Homeostasis of cell composition during prolonged darkness

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

The use of NH4+ rather than NO3- affects cell stoichiometry, C allocation, photosynthesis and growth in the cyanobacterium Synechococcus sp. UTEX LB 2380, only when energy is limiting

The assimilation of N-NO₃^- requires more energy than that of N-NH₄⁺ . This becomes relevant when energy is limiting and may impinge differently on cell energy budget depending on depth, time of the day and season. We hypothesize that N-limited and energy-limited cells of the oceanic cyanobacterium Synechococcus sp. differ in their response to the N source with respect to growth, elemental stoichiometry and carbon allocation. Under N limitation, cells retained almost absolute homeostasis of elemental and organic composition, and the use of NH₄⁺ did not stimulate growth. When energy was limiting, however, Synechococcus grew faster in NH₄⁺ than in NO₃^- and had higher C (20%), N (38%) and S (30%) cell quotas. Furthermore, more C was allocated to protein, whereas the carbohydrate and lipid pool size did not change appreciably. Energy limitation also led to a higher photosynthetic rate relative to N limitation. We interpret these results as an indication that, under energy limitation, the use of the least expensive N source allowed a spillover of the energy saved from N assimilation to the assimilation of other nutrients. The change in elemental stoichiometry influenced C allocation, inducing an increase in cell protein, which resulted in a stimulation of photosynthesis and growth.

Marine microalgae growth and carbon partitioning as a function of nutrient availability

To understand in which way the structural differences of three marine microalgae (Nannochloropsis gaditana, Rhodomonas marina and Isochrysis sp.) affect their carbon partitioning, growth and applicability; a stoichiometric imbalance was imposed by steady carbon and other nutrients variation. Towards high nutrients concentrations/low carbon availability a decrease of 12-51% in C/N microalgae ratio was observed and maximum cell densities were achieved. Moreover, linear correlation between the nutrient input and microalgae protein content were observed. The macromolecular ratios pointed that carbohydrate was the main contributor for the C/N decrement. Although lipid content in R. marina remained constant throughout the experiment, a rise of 37-107% in N. gaditana and Isochrysis sp. was verified. Lipid fractions revealed high percentages of glycolipids in all microalgae (57-73% of total lipids). The present study shows an easy way to understand and modulate microalgae carbon partitioning relying on the field of application.

Prokaryotic assemblages and metagenomes in pelagic zones of the South China Sea

BACKGROUND

Prokaryotic microbes, the most abundant organisms in the ocean, are remarkably diverse. Despite numerous studies of marine prokaryotes, the zonation of their communities in pelagic zones has been poorly delineated. By exploiting the persistent stratification of the South China Sea (SCS), we performed a 2-year, large spatial scale (10, 100, 1000, and 3000 m) survey, which included a pilot study in 2006 and comprehensive sampling in 2007, to investigate the biological zonation of bacteria and archaea using 16S rRNA tag and shotgun metagenome sequencing.

RESULTS

Alphaproteobacteria dominated the bacterial community in the surface SCS, where the abundance of Betaproteobacteria was seemingly associated with climatic activity. Gammaproteobacteria thrived in the deep SCS, where a noticeable amount of Cyanobacteria were also detected. Marine Groups II and III Euryarchaeota were predominant in the archaeal communities in the surface and deep SCS, respectively. Bacterial diversity was higher than archaeal diversity at all sampling depths in the SCS, and peaked at mid-depths, agreeing with the diversity pattern found in global water columns. Metagenomic analysis not only showed differential %GC values and genome sizes between the surface and deep SCS, but also demonstrated depth-dependent metabolic potentials, such as cobalamin biosynthesis at 10 m, osmoregulation at 100 m, signal transduction at 1000 m, and plasmid and phage replication at 3000 m. When compared with other oceans, urease at 10 m and both exonuclease and permease at 3000 m were more abundant in the SCS. Finally, enriched genes associated with nutrient assimilation in the sea surface and transposase in the deep-sea metagenomes exemplified the functional zonation in global oceans.

CONCLUSIONS

Prokaryotic communities in the SCS stratified with depth, with maximal bacterial diversity at mid-depth, in accordance with global water columns. The SCS had functional zonation among depths and endemically enriched metabolic potentials at the study site, in contrast to other oceans.

