The formation of iron plaques on roots and rhizomes of the seagrass Cymodocea serrulata (R. Brown) Ascherson with implications for sulphide intrusion

Despite the fact that iron plaque formation is ubiquitous in aquatic macrophytes and has been known for several decades, there are few reports of plaque occurrence in seagrasses to date. Herein we present the first microscopical observation and chemical quantification of iron (Fe) plaques on the shoots, rhizomes and roots of the seagrass Cymodocea serrulata (R. Brown) Ascherson collected from intertidal seagrass beds in Thailand. Plaques were observed on shoot bases, rhizomes and roots with the highest concentrations of iron in the plaques from the roots, reaching an average of 509 μmol gDW⁻¹. Interestingly, the most negative stable sulphur isotope (δ³⁴S) values, indicating H₂S intrusion into the plants occurred in the sampling site with the most intense root oxidizing capacity, as indicated by a greater Fe plaque formation. These apparently contradictory findings may be attributed to oxidizing capacity of root tips and root hairs sufficient to promote Fe(III) deposition in the rhizosphere, preceding deposition of plaques on the roots. While this rhizosphere oxidation may result in a more efficient sulphide detoxification during the day photosynthetic phase, root tips and hairs may serve as vulnerable sites for sulphide intrusion at night. The presence of Fe plaque on C. serrulata roots and rhizomes reveals the complexity of seagrass–sediment interactions and deserves further attention to understand if this is a local phenomenon or a newly discovered adaptive mechanism in seagrasses.     

Leaf gas films of Spartina anglica enhance rhizome and root oxygen during tidal submergence

Gas films on hydrophobic surfaces of leaves of some wetland plants can improve O₂ and CO₂ exchange when completely submerged during floods. Here we investigated the in situ aeration of rhizomes of cordgrass (Spartina anglica) during natural tidal submergence, with focus on the role of leaf gas films on underwater gas exchange. Underwater net photosynthesis was also studied in controlled laboratory experiments. In field experiments, O₂ microelectrodes were inserted into rhizomes and pO₂ measured throughout two tidal submergence events; one during daylight and one during night‐time. Plants had leaf gas films intact or removed. Rhizome pO₂ dropped significantly during complete submergence and most severely during night. Leaf gas films: (1) enhanced underwater photosynthesis and pO₂ in rhizomes remained above 10 kPa during submergence in light; and (2) facilitated O₂ entry from the water into leaves so that rhizome pO₂ was about 5 kPa during darkness. This study is the first in situ demonstration of the beneficial effects of leaf gas films on internal aeration in a submerged wetland plant. Leaf gas films likely contribute to submergence tolerance of S. anglica and this feature is expected to also benefit other wetland plant species when submerged.     

Global warming enhances sulphide stress in a key seagrass species (NW Mediterranean)

The build‐up of sulphide concentrations in sediments, resulting from high inputs of organic matter and the mineralization through sulphate reduction, can be lethal to the benthos. Sulphate reduction is temperature dependent, thus global warming may contribute to even higher sulphide concentrations and benthos mortality. The seagrass Posidonia oceanica is very sensitive to sulphide stress. Hence, if concentrations build up with global warming, this key Mediterranean species could be seriously endangered. An 8‐year monitoring of daily seawater temperature, the sulphur isotopic signatures of water (δ³⁴S_water), sediment (δ³⁴S_CRS) and P. oceanica leaf tissue (δ³⁴S_leaves), along with total sulphur in leaves (TS_leaves) and annual net population growth along the coast of the Balearic archipelago (Western Mediterranean) allowed us to determine if warming triggers P. oceanica sulphide stress and constrains seagrass survival. From the isotopic S signatures, we estimated sulphide intrusion into the leaves (F_sulphide) and sulphur incorporation into the leaves from sedimentary sulphides (SS_leaves). We observed lower δ³⁴S_leaves, higher F_sulphide and SS_leaves coinciding with a 6‐year period when two heat waves were recorded. Warming triggered sulphide stress as evidenced by the negative temperature dependence of δ³⁴S_leaves and the positive one of F_sulphide, TS_leaves and SS_leaves. Lower P. oceanica net population growth rates were directly related to higher contents of TS_leaves. At equivalent annual maximum sea surface water temperature (SST_max), deep meadows were less affected by sulphide intrusion than shallow ones. Thus, water depth acts as a protecting mechanism against sulphide intrusion. However, water depth would be insufficient to buffer seagrass sulphide stress triggered by Mediterranean seawater summer temperatures projected for the end of the 21st century even under scenarios of moderate greenhouse gas emissions, A1B. Mediterranean warming, therefore, is expected to enhance P. oceanica sulphide stress, and thus compromise the survival of this key habitat along its entire depth distribution range.     

