Comparative effects of heat stress on photosynthesis and oxidative stress in Halophila ovalis and Thalassia hemprichii under different light conditions.

This study investigated the physiological responses of two tropical seagrass species, Halophila ovalis and Thalassia hemprichii, to heat stress under varying light conditions in a controlled 5-day experiment. The experimental design included four treatments: control, saturating light, heat stress under sub-saturating light, and heat stress under saturating light (combined stress). We assessed various parameters, including chlorophyll fluorescence, levels of reactive oxygen species (ROS), antioxidant enzyme activities, and growth rates. In H. ovalis, heat stress resulted in a significant reduction in the maximum quantum yield of photosystem II (Fv/Fm) regardless of the light condition. However, the effects of heat stress on the effective quantum yield of photosystem II (ɸPSII) were more pronounced under saturating light conditions. In T. hemprichii, saturating irradiance exacerbated the heat stress effects on Fv/Fm and ɸPSII, although the overall photoinhibition was less severe than in H. ovalis. Heat stress led to ROS accumulation in H. ovalis and reduced the activity of superoxide dismutase (SOD) and ascorbate peroxidase in the sub-saturating light condition. Conversely, T. hemprichii exhibited elevated SOD activity under saturating light. Heat stress suppressed the growth of both seagrass species, regardless of the light environment. The Biomarker Response Index indicated that H. ovalis displayed severe effects in the heat stress treatment under both light conditions, while T. hemprichii exhibited moderate effects in sub-saturating light and major effects in saturating light conditions. However, the Effect Addition Index revealed an antagonistic interaction between heat stress and high light in both seagrass species. This study underscores the intricate responses of seagrasses, emphasizing the importance of considering both local and global stressors when assessing their vulnerability.

Comparative study on anatomical traits and gas exchange responses due to belowground hypoxic stress and thermal stress in three tropical seagrasses.

BACKGROUND: The ability to maintain sufficient oxygen levels in the belowground tissues and the rhizosphere is crucial for the growth and survival of seagrasses in habitats with highly reduced sediment. Such ability varies depending on plant anatomical features and environmental conditions. METHODS: In the present study, we compared anatomical structures of roots, rhizomes and leaves of the tropical intertidal seagrasses, Cymodocea rotundata, Thalassia hemprichii and Halophila ovalis, followed by an investigation of their gas exchange both in the belowground and aboveground tissues and photosynthetic electron transport rates (ETR) in response to experimental manipulations of O2 level (normoxia and root hypoxia) and temperature (30 °C and 40 °C). RESULTS: We found that C. rotundata and T. hemprichii displayed mostly comparable anatomical structures, whereas H. ovalis displayed various distinctive features, including leaf porosity, number and size of lacunae in roots and rhizomes and structure of radial O2 loss (ROL) barrier. H. ovalis also showed unique responses to root hypoxia and heat stress. Root hypoxia increased O2 release from belowground tissues and overall photosynthetic activity of H. ovalis but did not affect the other two seagrasses. More pronounced warming effects were detected in H. ovalis, measured as lower O2 release in the belowground tissues and overall photosynthetic capacity (O2 release and dissolved inorganic carbon uptake in the light and ETR). High temperature inhibited photosynthesis of C. rotundata and T. hemprichii but did not affect their O2 release in belowground tissues. Our data show that seagrasses inhabiting the same area respond differently to root hypoxia and temperature, possibly due to their differences in anatomical and physiological attributes. Halophila ovalis is highly dependent on photosynthesis and appears to be the most sensitive species with the highest tendency of O2 loss in hypoxic sediment. At the same time, its root oxidation capacity may be compromised under warming scenarios.

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.

Optimum Temperatures for Net Primary Productivity of Three Tropical Seagrass Species.