Fourier transform infrared with attenuated total reflectance applied to the discrimination of freshwater planktonic coccoid green microalgae

Despite the recent advances on fine taxonomic discrimination in microorganisms, namely using molecular biology tools, some groups remain particularly problematic. Fine taxonomy of green algae, a widely distributed group in freshwater ecosystems, remains a challenge, especially for coccoid forms. In this paper, we propose the use of the Fourier Transform Infrared (FTIR) spectroscopy as part of a polyphasic approach to identify and classify coccoid green microalgae (mainly order Sphaeropleales), using triplicated axenic cultures. The attenuated total reflectance (ATR) technique was tested to reproducibility of IR spectra of the biological material, a primary requirement to achieve good discrimination of microalgal strains. Spectral window selection was also tested, in conjunction with the first derivative treatment of spectra, to determine which regions of the spectrum provided better separation and clustering of strains. The non-metric multidimensional scaling (NMDS), analysis of similarities (ANOSIM) and hierarchical clusters (HCA), demonstrated a correct discrimination and classification of closely related strains of chlorophycean coccoid microalgae, with respect to currently accepted classifications. FTIR-ATR was highly reproducible, and provided an excellent discrimination at the strain level. The best separation was achieved by analyzing the spectral windows of 1500–1200 cm⁻¹ and 900–675 cm⁻¹, which differs from those used in previously studies for the discrimination of broad algal groups, and excluding spectral regions related to storage compounds, which were found to give poor discrimination. Furthermore, hierarchical cluster analyses have positioned the strains tested into clades correctly, reproducing their taxonomic orders and families. This study demonstrates that FTIR-ATR has great potential to complement classical approaches for fine taxonomy of coccoid green microalgae, though a careful spectrum region selection is needed.

Surveillance of C-allocation in microalgal cells

When microalgae are exposed to changing environmental conditions, e.g., light-dark cycles or oscillations in nutrient availability (CO₂, nitrogen, phosphate or silicate) they respond with metabolic changes in the carbon allocation pattern. Short time regulations in the time range of few seconds to minutes can be mirrored best by mass spectroscopy based metabolomics. However, these snap shots do not reflect the alterations in the carbon flow to the cellular macromolecules like protein, carbohydrate or lipid. In this review it is shown how the combination of FTIR spectroscopy and Chla-in-vivo-fluorescence based electron transport rates can reveal changes in the metabolic flux rates of carbon during a shift of the environmental conditions. The review will demonstrate in which time range FTIR spectroscopy can deliver significant information and how FTIR spectroscopy data can synergistically support metabolome analysis by mass-spectroscopy.

Growth rate affects the responses of the green alga Tetraselmis suecica to external perturbations

Acclimation to environmental changes involves a modification of the expressed proteome and metabolome. The reproductive advantage associated with the higher fitness that acclimation provides to the new conditions more than compensates for the costs of acclimation. To exploit such an advantage, however, the duration of the perturbation must be sufficiently long relative to the growth rate. Otherwise, a selective pressure may exist in favour of responses that minimize changes in carbon allocation and resource use and do not require reversal of the acclimation after the perturbation ceases (compositional homeostasis). We hypothesize that the choice between acclimation and homeostasis depends on the duration of the perturbation relative to the length of the cell cycle. To test this hypothesis, we cultured the green alga Tetraselmis suecica at two growth rates and subjected the cultures to three environmental perturbations. Carbon allocation was studied with Fourier transform infrared (FTIR) spectroscopy; elemental stoichiometry was investigated by total reflection X-ray fluorescence (TXRF) spectroscopy. Our data confirmed that growth rate is a crucial factor for C allocation in response to external changes, with a higher degree of compositional homeostasis in cells with lower growth rate.

Homeostasis: an underestimated focal point of ecology and evolution

BACKGROUND

The concept of homeostasis is often ill-defined, in the scientific literature. The word "homeostasis", literally, indicates the absence of changes and an absolute maintenance of the status quo. The multiplicity of possible examples of homeostasis suggests that it is essentially impossible that all aspects of the composition of the organism and the rate of processes carried out by the organism are simultaneously held constant, when the environment changes are in the non-lethal range.

SCOPE

In attempting to clarify the usage of the term homeostasis, I emphasize the probable contributions to evolutionary fitness of homeostasis main attributes: rate processes and compositions. I also attempted to identify the aspects of homeostasis that are most likely to be subject to natural selection.