Flow velocity affects internal oxygen conditions in the seagrass Cymodocea nodosa

The internal oxygen status of seagrass tissues, which is believed to play an important role in events of seagrass die-off, is partly determined by the rates of gas exchange between leaves and water column. In this study, we examined whether water column flow velocity has an effect on gas exchange, and hence on internal oxygen partial pressures (pO₂) in the Mediterranean seagrass, Cymodocea nodosa. We measured the internal pO₂ in the horizontal rhizomes of C. nodosa in darkness at different mainstream flow velocities, combined with different levels of water column oxygen pO₂ using an experimental flume in the laboratory. Flow velocity clearly had an effect on the internal oxygen status. In stagnant, but fully aerated water the mean internal pO₂ was 6.9 kPa, corresponding to about 30% of air saturation. The internal pO₂ increased with increasing flow velocity reaching saturation of around 12.2 kPa (60% of air saturation) at flow velocities ≥7 cm s⁻¹. Flow had a relatively larger influence on internal pO₂ at lower water column oxygen concentrations. By extrapolating linear relationships between internal and water column pO₂ in this experimental setup, rhizomes would become anoxic at a water column oxygen pO₂ of 4–4.5 kPa (∼20% of air saturation) in flowing water, but already at 6.4 kPa (∼30% of air saturation) in stagnant water. Water flow may play an important role for seagrass performance and survival in areas with poor water column oxygen conditions and may, in general, be of importance for the distribution of submerged rooted plants.     

Use of carbon monoxide and hydrogen by a bacteria–animal symbiosis from seagrass sediments

The gutless marine worm Olavius algarvensis lives in symbiosis with chemosynthetic bacteria that provide nutrition by fixing carbon dioxide (CO₂) into biomass using reduced sulfur compounds as energy sources. A recent metaproteomic analysis of the O. algarvensis symbiosis indicated that carbon monoxide (CO) and hydrogen (H₂) might also be used as energy sources. We provide direct evidence that the O. algarvensis symbiosis consumes CO and H₂. Single cell imaging using nanoscale secondary ion mass spectrometry revealed that one of the symbionts, the γ3‐symbiont, uses the energy from CO oxidation to fix CO₂. Pore water analysis revealed considerable in‐situ concentrations of CO and H₂ in the O. algarvensis environment, Mediterranean seagrass sediments. Pore water H₂ concentrations (89–2147 nM) were up to two orders of magnitude higher than in seawater, and up to 36‐fold higher than previously known from shallow‐water marine sediments. Pore water CO concentrations (17–51 nM) were twice as high as in the overlying seawater (no literature data from other shallow‐water sediments are available for comparison). Ex‐situ incubation experiments showed that dead seagrass rhizomes produced large amounts of CO. CO production from decaying plant material could thus be a significant energy source for microbial primary production in seagrass sediments.     

Small-scale removal of seagrass (Zostera marina L.): effects on the infaunal community

Eelgrass meadows are a common feature in shallow waters along the Norwegian coast, where they provide a habitat for a diverse infaunal community. Recreational boat anchoring and moorings physically scour seagrass and may affect the ecosystem functioning and resilience of the system to natural and anthropogenic disturbances. A small-scale eelgrass (Zostera marina) removal experiment was conducted to study the effects on macro- and meiofauna. Entire plants, including the rhizomes, were removed from 4?m² patches in three eelgrass meadows in the inner Oslofjord in October 2010. Core samples were taken after a recovery period of 10 months, from the removed patches as well as from the surrounding meadow. Macrofauna (>500?μm) and meiofauna (63–500?μm) in the sediment were investigated for possible effects of the eelgrass removal. Macrofauna and meiofauna composition were site specific and therefore location was identified as the main determinant for the infaunal community. The eelgrass did not regrow within the recovery period and bare sediment patches with only single eelgrass shoots were present during the sampling. Our analyses support an influence of the removal on individual species, but not the complete community. In particular one species, the gastropod Peringia ulvae, was encountered in higher numbers in samples from the removed patches than in control samples. From a management perspective, such minor removal of eelgrass, on the scale of square metres, appears to have no long-lasting detrimental effect to the infaunal community in sheltered meadows with muddy sediments.     

