Silja Frankenbach

High-to-Low CO2 Acclimation Reveals Plasticity of the Photorespiratory Pathway and Indicates Regulatory Links to Cellular Metabolism of Arabidopsis

Background Photorespiratory carbon metabolism was long considered as an essentially closed and nonregulated pathway with little interaction to other metabolic routes except nitrogen metabolism and respiration. Most mutants of this pathway cannot survive in ambient air and require CO2-enriched air for normal growth. Several studies indicate that this CO2 requirement is very different for individual mutants, suggesting a higher plasticity and more interaction of photorespiratory metabolism as generally thought. To understand this better, we examined a variety of high- and low-level parameters at 1% CO2 and their alteration during acclimation of wild-type plants and selected photorespiratory mutants to ambient air. Methodology and Principal Findings The wild type and four photorespiratory mutants of Arabidopsis thaliana (Arabidopsis) were grown to a defined stadium at 1% CO2 and then transferred to normal air (0.038% CO2). All other conditions remained unchanged. This approach allowed unbiased side-by-side monitoring of acclimation processes on several levels. For all lines, diel (24 h) leaf growth, photosynthetic gas exchange, and PSII fluorescence were monitored. Metabolite profiling was performed for the wild type and two mutants. During acclimation, considerable variation between the individual genotypes was detected in many of the examined parameters, which correlated with the position of the impaired reaction in the photorespiratory pathway. Conclusions Photorespiratory carbon metabolism does not operate as a fully closed pathway. Acclimation from high to low CO2 was typically steady and consistent for a number of features over several days, but we also found unexpected short-term events, such as an intermittent very massive rise of glycine levels after transition of one particular mutant to ambient air. We conclude that photorespiration is possibly exposed to redox regulation beyond known substrate-level effects. Additionally, our data support the view that 2-phosphoglycolate could be a key regulator of photosynthetic-photorespiratory metabolism as a whole.

Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress : evidence for latitudinal adaptation along the Atlantic coast of Southern Europe

Although estuarine microphytobenthos (MPB) is frequently exposed to excessive light and temperature conditions, little is known on their interactive effects on MPB primary productivity. Laboratory and in situ experiments were combined to investigate the short-term joint effects of high light (HL) and high temperature (37 °C versus 27 °C) on the operating efficiency of photoprotective processes [vertical migration versus non-photochemical quenching (NPQ)] exhibited by natural benthic diatom communities from two intertidal flats in France (FR) and Portugal (PT). A clear latitudinal pattern was observed, with PT biofilms being more resistant to HL stress, regardless the effect of temperature, and displaying a lower relative contribution of vertical migration to photoprotection and a stronger NPQ in situ. However, higher temperature leads to comparable effects, with photoinhibition increasing to about three times (i.e. from 3% to 10% and from 8% to 22% in PT and FR sites respectively). By using a number of methodological novelties in MPB research (lipid peroxidation quantification, Lhcx proteins immunodetection), this study brings a physiological basis to the previously reported depression of MPB photosynthetic productivity in summer. They emphasize the joint role of temperature and light in limiting, at least transiently (i.e. during emersion), MPB photosynthetic activity in situ.

Evidence for gravitactic behaviour in benthic diatoms

Vertical migration by diatoms is a well-known phenomenon, occurring in intertidal and subtidal benthic biofilms. It is partially endogenously driven, as cell movements can be observed in the absence of external stimuli such as light, temperature or water cover. Although vertical migration of diatoms under constant conditions has often been attributed to geotactic orientation, this hypothesis has never been experimentally demonstrated. Our study tested the gravitactic nature of the vertical migratory behaviour of benthic diatoms in sedimentary biofilms, using an experimental setup designed to distinguish gravitaxis from surface-oriented cell movements. The hourly variation of surface diatom biomass during migratory cycles was compared in homogenized sediment samples kept facing upwards (surface-oriented and gravity stimuli coinciding; controls) and facing sideways or downwards (surface-oriented and gravity stimuli not coinciding). During the experiments, sediment samples were kept in complete darkness in custom-made, sealed measuring chambers designed to avoid any contact with atmospheric air and the formation of physico-chemical gradients near the surface. Microalgal biomass was monitored non-intrusively using PAM fluorometry, by measuring dark-level fluorescence, Fo. The results showed a clear effect of sample orientation in relation to the gravitational stimulus. In the controls, a biphasic pattern in surface biomass was observed, with the formation of a clear biomass peak (three- to six-fold increase) followed by a slower decrease. In contrast, in samples facing sideways or downwards, surface biomass also varied but to a much lesser extent (typically < two-fold). These results strongly suggest that, in the absence of light, upward vertical migration of benthic diatoms is mostly guided by negative gravitaxis, supporting the often hypothesized capacity of these cells to sense and use gravity to move vertically within the sediment.

