Hypometabolism to survive the long polar night and subsequent successful return to light in the diatom Fragilariopsis cylindrus.

Diatoms, the main eukaryotic phytoplankton of the polar marine regions, are essential for the maintenance of food chains specific to Arctic and Antarctic ecosystems, and are experiencing major disturbances under current climate change. As such, it is fundamental to understand the physiological mechanisms and associated molecular basis of their endurance during the long polar night. Here, using the polar diatom Fragilariopsis cylindrus, we report an integrative analysis combining transcriptomic, microscopic and biochemical approaches to shed light on the strategies used to survive the polar night. We reveal that in prolonged darkness, diatom cells enter a state of quiescence with reduced metabolic and transcriptional activity, during which no cell division occurs. We propose that minimal energy is provided by respiration and degradation of protein, carbohydrate and lipid stores and that homeostasis is maintained by autophagy in prolonged darkness. We also report internal structural changes that manifest the morphological acclimation of cells to darkness, including the appearance of a large vacuole. Our results further show that immediately following a return to light, diatom cells are able to use photoprotective mechanisms and rapidly resume photosynthesis, demonstrating the remarkable robustness of polar diatoms to prolonged darkness at low temperature.

Taurine depletion impairs cardiac function and affects tolerance to hypoxia and high temperatures in brook char (Salvelinus fontinalis).

Physiological and environmental stressors can cause osmotic stress in fish hearts, leading to a reduction in intracellular taurine concentration. Taurine is a ß-amino acid known to regulate cardiac function in other animal models but its role in fish has not been well characterized. We generated a model of cardiac taurine deficiency (TD) by feeding brook char (Salvelinus fontinalis) a diet enriched in ß-alanine, which inhibits cardiomyocyte taurine uptake. Cardiac taurine levels were reduced by 21% and stress-induced changes in normal taurine handling were observed in TD brook char. Responses to exhaustive exercise and acute thermal and hypoxia tolerance were then assessed using a combination of in vivo, in vitro and biochemical approaches. Critical thermal maximum was higher in TD brook char despite significant reductions in maximum heart rate. In vivo, TD brook char exhibited a lower resting heart rate, blunted hypoxic bradycardia and a severe reduction in time to loss of equilibrium under hypoxia. In vitro function was similar between control and TD hearts under oxygenated conditions, but stroke volume and cardiac output were severely compromised in TD hearts under severe hypoxia. Aspects of mitochondrial structure and function were also impacted in TD permeabilized cardiomyocytes, but overall effects were modest. High levels of intracellular taurine are required to achieve maximum cardiac function in brook char and cardiac taurine efflux may be necessary to support heart function under stress. Taurine appears to play a vital, previously unrecognized role in supporting cardiovascular function and stress tolerance in fish.

Prochlorococcus marinus responses to light and oxygen.

Prochlorococcus marinus, the smallest picocyanobacterium, comprises multiple clades occupying distinct niches, currently across tropical and sub-tropical oligotrophic ocean regions, including Oxygen Minimum Zones. Ocean warming may open growth-permissive temperatures in new, poleward photic regimes, along with expanded Oxygen Minimum Zones. We used ocean metaproteomic data on current Prochlorococcus marinus niches, to guide testing of Prochlorococcus marinus growth across a matrix of peak irradiances, photoperiods, spectral bands and dissolved oxygen. MED4 from Clade HLI requires greater than 4 h photoperiod, grows at 25 µmol O2 L-1 and above, and exploits high cumulative diel photon doses. MED4, however, relies upon an alternative oxidase to balance electron transport, which may exclude it from growth under our lowest, 2.5 µmol O2 L-1, condition. SS120 from clade LLII/III is restricted to low light under full 250 µmol O2 L-1, shows expanded light exploitation under 25 µmol O2 L-1, but is excluded from growth under 2.5 µmol O2 L-1. Intermediate oxygen suppresses the cost of PSII photoinactivation, and possibly the enzymatic production of H2O2 in SS120, which has limitations on genomic capacity for PSII and DNA repair. MIT9313 from Clade LLIV is restricted to low blue irradiance under 250 µmol O2 L-1, but exploits much higher irradiance under red light, or under lower O2 concentrations, conditions which slow photoinactivation of PSII and production of reactive oxygen species. In warming oceans, range expansions and competition among clades will be governed not only by light levels. Short photoperiods governed by latitude, temperate winters, and depth attenuation of light, will exclude clade HLI (including MED4) from some habitats. In contrast, clade LLII/III (including SS120), and particularly clade LLIV (including MIT9313), may exploit higher light niches nearer the surface, under expanding OMZ conditions, where low O2 relieves the stresses of oxidation stress and PSII photoinhibition.

Quantitative and qualitative variability of airborne cyanobacteria and microalgae and their toxins in the coastal zone of the Baltic Sea.

Apart from viruses and bacteria, cyanobacteria and microalgae present in the atmosphere may pose a threat to the health of humans by inducing illnesses and diseases. Yet, they play an important role in the environment, influencing the Earth's radiation budget by absorbing and scattering solar radiation. The present study determined the daily and seasonal qualitative and qualitative variabilities of airborne cyanobacteria and microalgae during both vegetative and non-vegetative seasons in the coastal zone of the Baltic Sea. Samples were collected from January to December 2020 with a Tisch six-stage microbiological impactor which was used as a substitute for the respiratory tract. The stage levels of the impactor represented the respiratory tract and reproduced lung penetration by airborne particles, which allowed us to assess penetration of cyanobacteria and microalgae to the deepest parts of the human respiratory system. A total of 296 samples of cyanobacteria and microalgae were collected during the day and 276 samples during the night. The results showed that cyanobacteria and microalgae were present in the air all year, and their maximum abundance was 1685 cells m-3 in July. Furthermore, the ability of these microorganisms to produce the toxin microcystin-LR (MC-LR) was confirmed, which has a high potential negative impact on human health. MC-LR has been found in Nostoc sp., Pseudanabaena sp., Leptolyngbya sp., Synechococcus sp., Gloeocapsa sp., Aphanothece sp., and Rivularia sp. maintained at our Culture Collection of Airborne Algae (CCAA), as well as from air samples. The highest concentrations of MC-LR were recorded in airborne Synechococcus sp. CCAA 46 and amounted to as much as 420 fg cell-1. In turn, the highest mean concentration of 0.95 µg L-1 for MC-LR was recorded in an air sample taken in May. This research expands the knowledge on cyanobacteria and microalgae present in the atmosphere in the coastal zone of the southern Baltic Sea. We propose these microorganisms be used as indicators for further research on bioaerosols, which are potentially dangerous to human health.