Underwater Photosynthesis Research Articles

Response of underwater photosynthesis to light, CO2, temperature, and submergence time of Taxodium distichum, a flood-tolerant tree.

Partial or complete submergence of trees can occur in natural wetlands during times of high waters, but the submergence events have increased in severity and frequency over the past decades. Taxodium distichum is well-known for its waterlogging tolerance, but there are also numerous observations of this species becoming partially or complete submerged for longer periods of time. Consequently, the aims of the present study were to characterize underwater net photosynthesis (PN) and leaf anatomy of T. distichum with time of submergence. We completely submerged 6 months old seedling of T. distichum and diagnosed underwater (PN), hydrophobicity, gas film thickness, Chlorophyll concentration and needles anatomy at discrete time points during a 30-day submergence event. We also constructed response curves of underwater PN to CO2, light and temperature. During the 30-day submergence period, no growth or formation new leaves were observed, and therefore T. distichum shows a quiescence response to submergence. The hydrophobicity of the needles declined during the submergence event resulting in complete loss of gas films. However, the Chlorophyll concentration of the needles also declined significantly, and it was there not possible to identify the main cause of the corresponding significant decline in underwater PN. Nevertheless, even after 30 days of complete submergence, the needles still retained some capacity for underwater photosynthesis under optimal light and CO2 conditions. However, to fully understand the stunning submergence tolerance of T. distichum, we propose that future research concentrate on unravelling the finer details in needle anatomy and biochemistry as these changes occur during submergence.

Seasonal and spatial patterns of surface water quality in large-scale treatment wetlands with different vegetation communities

The Everglades Stormwater Treatment Areas (STAs), a network of large, constructed wetlands, were built to reduce phosphorus (P) in water discharged into downstream oligotrophic Everglades ecosystem. These wetlands are colonized by two main types of macrophyte communities: emergent aquatic vegetation (EAV) and submerged aquatic vegetation (SAV). The objective of this study was to compare the seasonal and spatial patterns of surface water quality within two flow-ways (FWs) in STA-2 (FW1 dominated by EAV and FW3 a mixed marsh with both EAV and SAV) and assess how surface water quality was affected by extreme storm events. Surface water samples were collected from three stations (near the inflow, middle of the FW, and near the outflow) along the flowpath of each FW four times from 2016 to 2018. Total P and its fractions generally decreased along the flowpath in both FWs while other elements did not show a consistent trend between seasons. Surface water quality in FW1 had more evident seasonal patterns and was less affected by 2017 storm events compared to that of FW3. The changes and patterns of water quality indicate that the processes (e.g., plant nutrient uptake, microbial assimilation, detritus decomposition and particle settling, etc.), which take the primary role of removing nutrients from the water column in FW1, vary not only due to the differences in plant growth but also to variations of hydraulic and nutrient loadings over wet and dry seasons. In FW3, on the contrary, our results indicate that the occlusion process facilitated by underwater photosynthesis of SAV likely enhanced the capacity of removing P and associated elements from water column and attenuated the seasonal differences. The SAV in FW3 also was susceptible to extreme events, with both short- and long-term adverse effects of storm events on surface water quality.

Cooperation of an external carbonic anhydrase and HCO3- transporter supports underwater photosynthesis in submerged leaves of the amphibious plant Hygrophila difformis.

