phosphate pathway synthesizing NADPH to meet the demand of chlororespiration during darkness. This phenomenon is distinct from the common assumption that chlororespiration is prone to consuming redundant reducing power during the short transition phase from light to dark.

Physiological responses of eelgrass (Zostera marina) to ambient stresses such as herbicide, insufficient light, and high water temperature.

This study aimed to elucidate the biological responses of eelgrass (Zostera marina) to artificially induced stresses such as herbicide (Irgarol 1051, Irg) exposure, insufficient light, and high water temperature (27 ± 1.0 °C) by evaluating growth inhibition, photosynthetic activity, and metabolomic profiles. After 14 days, all treatments inhibited growth, but photosynthetic activity was only reduced in the Irg-exposed group. In the Irg-exposed and insufficient light groups, the metabolomic profiles were characterized by decreased levels of sugar (sucrose) and increased levels of amino acids such as glutamine, glycine, and leucine. Biochemical and ultrastructural analyses revealed that the loss of sugar-derived metabolic energy was compensated for by energy generated during autophagic protein degradation. Furthermore, the level of myo-inositol, which has various biological roles and participates in several cellular processes such as cell wall synthesis, stress response, and mineral nutrient storage, was markedly increased in the Irg-exposed and insufficient light groups. A combination of metabolomic analysis with other analyses such as measurement of photosynthetic activity might further elucidate the response of eelgrass to ambient stresses in the natural environment.

Interaction of high seawater temperature and light intensity on photosynthetic electron transport of eelgrass (Zostera marina L.).

The interaction of widely recognized causes of eelgrass decline (high seawater temperature and limited light intensity) on photosynthetic electron transport was investigated via chlorophyll fluorescence technique. High seawater temperature combined light intensity significantly increasing the relative maximum electron transport rate (rETRmax); at critical temperature of 30 °C, the rETRmax increased with the enhancement of light intensity, indicating the elevation of overall photosynthetic performance. Based on the magnitude of effect size (η2), light intensity was the predominant factor affecting the performance index (PIABS), indicating that photosystem II (PSII) was sensitive to light intensity. Moreover, the donor side was severely damaged as evidenced by the higher decrease amplitude of fast component and its subsequent incomplete recovery. The reaction center exhibited limited flexibility due to the slight decrease amplitude in maximum photochemical quantum yield. In contrast with PSII, photosystem I (PSI) was more sensitive to high seawater temperature, based on the magnitude of η2 derived from the maximal decrease in slope. High seawater temperature significantly increased PSI activity, plastoquinol reoxidation capacity, and probability for electron transfer to final PSI electron acceptors. Moreover, it combined elevated light intensity significantly stimulated the activity of cyclic electron flow (CEF) around PSI. Higher activity of both PSI and CEF contributed to balancing the linear electron transport via alleviating the over-reduction of the plastoquinone pool, exhibiting flexible regulation of photosynthetic electron transport at critical temperature. Therefore, limited light intensity decreased the tolerance of eelgrass to critical temperature, which might be a factor contributing factor in the observed decline.

Losing a winner: thermal stress and local pressures outweigh the positive effects of ocean acidification for tropical seagrasses.

Seagrasses are globally important coastal habitat-forming species, yet it is unknown how seagrasses respond to the combined pressures of ocean acidification and warming of sea surface temperature. We exposed three tropical species of seagrass (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri) to increasing temperature (21, 25, 30, and 35°C) and pCO2 (401, 1014, and 1949 µatm) for 7 wk in mesocosms using a controlled factorial design. Shoot density and leaf extension rates were recorded, and plant productivity and respiration were measured at increasing light levels (photosynthesis-irradiance curves) using oxygen optodes. Shoot density, growth, photosynthetic rates, and plant-scale net productivity occurred at 25°C or 30°C under saturating light levels. High pCO2 enhanced maximum net productivity for Z. muelleri, but not in other species. Z. muelleri was the most thermally tolerant as it maintained positive net production to 35°C, yet for the other species there was a sharp decline in productivity, growth, and shoot density at 35°C, which was exacerbated by pCO2 . These results suggest that thermal stress will not be offset by ocean acidification during future extreme heat events and challenges the current hypothesis that tropical seagrass will be a 'winner' under future climate change conditions.

