[Show abstract][Hide abstract] ABSTRACT: The occurrence of active water transport (net transport against a free energy gradient) in photosynthetic organisms has been debated for several decades. Here, active water transport is considered in terms of its roles, where it is found, and the mechanisms by which it could occur. First there is a brief consideration of the possibility of active water transport into plant xylem in the generation of root pressure and the refilling of embolized xylem elements, and from an unsaturated atmosphere into terrestrial organisms living in habitats with limited availability of liquid water. There is then a more detailed consideration of volume and osmotic regulation in wall-less freshwater unicells, and the possiblity of generation of buoyancy in marine phytoplankton such as large-celled diatoms. Calculations show that active water transport is a plausible mechanism to assist cells in upwards vertical movements, requires less energy than synthesis of low-density organic solutes, and potentially on a par with excluding certain ions from the vacuole.
Journal of Experimental Botany 09/2014; · 5.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, the Kepler Space Telescope has detected several planets in orbit
around a close binary star system. These so-called circumbinary planets will
experience non-trivial spatial and temporal distributions of radiative flux on
their surfaces, with features not seen in their single-star orbiting
counterparts. Earthlike circumbinary planets inhabited by photosynthetic
organisms will be forced to adapt to these unusual flux patterns.
We map the flux received by putative Earthlike planets (as a function of
surface latitude/longitude and time) orbiting the binary star systems Kepler-16
and Kepler-47, two star systems which already boast circumbinary exoplanet
detections. The longitudinal and latitudinal distribution of flux is sensitive
to the centre of mass motion of the binary, and the relative orbital phases of
the binary and planet. Total eclipses of the secondary by the primary, as well
as partial eclipses of the primary by the secondary add an extra forcing term
to the system. We also find that the patterns of darkness on the surface are
equally unique. Beyond the planet's polar circles, the surface spends a
significantly longer time in darkness than latitudes around the equator, due to
the stars' motions delaying the first sunrise of spring (or hastening the last
sunset of autumn). In the case of Kepler-47, we also find a weak longitudinal
dependence for darkness, but this effect tends to average out if considered
over many orbits.
In the light of these flux and darkness patterns, we consider and discuss the
prospects and challenges for photosynthetic organisms, using terrestrial
analogues as a guide.
[Show abstract][Hide abstract] ABSTRACT: Algae frequently get a bad press. Pond slime is a problem in garden pools, algal blooms can produce toxins that incapacitate or kill animals and humans and even the term seaweed is pejorative - a weed being a plant growing in what humans consider to be the wrong place. Positive aspects of algae are generally less newsworthy - they are the basis of marine food webs, supporting fisheries and charismatic marine megafauna from albatrosses to whales, as well as consuming carbon dioxide and producing oxygen. Here we consider what algae are, their diversity in terms of evolutionary origin, size, shape and life cycles, and their role in the natural environment and in human affairs.
[Show abstract][Hide abstract] ABSTRACT: Multi-wavelength chlorophyll fluorescence analysis was utilised to examine the photosynthetic efficiency of the biofuel-producing alga Nannochloropsis oculata, grown under two light regimes; low (LL) and high (HL) irradiance levels. Wavelength dependency was evident in the functional absorption cross-section of Photosystem II (σII(λ)), absolute electron transfer rates (ETR(II)), and non-photochemical quenching (NPQ) of chlorophyll fluorescence in both HL and LL cells. While σII(λ) was not significantly different between the two growth conditions, HL cells upregulated ETR(II) 1.6-1.8-fold compared to LL cells, most significantly in the wavelength range of 440-540nm. This indicates preferential utilisation of blue-green light, a highly relevant spectral region for visible light in algal pond conditions. Under these conditions, the HL cells accumulated saturated fatty acids, whereas polyunsaturated fatty acids were more abundant in LL cells. This knowledge is of importance for the use of N. oculata for fatty acid production in the biofuel industry.