HCO3- can be a major carbon resource for photosynthesis in underwater environments. Here we investigate the underlying mechanism of uptake and membrane transport of HCO3- in submerged leaves of Hygrophila difformis, a heterophyllous amphibious plant. To characterize these mechanisms, we evaluated the sensitivity of underwater photosynthesis to an external carbonic anhydrase (CA) inhibitor and an anion exchanger protein inhibitor, and we attempted to identify components of the mechanism of HCO3- utilization. We evaluated the effects of the external CA inhibitor and anion exchanger protein inhibitor on the NaHCO3 response of photosynthetic O2 evolution in submerged leaves of H. difformis. Furthermore, we performed a comparative transcriptomic analysis between terrestrial and submerged leaves. Photosynthesis in the submerged leaves was decreased by both the external CA inhibitor and anion exchanger protein inhibitor, but no additive effect was observed. Among upregulated genes in submerged leaves, two α-CAs, Hdα-CA1 and Hdα-CA2, and one β-carbonic anhydrase, Hdβ-CA1, were detected. Based on their putative amino acid sequences, the α-CAs are predicted to be localized in the apoplastic region. Recombinant Hdα-CA1 and Hdβ-CA1 showed dominant CO2 hydration activity over HCO3- dehydration activity. We propose that the use of HCO3- for photosynthesis in submerged leaves of H. difformis is driven by the cooperation between an external CA, Hdα-CA1, and an unidentified HCO3- transporter.

Photosynthesis, growth, and distribution of plants in lowland streams—A synthesis and new data analyses of 40 years research

Abstract This paper synthesises insights and offers new quantitative analyses of data gathered during 40 years of stream research by Danish researchers and international associates. Lowland Danish streams mostly drain fertile cropland and contain high nutrient concentrations saturating maximum yield and growth rate of plants. Concentrations of carbon dioxide (CO2) are variable, although usually high, supporting a range of wetland and permanently submerged species. All terrestrial and most amphibious species are obligate CO2 users, while the majority of permanently submerged species can supplement their inorganic carbon demand with bicarbonate. In lake outlets, the average CO2 concentration was close to air saturation during summer, whereas sites with no lake influence were 9‐fold supersaturated. The 20% increase of atmospheric CO2 concentrations over the past 40 years has marginal influence on CO2 concentrations in the streams. In lake outlets with low CO2 concentrations, calculations on 33 stream species showed essentially no underwater photosynthesis by temporarily submerged terrestrial species, low rates by amphibious species, and high rates by permanently submerged bicarbonate users. Underwater photosynthetic rates increased at sites with high CO2 concentrations (no lake influence): they were lowest for terrestrial wetland species (mean 1.8 mg O2 g dry weight−1 hr−1), followed by homophyllous (3.0) and heterophyllous amphibious species (5.6), and highest among permanently submerged species (15.0). Terrestrial and amphibious species grew very slowly when CO2 levels were low, but rapidly in CO2 rich water, or when in contact with air above the water's surface. Decreasing CO2 concentrations from upstream to downstream caused lower photosynthesis rates of amphibious species, while photosynthesis by bicarbonate users was consistently high. The relative abundance of terrestrial and amphibious species decreased significantly as CO2 resources decline from upstream to downstream, while the abundance of permanently submerged species increased as streams progressed. However, plant abundance as a function of CO2 concentrations did not differ markedly among the plant groups. We conclude that photosynthesis and growth of species of different plant types under controlled experimental conditions resembling high in situ nutrient availability are closely related to inorganic carbon supplies, while their representation in plant assemblages is influenced by the spatially and temporally highly diverse ecological conditions from upstream to downstream in the mostly CO2‐rich lowland streams. Submerged terrestrial and amphibious species restricted to CO2 use for photosynthesis can partly escape the slow gas diffusion under water by growing in very CO2‐rich streams and having apical leaves in contact with air. Thus, strong restrictions on their growth and existence should be identified at sites where CO2 concentrations are consistently low and no air contact is possible, while co‐limitation by nutrients is likely to be marginal. Generally, rising atmospheric CO2 is irrelevant to photosynthesis of permanently submerged plants because of their ability to use bicarbonate and the high CO2 supersaturation in most stream sites. In contrast, photosynthesis and growth of amphibious and terrestrial plants under water are highly dependent on CO2 concentrations varying from close to air saturation in lake outlets to supersaturation in headwaters.