Light intensity dependent photosynthetic electron transport in eelgrass (Zostera marina L.).

Responses of electron transport to three levels of irradiation (20, 200, and 1200 µmol photons m-2 s-1 PAR; exposures called LL, ML and HL, respectively) were investigated in eelgrass (Zostera marina L.) utilizing the chlorophyll a fluorescence technique. Exposure to ML and HL reduced the maximum quantum yield of photosystem II (PSII) (Fv/Fm) and the maximum slope decrease of MR/MRO (VPSI), indicating the occurrence of photoinhibition of both PSII and photosystem I (PSI). A comparatively slow recovery rate of Fv/Fm due to longer half-life recovery time of PSII and 40% lower descending amplitude compared to other higher plants implied the poor resilience of the PSII. Comparatively, PSI demonstrated high resilience and cyclic electron transport (CEF) around PSI maintained high activity. With sustained exposure, the amplitudes of the kinetic components (L1 and L2), the probability of electron transfer from PSII to plastoquinone pool (ψET2o), and the connectivity among PSII units decreased, accompanied by an enhancement of energy dissipation. Principle component analysis revealed that both VPSI and Fv/Fm contributed to the same component, which was consistent with high connectivity between PSII and PSI, suggesting close coordination between both photosystems. Such coordination was likely beneficial for the adaption of high light. Exposure to LL significantly increased the activity of both PSI and CEF, which could lead to increased light harvesting. Moreover, smooth electron transport as indicated by the enhancement of L1, L2, ψET2o and the probability of electron transport to the final PSI acceptor sides, could contribute to an increase in light utilization efficiency.

The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.

This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2 mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3 % of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2 mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

Pollutant tracking for 3 Western North Atlantic sea grasses by remote sensing: Preliminary diminishing white light responses of Thalassia testudinum, Halodule wrightii, and Zostera marina.

Sea grasses are foundation species for estuarine ecosystems. The available light for sea grasses diminishes rapidly during pollutant spills, effluent releases, disturbances such as intense riverine input, and tidal changes. We studied how sea grasses' remote-sensing signatures and light-capturing ability respond to short term light alterations. In vivo responses were measured over the entire visible-light spectra to diminishing white-light on whole-living-plants' spectral reflectance, including 6h of full oceanic-light fluences from 10% to 100%. We analyzed differences by various reflectance indices. We compared the sea grasses species responses of tropical vs. temperate and intertidals (Halodule wrightii, and Zostera marina) vs. subtidal (Thalassia testudinum). Reflectance diminished with decreasing light intensity that coincided with greater accessory pigment stimulation (anthocyanin, carotenoids, xanthins). Chlorophyll a and Chlorophyll b differed significantly among species (Thalassia vs. Halodule). Photosynthetic efficiency diminished at high light intensities. The NDVI index was inadequate to perceive these differences. Our results demonstrate the leaf-level utility of data to remote sensing for mapping sea grass and sea grass stress.

Simultaneous measurements of H+ and O2 fluxes in Zostera marina and its physiological implications.

Zostera marina (eelgrass) is an important ecological component of many shallow, temperate lagoons. Evidence suggests that Z. marina has a high bicarbonate utilization capability, which could be promoted by possible proton extrusion and the consequent formation of an 'acid zone' in the apoplastic space (unstirred layer) of its leaves. It has been found that 50 mM of the buffer Tris significantly inhibited the photosynthetic O(2) evolution of Z. marina and it was proposed that this was because of Tris's ability to bond with protons outside the cell wall. To investigate if H(+) played an important role in the photosynthetic carbon utilization of Z. marina, it is very important to simultaneously monitor the photosynthesis status and possible H(+) fluxes. However, probably because of the lack of suitable techniques, this has never been attempted. In this study, experiments were undertaken on Z. marina by monitoring H(+) and O(2) fluxes and the relative electron transport rates during light-dark transition. During stable photosynthesis, in addition to an obvious O(2) outflow, there was a significant net H(+) influx connected to Z. marina photosynthesis. The inhibitory effects of both Tris and respiration inhibitors on apparent O(2) evolution of Z. marina were confirmed. However, evidence did not support the proposed Tris inhibition mechanism.