Rising sea water temperature will play a significant role in responses of the world's seagrass meadows to climate change. In this study, we investigated seasonal and latitudinal variation (spanning more than 1,500 km) in seagrass productivity, and the optimum temperatures at which maximum photosynthesis and net productivity (for the leaf and the whole plant) occurs, for three seagrass species (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri). To obtain whole plant net production, photosynthesis, and respiration rates of leaves and the root/rhizome complex were measured using oxygen-sensitive optodes in closed incubation chambers at temperatures ranging from 15 to 43°C. The temperature-dependence of photosynthesis and respiration was fitted to empirical models to obtain maximum metabolic rates and thermal optima. The thermal optimum (Topt) for gross photosynthesis of Z. muelleri, which is more commonly distributed in sub-tropical to temperate regions, was 31°C. The Topt for photosynthesis of the tropical species, H. uninervis and C. serrulata, was considerably higher (35°C on average). This suggests that seagrass species are adapted to water temperature within their distributional range; however, when comparing among latitudes and seasons, thermal optima within a species showed limited acclimation to ambient water temperature (Topt varied by 1°C in C. serrulata and 2°C in H. uninervis, and the variation did not follow changes in ambient water temperature). The Topt for gross photosynthesis were higher than Topt calculated from plant net productivity, which includes above- and below-ground respiration for Z. muelleri (24°C) and H. uninervis (33°C), but remained unchanged at 35°C in C. serrulata. Both estimated plant net productivity and Topt are sensitive to the proportion of below-ground biomass, highlighting the need for consideration of below- to above-ground biomass ratios when applying thermal optima to other meadows. The thermal optimum for plant net productivity was lower than ambient summer water temperature in Z. muelleri, indicating likely contemporary heat stress. In contrast, thermal optima of H. uninervis and C. serrulata exceeded ambient water temperature. This study found limited capacity to acclimate: thus the thermal optima can forewarn of both the present and future vulnerability to ocean warming during periods of elevated water temperature.

Model fit versus biological relevance: Evaluating photosynthesis-temperature models for three tropical seagrass species.

When several models can describe a biological process, the equation that best fits the data is typically considered the best. However, models are most useful when they also possess biologically-meaningful parameters. In particular, model parameters should be stable, physically interpretable, and transferable to other contexts, e.g. for direct indication of system state, or usage in other model types. As an example of implementing these recommended requirements for model parameters, we evaluated twelve published empirical models for temperature-dependent tropical seagrass photosynthesis, based on two criteria: (1) goodness of fit, and (2) how easily biologically-meaningful parameters can be obtained. All models were formulated in terms of parameters characterising the thermal optimum (Topt) for maximum photosynthetic rate (Pmax). These parameters indicate the upper thermal limits of seagrass photosynthetic capacity, and hence can be used to assess the vulnerability of seagrass to temperature change. Our study exemplifies an approach to model selection which optimises the usefulness of empirical models for both modellers and ecologists alike.

Light Levels Affect Carbon Utilisation in Tropical Seagrass under Ocean Acidification.

Under future ocean acidification (OA), increased availability of dissolved inorganic carbon (DIC) in seawater may enhance seagrass productivity. However, the ability to utilise additional DIC could be regulated by light availability, often reduced through land runoff. To test this, two tropical seagrass species, Cymodocea serrulata and Halodule uninervis were exposed to two DIC concentrations (447 µatm and 1077 µatm pCO2), and three light treatments (35, 100, 380 µmol m(-2) s(-1)) for two weeks. DIC uptake mechanisms were separately examined by measuring net photosynthetic rates while subjecting C. serrulata and H. uninervis to changes in light and addition of bicarbonate (HCO3-) use inhibitors (carbonic anhydrase inhibitor, acetazolamide) and TRIS buffer (pH 8.0). We observed a strong dependence on energy driven H+-HCO3- co-transport (TRIS, which disrupts H+ extrusion) in C. serrulata under all light levels, indicating greater CO2 dependence in low light. This was confirmed when, after two weeks exposure, DIC enrichment stimulated maximum photosynthetic rates (Pmax) and efficiency (α) more in C. serrulata grown under lower light levels (36-60% increase) than for those in high light (4% increase). However, C. serrulata growth increased with both DIC enrichment and light levels. Growth, NPP and photosynthetic responses in H. uninervis increased with higher light treatments and were independent of DIC availability. Furthermore, H. uninervis was found to be more flexible in HCO3- uptake pathways. Here, light availability influenced productivity responses to DIC enrichment, via both carbon fixation and acquisition processes, highlighting the role of water quality in future responses to OA.