Research Status and Trends of Underwater Photosynthesis

Underwater photosynthesis is the most important metabolic activity for submerged plants since it could utilize carbon fixation to replenish lost carbohydrates and improve internal aeration by producing O2. The present study used bibliometric methods to quantify the annual number of publications related to underwater photosynthesis. CiteSpace, as a visual analytic software for the literature, was employed to analyze the distribution of the subject categories, author collaborations, institution collaborations, international (regional) collaborations, and cocitation and keyword burst. The results show the basic characteristics of the literature, the main intellectual base, and the main research powers of underwater photosynthesis. Meanwhile, this paper revealed the research hotspots and trends of this field. This study provides an objective and comprehensive analysis of underwater photosynthesis from a bibliometric perspective. It is expected to provide reference information for scholars in related fields to refine the research direction, solve specific scientific problems, and assist scholars in seeking/establishing relevant collaborations in their areas of interest.

Adaptation of inorganic carbon utilization strategies in submerged and floating leaves of heteroblastic plant Ottelia cordata

During the life history, the aquatic heteroblastic plant undergoes submerged-leaf and floating/emerged-leaf phases, with dramatic changes in their surrounding environments. However, little is known about the physiological and biochemical changes of the two types of leaves adapting to submerged and aerial environments. In the present study, a heteroblastic plant Ottelia cordata was chosen to investigate the variations in morphology, anatomy, physiology, and biochemistry between the two types of leaves. The results showed that the floating leaf developed stomate to absorb atmospheric CO 2 for photosynthesis to adapt to the aerial habitat, while the submerged leaf showed longer, narrower, and thinner to maximize inorganic carbon uptake underwater. Through external carbonic anhydrase catalysis, both types of leaves could use HCO 3 - as an extra carbon source, and it was stronger in submerged leaves. Besides HCO 3 - use, the δ 13 C and the activity of C4 process key enzymes supported that the submerged leaf could perform C4 metabolism, while the floating leaves only carried on C3 metabolism. This is the first time to report an aquatic plant transiting from the C4 process underwater to the C3 process in the air. Based on the discrimination of stable carbon isotope, in the field, the main carbon source was atmospheric CO 2 for floating leaves and the free CO 2 underwater for submerged leaves. Additionally, HCO 3 - could approximately support 16% carbon source for underwater photosynthesis, which was the second important carbon source for submerged leaves. All our data indicated the heteroblastic aquatic plant adapts to underwater and aerial environments in morphology, anatomy, physiology, and biochemistry for completing life history and maintaining the population in the field. • Morphology and anatomy differed between submerged and floating leaves of O. cordata. • Both types of leaves could use HCO 3 - via external CA mediation for photosynthesis. • Submerged leaf performed C4 metabolism, floating leaf only carried on C3 metabolism. • Atmospheric CO 2 was the main carbon source for floating leaf. • Soluble CO 2 and HCO 3 - underwater were the two main carbon sources for submerged leaf.

Genome sequencing and transcriptome analyses provide insights into the origin and domestication of water caltrop (Trapa spp., Lythraceae).

SummaryHumans have domesticated diverse species from across the plant kingdom; however, our current understanding of plant domestication is largely founded on major cereal crops. Here, we examine the evolutionary processes and genetic basis underlying the domestication of water caltrop (Trapa spp., Lythraceae), a traditional, yet presently underutilized non‐cereal crop that sustained early Chinese agriculturalists. We generated a chromosome‐level genome assembly of tetraploid T. natans, and then divided the allotetraploid genome into two subgenomes. Based on resequencing data from 57 accessions, representing cultivated diploid T. natans, wild T. natans (2x and 4x) and diploid T. incisa, we showed that water caltrop was likely first domesticated in the Yangtze River Valley as early as 6300 yr BP, and experienced a second improvement c. 800 years ago. We also provided strong support for an allotetraploid origin of T. natans within the past 230 000–310 000 years. By integrating selective sweep and transcriptome profiling analyses, we identified a number of genes potentially selected and/or differentially expressed during domestication, some of which likely contributed not only to larger fruit sizes but also to a more vigorous root system, facilitating nutrient uptake, environmental stress response and underwater photosynthesis. Our results shed light on the evolutionary and domestication history of water caltrop, one of the earliest domesticated crops in China. This study has implications for genomic‐assisted breeding of this presently underutilized aquatic plant, and improves our general understanding of plant domestication.

