Aquatic phototrophs: efficient alternatives to land-based crops for biofuels.
Curr Opin Biotechnol. 19:235-40. Abstract
To mitigate some of the potentially deleterious environmental and agricultural consequences associated with current land-based-biofuel feedstocks, we propose the use of biofuels derived from aquatic microbial oxygenic photoautotrophs (AMOPs), more commonly known as cyanobacteria, algae, and diatoms. Herein we review their demonstrated productivity in mass culturing and aspects of their physiology that are particularly attractive for integration into renewable biofuel applications. Compared with terrestrial crops, AMOPs are inherently more efficient solar collectors, use less or no land, can be converted to liquid fuels using simpler technologies than cellulose, and offer secondary uses that fossil fuels do not provide. AMOPs pose a new set of technological challenges if they are to contribute as biofuel feedstocks.
Calcium controls the assembly of the photosynthetic water-oxidizing complex: a cadmium(II) inorganic mutant of the Mn4Ca core.
Philosophical Transactions of the Royal Society B-Biological Sciences. 363:1253-1261. Abstract
Perturbation of the catalytic inorganic core (Mn4Ca1OxCly) of the photosystem II-water-oxidizing complex (PSII-WOC) isolated from spinach is examined by substitution of Ca2+ with cadmium(II) during core assembly. Cd2+ inhibits the yield of reconstitution of O-2-evolution activity, called photoactivation, starting from the free inorganic cofactors and the cofactor-depleted apo-WOC-PSII complex. Ca2+ affinity increases following photooxidation of the first Mn2+ to Mn3+ bound to the 'high-affinity' site. Ca2+ binding occurs in the dark and is the slowest overall step of photoactivation (IM1/IM*(1) -> step). Cd2+ competitively blocks the binding of Ca2+ to its functional site with 10-to 30-fold higher affinity, but does not influence the binding of Mn2+ to its high-affinity site. By contrast, even 10-fold higher concentrations of Cd2+ have no effect on O-2-evolution activity in intact PSII-WOC. Paradoxically, Cd2+ both inhibits photoactivation yield, while accelerating the rate of photoassembly of active centres 10-fold relative to Ca2+. Cd2+ increases the kinetic stability of the photooxidized Mn3+ assembly intermediate(s) by twofold (mean lifetime for dark decay). The rate data provide evidence that Cd2+ binding following photooxidation of the first Mn3+, IM1/IM*(1), causes three outcomes: (i) a longer intermediate lifetime that slows IM1 decay to IM0 by charge recombination, (ii) 10-fold higher probability of attaining the degrees of freedom (either or both cofactor and protein d.f.) needed to bind and photooxidize the remaining 3 Mn2+ that form the functional cluster, and (iii) increased lability of Cd2+ following Mn-4 cluster assembly results in (re) exchange of Cd2+ by Ca2+ which restores active O-2-evolving centres. Prior EPR spectroscopic data provide evidence for an oxo-bridged assembly intermediate, Mn3+ (mu-O2-) Ca2+, for IM*(1). We postulate an analogous inhibited intermediate with Cd2+ replacing Ca2+.
Optimization of metabolic capacity and flux through environmental cues to maximize hydrogen production by the cyanobacterium "Arthrospira (Spirulina) maxima".
Applied and Environmental Microbiology. 74:6102-6113. Abstract
Environmental and nutritional conditions that optimize the yield of hydrogen (H-2) from water using a two-step photosynthesis/ fermentation (P/F) process are reported for the hypercarbonate-requiring cyanobacterium "Arthrospira maxima." Our observations lead to four main conclusions broadly applicable to fermentative H-2 production by bacteria: (i) anaerobic H-2 production in the dark from whole cells catalyzed by a bidirectional [NiFe] hydrogenase is demonstrated to occur in two temporal phases involving two distinct metabolic processes that are linked to prior light-dependent production of NADPH (photosynthetic) and dark/anaerobic production of NADH (fermentative), respectively; (ii) H-2 evolution from these reductants represents a major pathway for energy production (ATP) during fermentation by regenerating NAD(+) essential for glycolysis of glycogen and catabolism of other substrates; (iii) nitrate removal during fermentative H-2 evolution is shown to produce an immediate and large stimulation of H-2, as nitrate is a competing substrate for consumption of NAD(P) H, which is distinct from its slower effect of stimulating glycogen accumulation; (iv) environmental and nutritional conditions that increase anaerobic ATP production, prior glycogen accumulation (in the light), and the intracellular reduction potential (NADH/NAD(+) ratio) are shown to be the key variables for elevating H-2 evolution. Optimization of these conditions and culture age increases the H-2 yield from a single P/F cycle using concentrated cells to 36 ml of H-2/g (dry weight) and a maximum 18% H-2 in the headspace. H-2 yield was found to be limited by the hydrogenase-mediated H-2 uptake reaction.