Effects of copper enrichment on survival,growth and photosynthetic pigment of seedlings and young plants of the eelgrass Zostera marina

Copper (Cu²⁺) is an essential nutrient for plants but toxic at high concentrations. We subjected seedlings and young plants of eelgrass Zostera marina to different seawater Cu concentrations (3, 4, 5, 10, 30 and 50?µg?l^?1) for over 30 days under controlled laboratory conditions. Natural seawater without added Cu (3?µg?l^?1) was used as reference seawater. We measured plant response in terms of survivorship, morphology, growth, productivity and leaf pigment concentration. Survival analysis combined with morphological, dynamic and productive assessment suggested that the optimum seawater Cu concentration for the establishment of Z. marina seedlings and young plants is 4?μg?l^?1. The photosynthetic response of young plants to copper enrichment, including an increase in chlorophyll content under low Cu concentration treatment but significant decrease when treated with high concentrations of Cu, is similar to those reported for other seagrass species. NOEC (no observed effect concentration), LOEC (lowest observed effect concentration) and LC₅₀ (lethal concentration that caused an increase in mortality to 50% of that of the control) values of seedlings were significantly lower than those of young plants, implying a reduced Cu tolerance to high concentrations (>10?μg?l^?1). This study provides data that could prove helpful in the development of successful eelgrass restoration and conservation.     

Variable responses of native eelgrass Zostera marina to a non-indigenous bivalve Musculista senhousia

The transport and establishment of non-indigenous species in coastal marine environments are increasing worldwide, yet few studies have experimentally addressed the interactions between potentially dominant non-native species and native organisms. We studied the effects of the introduced mussel Musculista senhousia on leaf and rhizome growth and shoot density of eelgrass Zostera marina in San Diego Bay, California. We added M. senhousia over a natural range in biomass (0–1200 g dry mass/m²) to eelgrass in transplanted and established beds. The effects of the non-indigenous mussel varied from facilitation to interference depending on time, the abundance of M. senhousia, and the response variable considered. Consistent results were that mussel additions linearly inhibited eelgrass rhizome elongation rates. With 800 g dry mass/m² of M. senhousia, eelgrass rhizomes grew 40% less than controls in two eelgrass transplantations and in one established eelgrass bed. These results indicate that M. senhousia, could both impair the success of transplantations of eelgrass, which spread vegetatively by rhizomes, and the spread of established Z. marina beds to areas inhabited by M. senhousia. Although effects on leaf growth were not always significant, in August in both eelgrass transplantations and established meadows leaf growth was fertilized by mussels, and showed a saturation-type relationship to sediment ammonium concentrations. Ammonium concentrations and sediment organic content were linear functions of mussel biomass. We found only small, non-consistent effects of M. senhousia on shoot density of eelgrass over 6-month periods. In established eelgrass beds, but not in transplanted eelgrass patches (≈0.8 m in diameter), added mussels suffered large declines. Hence, eelgrass is likely to be affected by M. senhousia primarily where Z. marina beds are patchy and sparse. Our study has management and conservation implications for eelgrass because many beds are already seriously degraded and limited in southern California where the mussel is very abundant. Received: 31 May 1997 / Accepted: 4 September 1997     

Model‐based optimization and pO2 control of fed‐batch Escherichia coli and Saccharomyces cerevisiae cultivation processes

A model‐based approach for optimization and cascade control of dissolved oxygen partial pressure (pO₂) and maximization of biomass in fed‐batch cultivations is presented. The procedure is based on the off‐line model‐based optimization of the optimal feeding rate profiles and the subsequent automatic pO₂ control using a proposed cascade control technique. During the model‐based optimization of the process, feeding rate profiles are optimized with respect to the imposed technological constraints (initial and maximal cultivation volume, cultivation time, feeding rate range, maximal oxygen transfer rate and pO₂ level). The cascade pO₂ control is implemented using activation of cascades for agitation, oxygen enrichment, and correction of the preoptimized feeding rate profiles. The proposed approach is investigated in two typical fed‐batch processes with Escherichia coli and Saccharomyces cerevisiae. The obtained results show that it was possible to achieve sufficiently high biomass levels with respect to the given technological constraints and to improve controllability of the investigated processes.     