Transition From Proto-Kranz-Type Photosynthesis to HCO3 - Use Photosynthesis in the Amphibious Plant Hygrophila polysperma.

Hygrophila polysperma is a heterophyllous amphibious plant. The growth of H. polysperma in submerged conditions is challenging due to the low CO2 environment, increased resistance to gas diffusion, and bicarbonate ion (HCO3–) being the dominant dissolved inorganic carbon source. The submerged leaves of H. polysperma have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4′-Diisothiocyanatostilbene-2,2′-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that H. polysperma acclimates to the submerged condition by using HCO3– for photosynthesis. H. polysperma transports HCO3– into the leaf by a DIDS-sensitive HCO3– transporter and converted to CO2 by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that H. polysperma is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that H. polysperma performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO3– use photosynthesis in the submerged environments.

Two Brassica napus cultivars differ in gene expression, but not in their response to submergence.

Heavy rainfall causes flooding of natural ecosystems as well as farmland, negatively affecting plant performance. While the responses of the wild model organism Arabidopsis thaliana to such stress conditions is well understood, little is known about the responses of its relative, the important oil crop plant Brassica napus. For the first time, we analyzed the molecular response of Brassica napus seedlings to full submergence in a natural light-dark cycle. We used two cultivars in this study, a European hybrid cultivar and an Asian flood-tolerant cultivar. Despite their genomic differences, those genotypes showed no major differences in their responses to submergence. The molecular responses to submergence included the induction of defense- and hormone-related pathways and the repression of biosynthetic processes. Furthermore, RNAseq revealed a strong carbohydrate-starvation response under submergence in daylight, which corresponded with a fast depletion of sugars. Consequently, both B. napus cultivars exhibited a strong growth repression under water, but there was no indication of a low-oxygen response. The ability of the European hybrid cultivar to form a short-lived leaf gas film neither increased underwater net photosynthesis, underwater dark respiration nor growth during submergence. Due to the high sensitivity of both cultivars, the analysis of other cultivars or related species with higher submergence tolerance is required in order to improve flood tolerance of this crop species. One major target could be the improvement of underwater photosynthesis efficiency in order to enhance submergence survival.

Photosynthetic acclimation of terrestrial and submerged leaves in the amphibious plant Hygrophila difformis.

Hygrophila difformis, a heterophyllous amphibious plant, develops serrated or dissected leaves when grown in terrestrial or submerged conditions, respectively. In this study, we tested whether submerged leaves and ethylene-induced leaves of the heterophyllous, amphibious plant H. difformis have improved photosynthetic ability under submerged conditions. Also, we investigated how this amphibious plant photosynthesizes underwater and whether a HCO3− transport system is present. We have analysed leaf morphology, measured underwater photosynthetic rates and HCO3− affinity in H. difformis to determine if there are differences in acclimation ability dependent on growth conditions: terrestrial, submerged, terrestrial treated with ethylene and submerged treated with an ethylene inhibitor. Moreover, we measured time courses for changes in leaf anatomical characteristics and underwater photosynthesis in terrestrial leaves after submersion. Compared with the leaves of terrestrially grown plants, leaf thickness of submerged plants was significantly thinner. The stomatal density on the abaxial surface of submerged leaves was also reduced, and submerged plants had a significantly higher O2 evolution rate. When the leaves of terrestrially grown plants were treated with ethylene, their leaf morphology and underwater photosynthesis increased to levels comparable to those of submerged leaves. Underwater photosynthesis of terrestrial leaves was significantly higher by 5 days after submersion. In contrast, leaf morphology did not change after submergence. Submerged leaves and submerged terrestrial leaves were able to use bicarbonate but submerged terrestrial leaves had an intermediate ability to use HCO3− that was between terrestrial leaves and submerged leaves. Ethoxyzolamide, an inhibitor of intracellular carbonic anhydrase, significantly inhibited underwater photosynthesis in submerged leaves. This amphibious plant acclimates to the submerged condition by changing leaf morphology and inducing a HCO3− utilizing system, two processes that are regulated by ethylene.

