The potential role of plant oxygen and sulphide dynamics in die-off events of the tropical seagrass, Thalassia testudinum

1 Oxygen and sulphide dynamics were examined, using microelectrode techniques, in meristems and rhizomes of the seagrass Thalassia testudinum at three different sites in Florida Bay, and in the laboratory, to evaluate the potential role of internal oxygen variability and sulphide invasion in episodes of sudden die-off. The sites differed with respect to shoot density and sediment composition, with an active die-off occurring at only one of the sites. 2 Meristematic oxygen content followed similar diel patterns at all sites with high oxygen content during the day and hyposaturation relative to the water column during the night. Minimum meristematic oxygen content was recorded around sunrise and varied among sites, with values close to zero at the die-off site. 3 Gaseous sulphide was detected within the sediment at all sites but at different concentrations among sites and within the die-off site. Spontaneous invasion of sulphide into Thalassia rhizomes was recorded at low internal oxygen partial pressure during darkness at the die-off site. 4 A laboratory experiment showed that the internal oxygen dynamics depended on light availability, and hence plant photosynthesis, and on the oxygen content of the water column controlling passive oxygen diffusion from water column to leaves and belowground tissues in the dark. 5 Sulphide invasion only occurred at low internal oxygen content, and the rate of invasion was highly dependent on the oxygen supply to roots and rhizomes. Sulphide was slowly depleted from the tissues when high oxygen partial pressures were re-established through leaf photosynthesis. Coexistence of sulphide and oxygen in the tissues and the slow rate of sulphide depletion suggest that sulphide reoxidation is not biologically mediated within the tissues of Thalassia. 6 Our results support the hypothesis that internal oxygen stress, caused by low water column oxygen content or poor plant performance governed by other environmental factors, allows invasion of sulphide and that the internal plant oxygen and sulphide dynamics potentially are key factors in the episodes of sudden die-off in beds of Thalassia testudinum . Root anoxia followed by sulphide invasion may be a more general mechanism determining the growth and survival of other rooted plants in sulphate-rich aquatic environments.

Oxygen isotope ratios of cellulose-derived phenylglucosazone: An improved paleoclimate indicator of environmental water and relative humidity

Oxygen atoms within fossil wood provide high-resolution records of climate change, particularly for the Quaternary. However, current analysis methods of fossil cellulose do not differentiate between different positions of the oxygen atoms. Here, we propose a refinement to tree-cellulose paleoclimatology modeling, using the cellulose-derived compound phenylglucosazone as the isotopic substrate. Stem samples from trees were collected at northern latitudes as low as 24°37′N and as high as 69°00′N. We extracted stem water and cellulose from each stem sample and analyzed them for their 18O content. In addition, we derived the cellulose to phenylglucosazone, a compound which lacks the oxygen attached to the second carbon of the cellulose–glucose moieties. Oxygen isotope analysis of phenylglucosazone allowed us to calculate the 18O content of the oxygen attached to the second carbon of the cellulose–glucose moieties. By way of these analyses, we tested two hypotheses: first, that the 18O content of the oxygen attached to second carbon will more closely reflect the 18O content of the stem water, and will not resemble the 18O content of either cellulose or its derivative phenylglucosazone. Second, tree-ring models that incorporate the variable oxygen isotope fractionation shown here and elsewhere are more accurate than those that do not. Our first hypothesis was rejected on the basis that the oxygen isotope ratios of the oxygen attached to the second carbon of the glucose moieties had a noisy isotopic signal with a large standard deviation and gave the poorest correlation with the oxygen isotope ratios of stem water. Related to this isotopic noise, we observed that the correlation between oxygen isotope ratios of phenylglucosazone with both stem water and relative humidity were higher than those observed for cellulose. Our hypothesis about tree-ring models which account for changes in the oxygen isotopic fractionation during cellulose synthesis was consistent only for the 18O content of phenylglucosazone. We showed that the tree-ring model based on the 18O content of phenylglucosazone was an improvement over existing models that are based on whole cellulose. Additionally, this approach may be used in other cellulose based archives such as peat deposits and lacustrine sediments.

Lake metabolism and the diel oxygen technique: State of the science

Significant improvements have been made in estimating gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) from diel, “free-water” changes in dissolved oxygen (DO). Here we evaluate some of the assumptions and uncertainties that are still embedded in the technique and provide guidelines on how to estimate reliable metabolic rates from high-frequency sonde data. True whole-system estimates are often not obtained because measurements reflect an unknown zone of influence which varies over space and time. A minimum logging frequency of 30 min was sufficient to capture metabolism at the daily time scale. Higher sampling frequencies capture additional pattern in the DO data, primarily related to physical mixing. Causes behind the often large daily variability are discussed and evaluated for an oligotrophic and a eutrophic lake. Despite a 3-fold higher day-to-day variability in absolute GPP rates in the eutrophic lake, both lakes required at least 3 sonde days per week for GPP estimates to be within 20% of the weekly average. A sensitivity analysis evaluated uncertainties associated with DO measurements, piston velocity (k), and the assumption that daytime R equals nighttime R. In low productivity lakes, uncertainty in DO measurements and piston velocity strongly impacts R but has no effect on GPP or NEP. Lack of accounting for higher R during the day underestimates R and GPP but has no effect on NEP. We finally provide suggestions for future research to improve the technique.

Structural flexibility and oxygen diffusion pathways in monomeric fluorescent proteins

Fluorescent proteins are valuable tools as biochemical markers for studying cellular processes. Red fluorescent proteins (RFPs) are highly desirable for in vivo applications because they absorb and emit light in the red region of the spectrum where cellular autofluorescence is low. The naturally occurring fluorescent proteins with emission peaks in this region of the spectrum occur in dimeric or tetrameric forms. The development of mutant monomeric variants of RFPs has resulted in several novel FPs known as mFruits. Though oxygen is required for maturation of the chromophore, it is known that photobleaching of FPs is oxygen sensitive, and oxygen-free conditions result in improved photostabilities. Therefore, understanding oxygen diffusion pathways in FPs is important for both photostabilites and maturation of the chromophores. We used molecular dynamics calculations to investigate the protein barrel fluctuations in mCherry, which is one of the most useful monomeric mFruit variants, and its GFP homolog citrine. We employed implicit ligand sampling and locally enhanced sampling to determine oxygen pathways from the bulk solvent into the mCherry chromophore in the interior of the protein. The pathway contains several oxygen hosting pockets, which were identified by the amino acid residues that form the pocket. We calculated the free-energy of an oxygen molecule at points along the path. We also investigated an RFP variant known to be significantly less photostable than mCherry and find much easier oxygen access in this variant. We showed that oxygen pathways can be blocked or altered, and barrel fluctuations can be reduced by strategic amino acid substitutions. The results provide a better understanding of the mechanism of molecular oxygen access into the fully folded mCherry protein barrel and provide insight into the photobleaching process in these proteins.