Photosynthetic response to globally increasing CO2 of co‐occurring temperate seagrass species

Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO₂ could enhance seagrass photosynthesis and internal O₂ supply, and potentially change species competition through differential responses to increasing CO₂ availability among species. We used short‐term photosynthetic responses of nine seagrass species from the south‐west of Australia to test species‐specific responses to enhanced CO₂ and changes in HCO₃^?. Net photosynthesis of all species except Zostera polychlamys were limited at pre‐industrial compared to saturating CO₂ levels at light saturation, suggesting that enhanced CO₂ availability will enhance seagrass performance. Seven out of the nine species were efficient HCO₃^? users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO₃^?. Species responded differently to near saturating CO₂ implying that increasing atmospheric CO₂ may change competition among seagrass species if co‐occurring in mixed beds. Increasing CO₂ availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO₂ will have the strongest impact on seagrass recruits and sparsely vegetated beds, because densely vegetated seagrass beds are most often limited by light and not by inorganic carbon.     

Nitrogen Fixation Associated with Rinsed Roots and Rhizomes of the Eelgrass Zostera marina

Nitrogen fixation was associated with the rinsed roots and rhizomes of the seagrass, Zostera marina L. Nitrogenase activity (acetylene reduction) was greater on rhizomes compared to roots, and on older roots and rhizomes relative to younger tissue. Compared to aerobic assays, anaerobic or microaerobic conditions enhanced the rate of acetylene reduction by rhizomes with attached roots, with the highest activity (100 nanomoles per gram dry weight per hour) occurring at pO₂ = 0.01 atmosphere. Addition of glucose, sucrose, or succinate also increased the rate of acetylene reduction under anaerobic conditions, with glucose providing the most stimulation. In one experiment, comparison of acetylene reduction assays with ¹⁵N₂ incorporation yielded a ratio of about 2.6:1. Seagrass communities are thought to be limited by the availability of nitrogen and, therefore, nitrogenase activity directly associated with their roots and rhizomes suggests the possibility of a N₂-fixing flora which may subsidize their nutritional demand for nitrogen.     

Nitrate and nitrite in interstitial waters of eelgrass beds in relation to the rhizosphere

The distribution of nitrate and nitrite in the interstitial water of the sediment of eelgrass (Zostera marina) bed of Izembek Lagoon, Alaska, were investigated. Their concentrations were relatively high (0 to 9.8 μg-at.N·1^?1, average 4.8 for nitrate; 0 to 4.0 μ-at.N·1^?1, average 1.9 for nitrite) although the sediments were anoxic and contained hydrogen sulphide. The rates of bacterial denitrification measured by ¹⁵N tracer technique ranged from 0.49×10^?10 to 1.2 × 10^?9 g-atN·g^?1·h^?1. When a steady state is maintained, the loss of nitrate and nitrite must be balanced by their production by bacterial nitrification. Experimentally determined rate of nitrification in the sediment was of the same order. A model experiment demonstrated that oxygen is transported from leaves to rhizomes and roots of eelgrass and released into the sediment. The oxygen is used for nitrification in the rhizosphere in anoxic sediments.     

Sulphide detoxification in Hediste diversicolor and Marenzelleria viridis, two dominant polychaete worms within the shallow coastal waters of the southern Baltic Sea

The polychaete worms Marenzelleria viridis (Verrill 1873) and Hediste diversicolor (O.F. Müller) form the main part of the macro-zoobenthos in soft-bottomed shallow inlets of the Baltic Sea. Due to high eutrophication within these waters the animals are exposed to low oxygen and high sulphide concentrations. Specimens of both species from a low salinity location (S 8 ‰) were compared concerning their physiological abilities in coping with this hostile environment. Sulphide detoxification occurred in both polychaetes even during severe hypoxia with the main end-product being thiosulphate. In absence of sulphide nearly no end-products of anaerobic metabolism were found in the worms during moderate hypoxia (pO₂=7 kPa). In presence of hydrogen sulphide, succinate, a sensitive indicator of anaerobic metabolism, was accumulated in higher amounts at low sulphide concentrations (0.3 mM) already. Oxygen consumption and ATP production was determined in isolated mitochondria of both species. Both polychaetes were able to perform enzymatic sulphide oxidation in the mitochondria at concentrations up to 50 μM. This process was coupled with oxidative phosphorylation. At least in M. viridis sulphide respiration was not completely inhibited by cyanide, suggesting an alternative oxidation pathway, which by-passes the cytochrome-c-oxidase. The two species did not differ in the rate of sulphide detoxification, but H. diversicolor produced about as twice as much ATP from mitochondrial sulphide oxidation. Differences in mitochondrial sulphide oxidation are probably related to the different life strategies of the worms.     