CO2 and O2 dynamics in leaves of aquatic plants with C3 or CAM photosynthesis - application of a novel CO2 microsensor.

Leaf tissue CO2 partial pressure (pCO2) shows contrasting dynamics over a diurnal cycle in C3 and Crassulacean Acid Metabolism (CAM) plants. However, simultaneous and continuous monitoring of pCO2 and pO2 in C3 and CAM plants under the same conditions was lacking. Our aim was to use a new CO2 microsensor and an existing O2 microsensor for non-destructive measurements of leaf pCO2 and pO2 dynamics to compare a C3 and a CAM plant in an aquatic environment. A new amperometric CO2 microsensor and an O2 microsensor elucidated with high temporal resolution the dynamics in leaf pCO2 and pO2 during light-dark cycles for C3Lobelia dortmanna and CAM Littorella uniflora aquatic plants. Underwater photosynthesis, dark respiration, tissue malate concentrations and sediment CO2 and O2 were also measured. During the dark period, for the C3 plant, pCO2 increased to approx. 3.5 kPa, whereas for the CAM plant CO2 was mostly below 0.05 kPa owing to CO2 sequestration into malate. Upon darkness, the CAM plant had an initial peak in pCO2 (approx. 0.16 kPa) which then declined to a quasi-steady state for several hours and then pCO2 increased towards the end of the dark period. The C3 plant became severely hypoxic late in the dark period, whereas the CAM plant with greater cuticle permeability did not. Upon illumination, leaf pCO2 declined and pO2 increased, although aspects of these dynamics also differed between the two plants. The continuous measurements of pCO2 and pO2 highlighted the contrasting tissue gas compositions in submerged C3 and CAM plants. The CAM leaf pCO2 dynamics indicate an initial lag in CO2 sequestration to malate, which after several hours of malate synthesis then slows. Like the use of O2 microsensors to resolve questions related to plant aeration, deployment of the new CO2 microsensor will benefit plant ecophysiology research.

Rice leaf hydrophobicity and gas films are conferred by a wax synthesis gene (LGF1) and contribute to flood tolerance.

Floods impede gas (O2 and CO2 ) exchange between plants and the environment. A mechanism to enhance plant gas exchange under water comprises gas films on hydrophobic leaves, but the genetic regulation of this mechanism is unknown. We used a rice mutant (dripping wet leaf 7, drp7) which does not retain gas films on leaves, and its wild-type (Kinmaze), in gene discovery for this trait. Gene complementation was tested in transgenic lines. Functional properties of leaves as related to gas film retention and underwater photosynthesis were evaluated. Leaf Gas Film 1 (LGF1) was identified as the gene determining leaf gas films. LGF1 regulates C30 primary alcohol synthesis, which is necessary for abundant epicuticular wax platelets, leaf hydrophobicity and gas films on submerged leaves. This trait enhanced underwater photosynthesis 8.2-fold and contributes to submergence tolerance. Gene function was verified by a complementation test of LGF1 expressed in the drp7 mutant background, which restored C30 primary alcohol synthesis, wax platelet abundance, leaf hydrophobicity, gas film retention, and underwater photosynthesis. The discovery of LGF1 provides an opportunity to better understand variation amongst rice genotypes for gas film retention ability and to target various alleles in breeding for improved submergence tolerance for yield stability in flood-prone areas.