A chlorophyll fluorescence-based method for the integrated characterization of the photophysiological response to light stress

&NA; This work introduces a new experimental method for the comprehensive description of the physiological responses to light of photosynthetic organisms. It allows the integration in a single experiment of the main established manipulative chlorophyll fluorescence‐based protocols. It enables the integrated characterization of the photophysiology of samples regarding photoacclimation state (generating non‐sequential light‐response curves of effective PSII quantum yield, electron transport rate or non‐photochemical quenching), photoprotection capacity (running light stress‐recovery experiments, quantifying non‐photochemical quenching components) and the operation of photoinactivation and photorepair processes (measuring rate constants of photoinactivation and repair for different light levels and the relative quantum yield of photoinactivation). The new method is based on a previously introduced technique, combining the illumination of a set of replicated samples with spatially separated actinic light beams of different intensity, and the simultaneous measurement of the fluorescence emitted by all samples using an imaging fluorometer. The main novelty described here is the independent manipulation of light intensity and duration of exposure for each sample, and the control of the cumulative light dose applied. The results demonstrate the proof of concept for the method, by comparing the responses of cultures of Chlorella vulgaris acclimated to low and high light regimes, highlighting the mapping of light stress responses over a wide range of light intensity and exposure conditions, and the rapid generation of paired light‐response curves of photoinactivation and repair rate constants. This approach represents a chlorophyll fluorescence ‘protocol of everything’, contributing towards the high throughput characterization of the photophysiology of photosynthetic organisms.

Nematode assemblages in banana (Musa acuminata) monocultures and banana plantations with Juçara palms (Euterpe edulis) in the southern Mata Atlântica, Brazil

Summary – The composition of the nematode fauna at two different agrosystems (banana monocultures and mixed banana-palmito plantations) was investigated at six study sites on the coastal plain of southern Brazil. Nematode abundance was higher and the number of families was lower (25 compared to 33) in the soil of banana monocultures. The assemblages in the soil of the banana monocultures were dominated by bacterial feeders and plant parasites, whereas in the soil of the mixed plantations the proportion of the other trophic groups was higher. In the monocultures, the percentage of families categorised as colonisers (c-p 1 families) was higher and the persisters lower. The difference in the assemblage of c-p groups was significant between sites of the two different systems. Principal component analysis (PCA) ordination of the samples by nematode family composition showed site-specific assemblages, similarity between two sites of each system and significant dissimilarities between the two systems. The MI 1-5 and the MI 2-5 were lower in the banana monocultures. Additionally, the MI 2-5 of banana site B2 was significantly higher than that of the two other banana sites. The ratio F/B was higher and the ratio (F + B)/plant feeders was lower in the banana-palmito plantations. The differences between the systems in nearly all measured parameters indicate a higher degree of disturbance and nutrient enrichment of the soil under monocultures. However, a low number of plant parasites and dominance of c-p 3 taxa at both agroecosystems show that the soil of both agroecosystems seems to be of an advanced successional stage. This may be a result of a less intensive ‘organic’ cultivation without the use of plant protection products and fertilisers and with additional non-host plants. Despite many non-controlled variables in the smallholder systems, according to the results the nematodes can be regarded as suitable indicators of soil disturbance in banana and banana-palmito agro-ecosystems.