Induction of photosynthesis under anoxic condition in Thalassiosira pseudonana and Euglena gracilis: interactions between fermentation and photosynthesis

INTRODUCTION

In their natural environment, microalgae can be transiently exposed to hypoxic or anoxic environments. Whereas fermentative pathways and their interactions with photosynthesis are relatively well characterized in the green alga model Chlamydomonas reinhardtii, little information is available in other groups of photosynthetic micro-eukaryotes. In C. reinhardtii cyclic electron flow (CEF) around photosystem (PS) I, and light-dependent oxygen-sensitive hydrogenase activity both contribute to restoring photosynthetic linear electron flow (LEF) in anoxic conditions.

METHODS

Here we analyzed photosynthetic electron transfer after incubation in dark anoxic conditions (up to 24 h) in two secondary microalgae: the marine diatom Thalassiosira pseudonana and the excavate Euglena gracilis.

RESULTS

Both species showed sustained abilities to prevent over-reduction of photosynthetic electron carriers and to restore LEF. A high and transient CEF around PSI was also observed specifically in anoxic conditions at light onset in both species. In contrast, at variance with C. reinhardtii, no sustained hydrogenase activity was detected in anoxic conditions in both species.

DISCUSSION

Altogether our results suggest that another fermentative pathway might contribute, along with CEF around PSI, to restore photosynthetic activity in anoxic conditions in E. gracilis and T. pseudonana. We discuss the possible implication of the dissimilatory nitrate reduction to ammonium (DNRA) in T. pseudonana and the wax ester fermentation in E. gracilis.

Long-term acclimation to cadmium exposure reveals extensive phenotypic plasticity in Chlamydomonas

Increasing industrial and anthropogenic activities are producing and releasing more and more pollutants in the environment. Among them, toxic metals are one of the major threats for human health and natural ecosystems. Because photosynthetic organisms play a critical role in primary productivity and pollution management, investigating their response to metal toxicity is of major interest. Here, the green microalga Chlamydomonas (Chlamydomonas reinhardtii) was subjected to short (3 d) or chronic (6 months) exposure to 50 µM cadmium (Cd), and the recovery from chronic exposure was also examined. An extensive phenotypic characterization and transcriptomic analysis showed that the impact of Cd on biomass production of short-term (ST) exposed cells was almost entirely abolished by long-term (LT) acclimation. The underlying mechanisms were initiated at ST and further amplified after LT exposure resulting in a reversible equilibrium allowing biomass production similar to control condition. This included modification of cell wall-related gene expression and biofilm-like structure formation, dynamics of metal ion uptake and homeostasis, photosynthesis efficiency recovery and Cd acclimation through metal homeostasis adjustment. The contribution of the identified coordination of phosphorus and iron homeostasis (partly) mediated by the main phosphorus homeostasis regulator, Phosphate Starvation Response 1, and a basic Helix-Loop-Helix transcription factor (Cre05.g241636) was further investigated. The study reveals the highly dynamic physiological plasticity enabling algal cell growth in an extreme environment.

Trophic state alters the mechanism whereby energetic coupling between photosynthesis and respiration occurs in Euglena gracilis

The coupling between mitochondrial respiration and photosynthesis plays an important role in the energetic physiology of green plants and some secondary-red photosynthetic eukaryotes (diatoms), allowing an efficient CO₂ assimilation and optimal growth. Using the flagellate Euglena gracilis, we first tested if photosynthesis-respiration coupling occurs in this species harbouring secondary green plastids (i.e. originated from an endosymbiosis between a green alga and a phagotrophic euglenozoan). Second, we tested how the trophic state (mixotrophy and photoautotrophy) of the cell alters the mechanisms involved in the photosynthesis-respiration coupling. Energetic coupling between photosynthesis and respiration was determined by testing the effect of respiratory inhibitors on photosynthesis, and measuring the simultaneous variation of photosynthesis and respiration rates as a function of temperature (i.e. thermal response curves). The mechanism involved in the photosynthesis-respiration coupling was assessed by combining proteomics, biophysical and cytological analyses. Our work shows that there is photosynthesis-respiration coupling and membrane contacts between mitochondria and chloroplasts in E. gracilis. However, whereas in mixotrophy adjustment of the chloroplast ATP/NADPH ratio drives the interaction, in photoautotrophy the coupling is conditioned by CO₂ limitation and photorespiration. This indicates that maintenance of photosynthesis-respiration coupling, through plastic metabolic responses, is key to E. gracilis functioning under changing environmental conditions.

Gastrointestinal Manifestations among COVID-19 Patients in Bangladesh: A Cross Sectional Study

This study was done to find out the frequency of gastrointestinal manifestations among adult COVID-19 patients in Bangladesh. This descriptive cross sectional retrospective study was conducted from 27 May till 20 June 2020. Data were collected retrospectively from three different hospitals of Dhaka, Bangladesh. Patients were interviewed over telephone and verbal consents were taken. Their demographic and clinical profiles were put in the questionnaires by the investigators themselves. Statistical analysis was done thereafter. Out of 226 patients diagnosed with COVID-19, 145(61.4%) patients had gastrointestinal symptoms. Five of them had none other than gastrointestinal symptoms. Twenty six patients were asymptomatic. Mean age of the patients with were 41.6±14.8 years. Males were affected almost equally as the females (51.72% vs. 48.27%) (Ratio 1.1:1). Anorexia (44.7%) followed by diarrhoea (35%) and nausea (22.6%) were the predominant symptoms. Patients with co-morbidities (74.7%) were more prone to develop GI symptoms. Family members (71.6%) of COVID-19 patients with GI symptomatic patients were more affected than the others. Hospitalizations (77.6%) were more among GI symptomatic patients than the others. Thirty Five percent (35%) patients had diarrhoea. Mean duration of diarrhoea was 2.7±1.7 days with a frequency of were 4±1.8 days per day. Gastrointestinal manifestations are common among patients with COVID-19. Clinicians need to be aware of the GI features amongst COVID-19 patients so that they can be addressed and treated effectively and immediately. Further large scale study is needed to predict the disease outcome among COVID 19 patients with GI symptoms.