Pseudomonas putida inoculation promotes submerged plant Vallisneria natans growth by carbon conversion in a plant–microbe interaction

Submerged plant growth is limited by the reduction of underwater photosynthesis attributed to low CO2 availability, as well as light limitation associated with underwater conditions. Heterotrophic bacteria and fungi play an important role in local aqueous dissolved inorganic carbon (DIC) content surrounding submerged plants. In order to investigate the effects of carbon conversion in plant–microbe interactions on plant growth, in the present study we inoculated the plant medium of Vallisneria natans with Pseudomonas putida KT2440 and measured carbon conversion in the system, as well as several indices of plant growth. The quantity of P. putida KT2440 increased twofold because of the availability of organic matter produced by V. natans. Similarly, P. putida KT2440 supplied DIC for V. natans, improving its photosynthetic rate. Moreover, the significantly higher leaf area, specific leaf area and fresh biomass of V. natans attributed to the presence of P. putida KT2440 demonstrated that the interaction between V. natans and P. putida enhanced the efficiency of nutrient and CO2 uptake by V. natans, promoting V. natans growth. Therefore, we suggest that the carbon and oxygen microcycle based on the protocooperation of V. natans and P. putida KT2440 may accelerate the transformation of carbon to increase carbon availability to promote the growth of both plant and microbe.

Leaf gas films contribute to rice (Oryza sativa) submergence tolerance during saline floods.

Floods and salinization of agricultural land adversely impact global rice production. We investigated whether gas films on leaves of submerged rice delay salt entry during saline submergence. Two-week-old plants with leaf gas films (+GF) or with gas films experimentally removed (-GF) were submerged in artificial floodwater with 0 or 50 mm NaCl for up to 16 d. Gas films were present >9 d on GF plants after which gas films were diminished. Tissue ion analysis (Na+ , Cl- and K+ ) showed that gas films caused some delay of Na+ entry, as leaf Na+ concentration was 36-42% higher in -GF leaves than +GF leaves on days 1-5. However, significant net uptakes of Na+ and Cl- , and K+ net loss, occurred despite the presence of gas films, indicating the likely presence of some leaf-to-floodwater contact, so that the gas layer must not have completely separated the leaf surfaces from the water. Natural loss and removal of gas films resulted in severe declines in growth, underwater photosynthesis, chlorophylla and tissue porosity. Submergence was more detrimental to leaf PN and growth than the additional effect of 50 mm NaCl, as salt did not significantly affect underwater PN at 200 μm CO2 nor growth.

Leaf gas films, underwater photosynthesis and plant species distributions in a flood gradient.

Traits for survival during flooding of terrestrial plants include stimulation or inhibition of shoot elongation, aerenchyma formation and efficient gas exchange. Leaf gas films form on superhydrophobic cuticles during submergence and enhance underwater gas exchange. The main hypothesis tested was that the presence of leaf gas films influences the distribution of plant species along a natural flood gradient. We conducted laboratory experiments and field observations on species distributed along a natural flood gradient. We measured presence or absence of leaf gas films and specific leaf area of 95 species. We also measured, gas film retention time during submergence and underwater net photosynthesis and dark respiration of 25 target species. The presence of a leaf gas film was inversely correlated to flood frequency and duration and reached a maximum value of 80% of the species in the rarely flooded locations. This relationship was primarily driven by grasses that all, independently of their field location along the flood gradient, possess gas films when submerged. Although the present study and earlier experiments have shown that leaf gas films enhance gas exchange of submerged plants, the ability of species to form leaf gas films did not show the hypothesized relationship with species composition along the flood gradient.

Harnessing facilitation: Why successful re-introduction of Stratiotes aloides requires high densities under high nitrogen loading