CONCLUSIONS

The tendency to retain the status quo derives from the interplay of functions (among which growth), metabolic pools and elemental stoichiometry. The set points around which oscillations occur in biological system and their control mechanisms are determined by evolutionary processes; consequently, also the tendency of a cell to be homeostatic with respect to a given set point is selectable. A homeostatic response to external perturbations may be selectively favored when the potential reproductive advantage offered by a reorganization of cell resources cannot be exploited. This is most likely to occur in the case of environmental perturbations of moderate intensity and short duration relative to the growth rate. Under these circumstances, homeostasis may be an energetically and competitively preferable option, because it requires no alteration of the expressed proteome and eliminates the requirement for reverse acclimation, upon cessation of the perturbation. This review also intends to be a stimulus to "ad hoc" experiments to assess the ecological and evolutionary relevance of homeostasis.

Life and light: exotic photosynthesis in binary and multiple-star systems

The potential for Earth-like planets within binary/multiple-star systems to host photosynthetic life was evaluated by modeling the levels of photosynthetically active radiation (PAR) such planets receive. Combinations of M and G stars in (i) close-binary systems; (ii) wide-binary systems, and (iii) three-star systems were investigated, and a range of stable radiation environments were found to be possible. These environmental conditions allow for the possibility of familiar, but also more exotic, forms of photosynthetic life, such as IR photosynthesizers and organisms that are specialized for specific spectral niches.

Impact of irradiance on the C allocation in the coastal marine diatom Skeletonema marinoi Sarno and Zingone

Elemental stoichiometry and organic composition were investigated in an Adriatic strain of Skeletonema marinoi, cultured at 25 [low light (LL)] and 250 [high light (HL)]µmol photon m⁻² s⁻¹. Inorganic carbon acquisition, fixation and allocation, and silicic acid and orthophosphate uptake were also studied. The C:P ratio was below the Redfield ratio, especially at LL. In HL cells, N quota was halved, C quota was similar, silica quota was lower, growth rate and long-term net primary productivity were almost doubled, relative to LL cells. The HL:LL cell quota ratios were 6 for lipid, 0.5 for protein and 0.4 for carbohydrate. Phosphoenolpyruvate carboxylase (PEPc) and glutamine synthetase (GS) activities were unaffected by the growth irradiance; phosphoenolpyruvate carboxykinase (PEPck) was 2.5-fold more active in LL cells. This suggests that in S. marinoi, C₄ photosynthesis is unlikely, PEPc is anaplerotic and PEPck may be involved in the conversion of lipid C to carbohydrates, especially in LL cells. Because about 50% of the cost for the production of an HL cell is caused by lipid biosynthesis, we propose that the preferential allocation of C to lipid at HL takes advantage of the relatively high volume-based energy content of lipids, in an organism that reduces its size at each vegetative cell division.

ECOLOGICAL AND EVOLUTIONARY IMPLICATIONS OF CARBON ALLOCATION IN MARINE PHYTOPLANKTON AS A FUNCTION OF NITROGEN AVAILABILITY: A FOURIER TRANSFORM INFRARED SPECTROSCOPY APPROACH(1)

An imbalance in the cellular C:N ratio may appreciably affect C allocation in algal cells. The consequences of these rearrangements of cellular pools on cell energetics, ecological fitness, and evolutionary trajectories are little known, although they are expected to be substantial. We investigated the fate of C in 11 microalgae cultured semicontinuously at three [NO3 (-) ] and constant pCO2 . We developed a new computational method for the semiquantitative use of Fourier transform infrared (FTIR) spectroscopy data for the determination of macromolecular composition. No obvious relationship was observed between the taxonomy and the allocation strategies adopted by the 11 species considered in this study. Not all species responded to a lower N availability by accumulating lipids or carbohydrates: Dunaliella parva W. Lerche and Thalassiosira pseudonana Hasle et Heimdal were homeostatic with respect to organic cell composition. A hyperbolic dependence of the lipid concentration from cell volume was observed. The level of reduction of organic constituents of green algae was parabolically related to size and was modulated in response to changes in N availability; the same was not true for the species bearing a "red" chloroplast. The above observations are discussed with respect to phytoplankton species composition and palatability for grazers, oleogenesis, and overall cell energetics.

