[Show abstract][Hide abstract]ABSTRACT: Little information is available on the energetics of buoyancy modulation in aflagellate phytoplankton, which comprise the majority of autotrophic cells found in the ocean. Here we computed for three aflagellate species of marine phytoplankton (Emiliania huxleyi, Thalassiosira pseudonana, and Ethmodiscus rex) the theoretical minimum energy cost as photons absorbed and nitrogen resource required of the key physiological mechanisms (i.e. replacement of quaternary ammonium by dimethylsulfoniopropionate, storage of polysaccharides, and cell wall biosynthesis) affecting the cell's vertical movement as a function of nitrogen (N) availability. These energy costs were also normalized to the capacity of each buoyancy mechanism to modulate sinking or rising rates based on Stokes’ law. The three physiological mechanisms could act as ballast in the three species tested in conditions of low N availability at a low fraction (<11.9%) of the total photon energy cost for growth. Cell wall formation in E. huxleyi was the least costly ballast strategy whereas in T. pseudonana the photon energy cost of the three ballast strategies were similar. In E. rex, carbohydrate storage and mobilization appear to be energetically cheaper than modulations in organic solute synthesis to achieve vertical migration. This supports the carbohydrate ballast strategy for vertical migration for this species, but argues against the theory of replacement of low or high density organic solutes. The present study brings new insights into the energy cost and potential selective advantage of several strategies modulating the buoyancy of aflagellate marine phytoplankton. This article is protected by copyright. All rights reserved.
Full-text Article · Apr 2016 · Journal of Phycology
[Show abstract][Hide abstract]ABSTRACT: Mosses are among the earliest branching embryophytes and probably originated not later than the early Ordovician when atmospheric
CO2 was higher and O2 was lower than today. The C3 biochemistry and physiology of their photosynthesis suggests, by analogy with tracheophytes, that growth of extant bryophytes
in high CO2 approximating Ordovician values would increase the growth rate. This occurs for many mosses, including Physcomitrella patens in suspension culture, although recently published transcriptomic data on this species at high CO2 and present-day CO2 show down-regulation of the transcription of several genes related to photosynthesis. It would be useful if transcriptomic
(and proteomic) data comparing growth conditions are linked to measurements of growth and physiology on the same, or parallel,
cultures. Mosses (like later-originating embryophytes) have been subject to changes in bulk atmospheric CO2 and O2 throughout their existence, with evidence, albeit limited, for positive selection of moss Rubisco. Extant mosses are subject
to a large range of CO2 and O2 concentrations in their immediate environments, especially aquatic mosses, and mosses are particularly influenced by CO2 generated by, and O2 consumed by, soil chemoorganotrophy from organic C produced by tracheophytes (if present) and bryophytes.
Article · Feb 2016 · Journal of Experimental Botany
[Show abstract][Hide abstract]ABSTRACT: Arranging organisms into functional groups aids ecological research by grouping organisms (irrespective of phylogenetic origin) that interact with environmental factors in similar ways. Planktonic protists traditionally have been split between photoautotrophic “phytoplankton” and phagotrophic “microzooplankton”. However, there is a growing recognition of the importance of mixotrophy in euphotic aquatic systems, where many protists often combine photoautotrophic and phagotrophic modes of nutrition. Such organisms do not align with the traditional dichotomy of phytoplankton and microzooplankton. To reflect this understanding, we propose a new functional grouping of planktonic protists in an eco-physiological context: (i) phagoheterotrophs lacking phototrophic capacity, (ii) photoautotrophs lacking phagotrophic capacity, (iii) constitutive mixotrophs (CMs) as phagotrophs with an inherent capacity for phototrophy, and (iv) non-constitutive mixotrophs (NCMs) that acquire their phototrophic capacity by ingesting specific (SNCM) or general non-specific (GNCM) prey. For the first time, we incorporate these functional groups within a foodweb structure and show, using model outputs, that there is scope for significant changes in trophic dynamics depending on the protist functional type description. Accordingly, to better reflect the role of mixotrophy, we recommend that as important tools for explanatory and predictive research, aquatic food-web and biogeochemical models need to redefine the protist groups within their structures.
[Show abstract][Hide abstract]ABSTRACT: Algae can use a wide range of combined N sources. All of them can use NH4+, and probably also use urea and one of more amino acids; most of them can also use NO2− and NO3−, and some can use betaines and/or purines. Transport of combined N into cells very often uses H+ or Na+ symport. Two cations transported per anionic N species, and one cation transported per neutral N species, enables the electrical potential difference across the membrane generated by active cation efflux to be used to increase the accumulation ratio inside:outside of the combined N species. Cationic N forms, e.g. NH4+, sometimes occur at very low concentrations in the natural environment, and cation symport can increase the steady-state NH4+ concentration. The transporters have been to some extent characterised at the molecular level, especially for the plasmalemma. Assimilation of inorganic N species into organic N within the cell use well-established pathways, i.e. NO3− reductase, NO2− reductase, and glutamine synthetase – glutamine-oxoglutarate aminotransferase enzymes. N assimilation, and especially the initial step (NO3− reductase), are under more direct redox control in microalgae than in vascular plants. Combined N species which as NO and NO3− are involved in signalling within the cell, but extent to which they modulate metabolism in response to internal and external cues needs clarification. It is important to bear in mind that the conclusions drawn here generally come from work on relatively few microalgal species, and generalisations should be viewed with caution.