Dense mats of floating vegetation form complex structures that support high biodiversity in wetlands. Since the 1960s, however, high-density stands of Stratiotes aloides have shown strong declines throughout Europe and restoration efforts have often failed, both of which have been linked to high nitrogen (N) input rates. The low success rate of reintroduction is striking, since seemingly healthy, high-density stands are observed to grow under similar environmental conditions. We therefore hypothesise that N tolerance of S. aloides increases with density due to joint N uptake and detoxification. To test our hypothesis, we set up a controlled full factorial mesocosm experiment in which we manipulated N loading and S. aloides density, and investigated interacting effects on habitat biogeochemistry and plant performance. High-density S. aloides stands strongly lowered ammonium availability through shared uptake, even at extremely high N loads. Furthermore, dense stands strongly reduced water layer oxygen concentrations, which stimulated sediment phosphorus mobilisation, and increased carbon dioxide concentrations, which enhanced underwater photosynthesis. High-density stands thus not only detoxify ammonium, but also facilitate population growth through habitat manipulation. Combined, these mechanisms enabled S. aloides to remain viable at high loads of 800kgNha−1y−1, whereas low-density stands already collapsed at 200kgNha−1y−1. Overall, our results show that under similar environmental conditions, high-density stands can thrive, while restoration or natural recruitment by a low number of individuals may be impossible. We therefore conclude that acknowledging and harnessing intraspecific facilitation can be vital for successful conservation and restoration.

To Float or Not to Float: How Interactions between Light and Dissolved Inorganic Carbon Species Determine the Buoyancy of Stratiotes aloides.

Structural diversity formed by dense, floating Stratiotes aloides stands, generates hotspots of biodiversity of flora and fauna in wetlands. However, only part of the populations become emergent and provide this important facilitation. Since it has been hypothesised that its buoyancy depends on the rates of underwater photosynthesis, we investigated the role of dissolved CO2 availability and PAR on photosynthesis, biomass production and buoyancy in a controlled greenhouse experiment. Photosynthesis and growth were strongly influenced by both PAR and CO2 availability. At low PAR, plants formed less biomass and produced no emergent leaves, even when CO2 was abundant. At low CO2 levels, S. aloides switched to HCO3 - use, resulting in a lower photosynthetic O2 production, decreased emergent leaf formation and increased CaCO3 precipitation on its leaves, all of which impaired buoyancy. At high PAR, low CO2 availability resulted in slower colonisation of the water layer, whereas CO2 availability did not influence PAR-limited plants. Our study shows that site conditions, rather than the sole abundance of potentially facilitating species, may strongly determine whether or not they form the structure necessary to act as a facilitator for biodiversity in aquatic environments.

Diurnal hydrochemical variations in a karst spring and two ponds, Maolan Karst Experimental Site, China: Biological pump effects

• There are almost no diurnal hydrochemical variations in the spring waters. • In the surface waters, however, all parameters show distinct diurnal changes. • Hydrochemistry of land surface waters may be strongly modified by submerged photosynthesis. • Land surface water systems may constitute an important sink of carbon. A karst spring and two downstream ponds fed by the spring at the Maolan Karst Experimental Site, Guizhou Province, China, were used to investigate the effect of submerged plants on the CO 2 –H 2 O–CaCO 3 system during a time of spring base flow in summer when underwater photosynthesis was strongest. Temperature, pH, electrical conductivity (EC) and dissolved oxygen (DO) were recorded at 15 min intervals for a period of 30 h (12:00 29 August–18:00 30 August, 2012). [Ca 2+ ], HCO 3 - , CO 2 partial pressure ( p CO 2 ) and saturation index of calcite (SI C ) were estimated from the high-frequency measurements. Water samples were also collected three times a day (early morning, midday and evening) for δ 13 C DIC determination. A floating CO 2 -flux monitoring chamber was used to measure CO 2 flux at the three locations. Results show that there was little or no diurnal variation in the spring water parameters. In the midstream pond with flourishing submerged plants, however, all parameters show distinct diurnal changes: temperature, pH, DO, SIc, δ 13 C DIC increased during the day and decreased at night, while EC, HCO 3 - , [Ca 2+ ], and p CO 2 behaved in the opposite sense. In addition, maximum DO values (16–23 mg/L) in the midstream pond at daytime were two to three times those of water equilibrated with atmospheric O 2 , indicating strong aquatic photosynthesis. The proposed photosynthesis is corroborated by the low calculated p CO 2 of 20–200 ppmv, which is much less than atmospheric p CO 2 . In the downstream pond with fewer submerged plants but larger volume, all parameters displayed similar trends to the midstream pond but with much less change, a pattern that we attribute to the lower biomass/water volume ratio. The diurnal hydrobiogeochemical variations in the two ponds depended essentially on illumination, indicating that photosynthesis and respiration by the submerged plants are the dominant controlling processes. The large loss of DIC between the spring and midstream pond, attributed to biological pump effects, demonstrates that natural surface water systems may constitute an important sink of carbon (on the order of a few hundred tons of C km 2 /a) as DIC is transformed to autochthonous organic matter. The rates of sedimentary deposition and preservation of this organic matter in the ponds, however, require quantification in future work to fully assess the karst processes-related carbon sink, especially under global climate and land use changes.