Genetic effects of eelgrass restoration efforts by fishers' seeding to recover seagrass beds as an important natural capital for coastal ecosystem services

The reestablishment of seagrass vegetation is a vital part of recovering coastal marine ecosystem services. Historically the Hinase area was a famous for the fishing by coastal pound netting in eelgrass beds, but this practice was progressively displaced with oyster farming due to an enormous decline in seagrass vegetation. For several decades, the local fishers' cooperative has worked to restore eelgrass beds by a seeding method. Through these efforts, seagrass vegetation in their fishing area has increased to about half of their previous area. This study examined the effect of long-term seeding by fishers on the recovery of eelgrass beds in the Hinase area, based on analysis of eelgrass genetic structure using microsatellite markers. Specimens for the DNA analysis were collected from each of all eelgrass meadows that the fishers conducted sowing eelgrass seeds as well as from the source sites where they collected the seeds. The results found that restored beds in the study area have high genetic diversity comparable to natural ones. The multiple regression analysis revealed that a combined model of seedling intensity and geographic distance (R² = .457) better explained genetic structure across our sampling sites than models of seedling intensity (R² = .092) or geographic distance only (R² = .344). This supports that the eelgrass seeds they sowed did not disturb the genetic structure but rather supplemented natural dispersal, suggesting that the fishers' seeding did not develop nonnatural seagrass meadows but certainly contributed to the recovery of natural seagrass meadows.     

Development of a ‘sediment-stress’ functional-level indicator for the seagrass Halophila ovalis

Understanding mechanistic relationships between seagrass and their environmental stressors should be considered for effective management of estuaries and may inform on why change has occurred. We aimed to develop indicators for seagrass health in response to sediment conditions for the Swan-Canning Estuary, south-west Australia. This article describes the development of a new sediment-stress indicator, relating aspects of seagrass productivity with sediment sulfur dynamics. Sulfur stable isotope ratio and total sulfur were measured monthly within the roots, rhizomes and leaves of Halophila ovalis, and significantly varied across sites and months. The growth of seagrass over the summer months appeared restricted by sediment condition, with growth of seagrass lower when sediment derived sulfur and/or total sulfur within rhizome of leaf tissues was higher. H. ovalis appeared quite tolerant of sulfide intrusion within the root compartment, but growth was compromised when sulfide breached the root–rhizome barrier. The tightest correlation between potential sulfur metrics and seagrass growth was observed for the ratio (δ³⁴S_leaf + 30)/(TS_leaf), and it is this ratio that we propose may be a useful sediment-stress indicator for seagrass. The study also highlights that sediment condition needs to be considered at the meadow scale.     

Nanoparticle‐based measurements of pH and O2 dynamics in the rhizosphere of Zostera marina L.: effects of temperature elevation and light‐dark transitions

Seagrasses can modulate the geochemical conditions in their immediate rhizosphere through the release of chemical compounds from their below‐ground tissue. This is a vital chemical defence mechanism, whereby the plants detoxify the surrounding sediment. Using novel nanoparticle‐based optical O₂ and pH sensors incorporated in reduced and transparent artificial sediment, we investigated the spatio‐temporal dynamics of pH and O₂ within the entire rhizosphere of Zostera marina L. during experimental manipulations of light and temperature. We combined such measurements with O₂ microsensor measurements of the photosynthetic productivity and respiration of seagrass leaves. We found pronounced pH and O₂ microheterogeneity within the immediate rhizosphere of Z. marina, with higher below‐ground tissue oxidation capability and rhizoplane pH levels during both light exposure of the leaf canopy and elevated temperature, where the temperature‐mediated stimuli of biogeochemical processes seemed to predominate. Low rhizosphere pH microenvironments appeared to correlate with plant‐derived oxic microzones stimulating local sulphide oxidation and thus driving local proton generation, although the rhizoplane pH levels generally where much higher than the bulk sediment pH. Our data show that Z. marina can actively alter its rhizosphere pH microenvironment alleviating the local H₂S toxicity and enhancing nutrient availability in the adjacent sediment via geochemical speciation shift.     