[Show abstract][Hide abstract]ABSTRACT: It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is
known and where there are gaps in knowledge. Irreversible decarboxylases produce CO2, and CO2 is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO2; others use HCO3
–. The relative role of permeation through the lipid bilayer versus movement through CO2-selective membrane proteins in the downhill, non-energized, movement of CO2 is not clear. Passive permeation explains most CO2 entry, including terrestrial and aquatic organisms with C3 physiology and biochemistry, terrestrial C4 plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO3
– use in CO2 concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there
is some evidence of active CO2 influx at the plasmalemma of algae. HCO3
– active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO3
– can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO2 and HCO3
– pools consequent upon CCMs result in leakage involving CO2, and occasionally HCO3
–. Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering
in the CCM; leakage from terrestrial C4 plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage
better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.
Article · Oct 2015 · Journal of Experimental Botany
[Show abstract][Hide abstract]ABSTRACT: Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably . Long thought to be the preferred form of N for phytoplankton uptake, may actually suppress overall growth when concentrations are sufficiently high. has been well known to be inhibitory or repressive for uptake and assimilation, but the concentrations of that promote vs. repress uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a “generic” eukaryotic cell, and the contrasting metabolic pathways and regulation of and when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory / reduction to and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in and metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are presented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing.
[Show abstract][Hide abstract]ABSTRACT: The Ordovician and Silurian periods were times of major geological activity as regards palaeogeography, volcanism and climate change, the last of these evidenced by a series of cooling episodes and glaciations that climaxed in the Hirnantian (Late Ordovician). The presence of cryptospores in the Darriwilian (Middle Ordovician) marked the advent of higher plants on land. A critical survey of direct (mega- and microfossils) and some indirect evidence in succeeding rocks indicates the presence of algae, Bacteria, Cyanobacteria, Fungi, probable lichens, cryptophytes and basal tracheophytes. Similar associations of photosynthesizers and decomposers occur today in cryptogamic covers (CCs), for example biological crusts, except that bryophytes replace cryptophytes (basal embryophytes) and tracheophytes are absent. Thus, extant CCs, which make significant contributions today to global carbon and nitrogen fixation and prevention of erosion, provide an excellent analogue for the impacts of early land vegetation on both lithosphere and atmosphere. As a prerequisite to assessing impacts in Ordovician–Silurian times, with particular consideration of parameters used by climate modellers, the effects of a number of abiotic factors on the growth and survival of extant cryptogamic ground covers and their environmental impacts are reviewed. Factors include photosynthetically active radiation, ultraviolet radiation, temperature, water, oxygen, carbon dioxide, nitrogen, phosphorus, iron, surface roughness and albedo. A survey of the nature and extent of weathering facilitated by such vegetation concludes that it was limited based on depth of weathering when compared with that from rooted tracheophytes today, with minor effects on carbon dioxide drawdown. As global net productivity from Ordovician–Silurian CCs was very probably lower than today, and while the small fraction of intractable material in their organic carbon would have resulted in a more rapid turnover of terrestrial biomass, we conclude that there was decreased possibility of long-term organic carbon burial. Hence, there would have been very limited increase in atmospheric oxygen and decrease in carbon dioxide resulting from carbon burial.
[Show abstract][Hide abstract]ABSTRACT: It was with a deep sense of loss and immense sadness that those at Plant, Cell and Environment learnt of the death, on 9 February 2015, of Harry Smith FRS. Harry was one of the four founding editors of the journal, and its first Chief Editor.
This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract]ABSTRACT: The Growth Rate Hypothesis (GRH) predicts a positive correlation between growth rate and RNA content, because growth depends on the protein synthesis machinery. The application of this hypothesis to photoautotrophic organisms has been questioned. We tested the GRH on one prasinophycean, Tetraselmis suecica and one chlorophycean Dunaliella salina, grown at three sulfate concentrations. Sulfate was chosen because its concentration in the oceans increased through geological time and apparently had a role in the evolutionary trajectories of phytoplankton. Cell protein content and P quota were positively related to the RNA content (r = 0.62 and r = 0.74, respectively). The correlation of the RNA content with growth rates (r = 0.95) indicates that the GRH was valid for these species, when growth rates were below 0.82 d(-1) .
This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract]ABSTRACT: Earth will become uninhabitable within 2-3 Gyr as a result of the moving
boundaries of the habitable zone caused by the increasing luminosity of the
Sun. Predictions about the future of habitable conditions on Earth include a
decline in species diversity and habitat extent, ocean loss and changes in the
magnitudes of geochemical cycles. However, testing these predictions on the
present-day Earth is difficult. The discovery of a planet that is a near
analogue to the far future Earth could provide a means to test these
predictions. Such a planet would need to have an Earth-like biosphere history,
requiring it to have been in its system's habitable zone (HZ) for Gyr-long
periods during the system's past, and to be approaching the inner-edge of the
HZ at present. Here we assess the possibility of finding this very specific
type of exoplanet and discuss the benefits of analysing older Earths in terms
of improving our understanding of long-term geological and bio-geological
processes. As an illustrative example, G stars within 10 parsecs are assessed
as potential old-Earth-analogue hosts. Surface temperature estimates for
hypothetical inner-HZ Earth analogues are used to determine whether any such
planets in these systems would be at the right stage in their late-habitable
lifetimes to exhibit detectable biosignatures. Predictions from planet
formation studies and biosphere evolution models suggest that only 0.36% of G
stars in the solar neighbourhood could host an old-Earth-analogue. However, if
the development of an Earth-like biosphere is assumed to be rare, requiring a
sequence of low-probability events to occur, then such planets are unlikely to
be found in the solar neighbourhood - although 1000s could be present in the
galaxy as a whole.