Compositional homeostasis of the dinoflagellate Protoceratium reticulatum grown at three different pCO2

In the CO2-richer world that awaits us, the impact of elevated pCO2 on the allocation of resources in phytoplankton may have profound repercussions on the physiology of the microalgae and on the ecology of the ecosystems of which they are part. We studied the overall physiology and cell composition of the potentially toxic dinoflagellate Protoceratium reticulatum subjected to a medium-term increase of CO2. The physiological responses investigated were growth rates, cell size, photosynthetic and respiratory rates, and key enzyme activities. Cell composition was assessed by conventional analytical methods and FTIR spectroscopy. After 3 generations of incubation at current atmospheric, high and very high pCO2 (380, 1000, 5000ppm CO2), growth, photosynthesis, and dark respiration rates increased significantly, but the internal composition was only slightly affected. We propose the homeostasis of cell composition as a strategy that organisms can use to tackle environmental perturbations, especially when they are of relatively short duration.

FOURIER TRANSFORM INFRARED SPECTROSCOPY OF MICROALGAE AS A NOVEL TOOL FOR BIODIVERSITY STUDIES, SPECIES IDENTIFICATION, AND THE ASSESSMENT OF WATER QUALITY(1)

Fourier transform infrared (FTIR) spectrometry was used to study the spectral features of 12 eukaryotic and two prokaryotic species of microalgae. The algae were cultured in liquid media containing either NO3 (-) or NH4 (+) as the sole N-source; for the NH4 (+) treatment, the algae were subjected to short-term (24 h) or long-term (1 month) incubations; for the hypersaline species, cells were also grown in the presence of 2 M NaCl. Over 500 spectra, acquired from at least three distinct cultures for each species, in each growth regime, were subjected to hierarchical cluster analysis (HCA) and were successfully separated according to their taxonomy, showing that the overall spectra were characteristic of each species and that this technique could be fruitfully employed to separate microalgal species living in a similar condition (as would be the case for a natural assemblage). In addition, in most cases, it was possible to differentiate between algae subjected to different growth treatments although belonging to the same species. We also demonstrated that it is possible to accurately identify species and determine the nutritional status of their environment of origin (e.g., N-source), provided that suitable FTIR spectral libraries are available. This study aims to provide the basis for the development of rapid, easy, and inexpensive methods for the evaluation of biodiversity in natural phytoplankton samples and to monitor the water quality of natural environments.

The odd behaviour of carbonic anhydrase in the terrestrial cyanobacterium Nostoc flagelliforme during hydration-dehydration cycles

The terrestrial cyanobacterium Nostoc flagelliforme, inhabiting arid areas, withstands prolonged periods of dehydration. How dehydration and occasional wetting affect inorganic C acquisition in this organism is not well known. As inorganic C acquisition in cyanobacteria often involves carbonic anhydrases (CA), we studied the effect of cycles of hydration and dehydration on the extracellular and intracellular CA activities, at the pH values presumably associated with dew or rain wetting. The external CA of N. flagelliforme (or of the microorganismal consortium of which N. flagelliforme is the main component) is activated by hydration, especially at low pH, and it may facilitate inorganic C acquisition when N. flagelliforme colonies are wetted by dew. Internal CA is present in dry colonies and is rapidly inactivated upon rehydration, therefore an anaplerotic role for this enzyme is proposed.

Physiological responses of the green alga Dunaliella parva (Volvocales, Chlorophyta) to controlled incremental changes in the N source

This work is aimed at obtaining information on the acclimation processes of the green flagellate Dunaliella parva Lerche to gradual changes in the N source from NO₃^- to NH₄⁺, in continuous cultures. Photosynthesis, dark respiration, and light-independent carbon fixation (LICF) rates, chlorophyll a fluorescence, RUBISCO and phosphoenolpyruvate carboxylase (PEPc) activities, plasmalemma electrical potential difference, cell volume, and absolute or relative amounts of major cell constituents were measured. Two phases characterised the response to the transition from NO₃^- to NH₄⁺: (1) an initial phase in which photosynthesis and anaplerosis were stimulated and protein increased; (2) a subsequent phase in which most parameters reached new values that were close to those at the beginning of the experiment (100% NO₃^-). The only exceptions were PEPc activity and LICF, whose rates remained at least 2-fold higher than at 100% NO₃^-, when NH₄⁺ was the sole N source. The results are indicative of a tendency to re-establish homeostasis, after an initial perturbation of the intracellular composition. The roles of different metabolic processes during acclimation are discussed.