[Show abstract][Hide abstract] ABSTRACT: Proteaceae species in south-western Australia occur on phosphorus- (P) impoverished soils. Their leaves contain very low P levels, but have relatively high rates of photosynthesis. We measured ribosomal RNA (rRNA) abundance, soluble protein, activities of several enzymes, and glucose 6-phosphate (Glc6P) levels in expanding and mature leaves of six Proteaceae species in their natural habitat. The results were compared with those for Arabidopsis thaliana. Compared with A. thaliana, immature leaves of Proteaceae species contained very low levels of rRNA, especially plastidic rRNA. Proteaceae species showed slow development of the photosynthetic apparatus (“delayed greening”), with young leaves having very low levels of chlorophyll and Calvin-Benson cycle enzymes. In mature leaves, soluble protein and Calvin-Benson cycle enzyme activities were low, but Glc6P levels were similar to those in A. thaliana. We propose that low ribosome abundance contributes to the high P efficiency of these Proteaceae species in three ways: less P is invested in ribosomes; the rate of growth and, hence, demand for P is low; the especially low plastidic ribosome abundance in young leaves delays formation of the photosynthetic machinery, spreading investment of P in rRNA. Although Calvin-Benson cycle enzyme activities are low, Glc6P levels are maintained, allowing their effectively use.
Plant Cell and Environment 04/2014; 37:1276-1298. · 5.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent work has made progress in relating the size of stomata to stomatal functioning and, in particular, the speed of opening and closing and its implications. Calculations of the influence of stomatal size on the potential rate of osmolarity increase, assuming size-independent ion influx rate per unit area of guard cell plasmalemma set at the value found in large (60 μm long) stomata, show that 10 μm long stomata could have at least a 6-fold higher rate of osmolarity increase. There could be a corresponding decrease in the time taken in going from the closed to the fully open state from about 1h to about 10min; this is approximately the range found for stomata.. However, there are no data on the rate of stomatal movement over a sufficient size range to test this suggestion. Faster opening requires, assuming optimal allocation, a higher activity of the required enzymes per unit volume of guard cells. This is explored for cytosolic carbonic anhydrase which is needed in guard cells, at least in the light, for malic acid synthesis which is involved in stomatal opening in most stomata. Faster opening and closing of smaller than of larger stomata could allow closer tracking of environmental (mainly light) variations, although the available data are not adequate to determine if such a greater tracking occurs. The range of speeds of stomatal movement is similar to that for photoinhibition-related phenomena, despite the very different mechanisms involved.
Journal of Experimental Botany 03/2014; · 5.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Neoproterozoic (1000-542 million years ago, Mya) was characterized by profound global environmental and evolutionary changes, not least of which included a major rise in atmospheric oxygen concentrations [1, 2], extreme climatic fluctuations and global-scale glaciation , and the emergence of metazoan life in the oceans [4, 5]. We present here phylogenomic (135 proteins and two ribosomal RNAs, SSU and LSU) and relaxed molecular clock (SSU, LSU, and rpoC1) analyses that identify this interval as a key transition in the marine nitrogen cycle. Specifically, we identify the Cryogenian (850-635 Mya) as heralding the first appearance of both marine planktonic unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing picocyanobacteria (Synechococcus and Prochlorococcus ). Our findings are consistent with the existence of open-ocean environmental conditions earlier in the Proterozoic adverse to nitrogen-fixers and their evolution-specifically, insufficient availability of molybdenum and vanadium, elements essential to the production of high-yielding nitrogenases. As these elements became more abundant during the Cryogenian [7, 8], both nitrogen-fixing cyanobacteria and planktonic picocyanobacteria diversified. The subsequent emergence of a strong biological pump in the ocean implied by our evolutionary reconstruction may help in explaining increased oxygenation of the Earth's surface at this time, as well as tendency for glaciation.
Current biology: CB 02/2014; · 10.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photosynthetic life requires sufficient photosynthetically active radiation
(PAR) to metabolise. On Earth, plant behaviour, physiology and metabolism are
sculpted around the night-day cycle by an endogenous biological circadian
The evolution of life was influenced by the Earth-Sun orbital dynamic, which
generates the photo-environment incident on the planetary surface. In this work
the unusual photo-environment of an Earth-like planet (ELP) in 3:2 spin orbit
resonance is explored. Photo-environments on the ELP are longitudinally
differentiated, in addition to differentiations relating to latitude and depth
(for aquatic organisms) which are familiar on Earth. The light environment on
such a planet could be compatible with Earth's photosynthetic life although the
threat of atmospheric freeze-out and prolonged periods of darkness would
present significant challenges. We emphasise the relationship between the
evolution of life on a planetary body with its orbital dynamics.
International Journal of Astrobiology 02/2014; · 1.45 Impact Factor