Contrasting submergence tolerance in two species of stem-succulent halophytes is not determined by differences in stem internal oxygen dynamics.

Many stem-succulent halophytes experience regular or episodic flooding events, which may compromise gas exchange and reduce survival rates. This study assesses submergence tolerance, gas exchange and tissue oxygen (O2) status of two stem-succulent halophytes with different stem diameters and from different elevations of an inland marsh. Responses to complete submergence in terms of stem internal O2 dynamics, photosynthesis and respiration were studied for the two halophytic stem-succulents Tecticornia auriculata and T. medusa. Plants were submerged in a glasshouse experiment for 3, 6 and 12 d and O2 levels within stems were measured with microelectrodes. Photosynthesis by stems in air after de-submergence was also measured. Tecticornia medusa showed 100 % survival in all submergence durations whereas T. auriculata did not survive longer than 6 d of submergence. O2 profiles and time traces showed that when submerged in water at air-equilibrium, the thicker stems of T. medusa were severely hypoxic (close to anoxic) when in darkness, whereas the smaller diameter stems of T. auriculata were moderately hypoxic. During light periods, underwater photosynthesis increased the internal O2 concentrations in the succulent stems of both species. Stems of T. auriculata temporally retained a gas film when first submerged, whereas T. medusa did not. The lower O2 in T. medusa than in T. auriculata when submerged in darkness was largely attributed to a less permeable epidermis. The submergence sensitivity of T. auriculata was associated with swelling and rupturing of the succulent stem tissues, which did not occur in T. medusa. The higher submergence tolerance of T. medusa was not associated with better internal aeration of stems. Rather, this species has poor internal aeration of the succulent stems due to its less permeable epidermis; the low epidermal permeability might be related to resistance to swelling of succulent stem tissues when submerged.

Gas film retention and underwater photosynthesis during field submergence of four contrasting rice genotypes.

Floods can completely submerge some rice (Oryza sativa L.) fields. Leaves of rice have gas films that aid O2 and CO2 exchange under water. The present study explored the relationship between gas film persistence and underwater net photosynthesis (PN) as influenced by genotype and submergence duration. Four contrasting genotypes (FR13A, IR42, Swarna, and Swarna-Sub1) were submerged for 13 days in the field and leaf gas films, chlorophyll, and the capacity for underwater PN at near ambient and high CO2 were assessed with time of submergence. At high CO2 during the PN assay, all genotypes initially showed high rates of underwater PN, and this rate was not affected by time of submergence in FR13A. This superior photosynthetic performance of FR13A was not evident in Swarna-Sub1 (carrying the SUB1 QTL) and the declines in underwater PN in both Swarna-Sub1 and Swarna were equal to that in IR42. At near ambient CO2 concentration, underwater PN declined in all four genotypes and this corresponded with loss of leaf gas films with time of submergence. FR13A retained leaf gas films moderately longer than the other genotypes, but gas film retention was not linked to SUB1. Diverse rice germplasm should be screened for gas film persistence during submergence, as this trait could potentially increase carbohydrate status and internal aeration owing to increased underwater PN, which contributes to submergence tolerance in rice.