Fluorescent silica particles for monitoring oxygen levels in three-dimensional heterogeneous cellular structures

Bacterial biofilms are a major obstacle challenging the development of more effective therapies to treat implant infections. Oxygen availability to bacterial cells has been implicated in biofilm formation and planktonic cell detachment; however, there are insufficient tools available to measure oxygen concentrations within complex three‐dimensional structures with ～1 µm resolution. Such measurements may complement measures of biofilm structure and cell activity to provide a more comprehensive understanding of biofilm biology. Thus, we developed oxygen‐sensing microparticles specifically designed to characterize oxygen transport through the volume of bacterial biofilms. The Stöber method was used to synthesize monodisperse silica microparticles of approximately the same size as a bacterium (～1 µm). Two fluorophores, oxygen‐sensitive Ru(Ph₂phen₃)Cl₂, and the reference fluorophore Nile blue chloride were immobilized on the surface of the particles. We demonstrate application of the microparticles toward measuring the oxygen concentration profiles within a live Staphylococcus aureus biofilm. Biotechnol. Bioeng. 2012; 109: 2663–2670. © 2012 Wiley Periodicals, Inc.     

Accurate control of oxygen level in cells during culture on silicone rubber membranes with application to stem cell differentiation

Oxygen level in mammalian cell culture is often controlled by placing culture vessels in humidified incubators with a defined gas phase partial pressure of oxygen (pO_2gas). Because the cells are consuming oxygen supplied by diffusion, a difference between pO_2gas and that experienced by the cells (pO_2cell) arises, which is maximal when cells are cultured in vessels with little or no oxygen permeability. Here, we demonstrate theoretically that highly oxygen‐permeable silicone rubber membranes can be used to control pO_2cell during culture of cells in monolayers and aggregates much more accurately and can achieve more rapid transient response following a disturbance than on polystyrene and fluorinated ethylene‐propylene copolymer membranes. Cell attachment on silicone rubber was achieved by physical adsorption of fibronectin or Matrigel. We use these membranes for the differentiation of mouse embryonic stem cells to cardiomyocytes and compare the results with culture on polystyrene or on silicone rubber on top of polystyrene. The fraction of cells that are cardiomyocyte‐like increases with decreasing pO₂ only when using oxygen‐permeable silicone membrane‐based dishs, which contract on silicone rubber but not polystyrene. The high permeability of silicone rubber results in pO_2cell being equal to pO_2gas at the tissue‐membrane interface. This, together with geometric information from histological sections, facilitates development of a model from which the pO₂ distribution within the resulting aggregates is computed. Silicone rubber membranes have significant advantages over polystyrene in controlling pO_2cell, and these results suggest they are a valuable tool for investigating pO₂ effects in many applications, such as stem cell differentiation. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

In vitro cell culture pO2 is significantly different from incubator pO2

Continuous noninvasive monitoring of peri‐cellular liquid phase pO₂ in adherent cultures is described. For neurons and astrocytes, this approach demonstrates that there is a significant difference between predicted and observed liquid phase pO₂. Particularly at low gas phase pO₂s, cell metabolism shifts liquid phase pO₂ significantly lower than would be predicted from the O₂ gas/air equilibrium coefficient, indicating that the cellular oxygen uptake rate exceeds the oxygen diffusion rate. The results demonstrate the need for direct pO₂ measurements at the peri‐cellular level, and question the widely adopted current practice of relying on setting the incubator gas phase level as means of controlling pericellular oxygen tension, particularly in static culture systems that are oxygen mass transfer limited. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

Eelgrass (Zostera spp.) associated phytomyxids are host-specific congeneric parasites and predominant eukaryotes in the eelgrass rhizosphere on a global scale

Together with increasing environmental and anthropogenic pressures, pathogenic diseases are one of the important factors contributing to the ongoing decline of seagrass meadows worldwide; yet the diversity and ecology of the microorganisms acknowledged as seagrass parasites remain critically understudied. Here, we investigate phytomyxid parasites (Rhizaria: Endomyxa: Phytomyxea) of three different eelgrass (Zostera spp.) species found in the Northern hemisphere. We present molecular evidence that Plasmodiophora bicaudata, a long-recognized parasite of dwarf eelgrass taxa, is closely related to the novel phytomyxid recently discovered in root hairs of Zostera marina, and together they form a distinct clade within the order Phagomyxida, proposed here as Feldmanniella gen. nov. A full life cycle is systematically described in a phagomyxid representative for the first time, proving its conformity with the generalized phytomyxid life history, despite previous uncertainties. The presence of primary infection stages in nearly all collected eelgrass specimens, and subsequent analysis of amplicon sequences from a global Z. marina dataset, reveal phytomyxids to be ubiquitous and one of the predominant microeukaryotes associated with eelgrass roots on a global scale. Our discoveries challenge the current view of Phytomyxea as rare entities in seagrass meadows and suggest their generally low pathogenicity in natural ecosystems.