Kevin J. Flynn

Placing microalgae on the biofuels priority list: a review of the technological challenges

Microalgae provide various potential advantages for biofuel production when compared with ‘traditional’ crops. Specifically, large-scale microalgal culture need not compete for arable land, while in theory their productivity is greater. In consequence, there has been resurgence in interest and a proliferation of algae fuel projects. However, while on a theoretical basis, microalgae may produce between 10- and 100-fold more oil per acre, such capacities have not been validated on a commercial scale. We critically review current designs of algal culture facilities, including photobioreactors and open ponds, with regards to photosynthetic productivity and associated biomass and oil production and include an analysis of alternative approaches using models, balancing space needs, productivity and biomass concentrations, together with nutrient requirements. In the light of the current interest in synthetic genomics and genetic modifications, we also evaluate the options for potential metabolic engineering of the lipid biosynthesis pathways of microalgae. We conclude that although significant literature exists on microalgal growth and biochemistry, significantly more work needs to be undertaken to understand and potentially manipulate algal lipid metabolism. Furthermore, with regards to chemical upgrading of algal lipids and biomass, we describe alternative fuel synthesis routes, and discuss and evaluate the application of catalysts traditionally used for plant oils. Simulations that incorporate financial elements, along with fluid dynamics and algae growth models, are likely to be increasingly useful for predicting reactor design efficiency and life cycle analysis to determine the viability of the various options for large-scale culture. The greatest potential for cost reduction and increased yields most probably lies within closed or hybrid closed–open production systems.

End-To-End Models for the Analysis of Marine Ecosystems: Challenges, Issues, and Next Steps

Abstract There is growing interest in models of marine ecosystems that deal with the effects of climate change through the higher trophic levels. Such end-to-end models combine physicochemical oceanographic descriptors and organisms ranging from microbes to higher-trophic-level (HTL) organisms, including humans, in a single modeling framework. The demand for such approaches arises from the need for quantitative tools for ecosystem-based management, particularly models that can deal with bottom-up and top-down controls that operate simultaneously and vary in time and space and that are capable of handling the multiple impacts expected under climate change. End-to-end models are now feasible because of improvements in the component submodels and the availability of sufficient computing power. We discuss nine issues related to the development of end-to-end models. These issues relate to formulation of the zooplankton submodel, melding of multiple temporal and spatial scales, acclimation and adaptation, behavioral movement, software and technology, model coupling, skill assessment, and interdisciplinary challenges. We urge restraint in using end-to-end models in a true forecasting mode until we know more about their performance. End-to-end models will challenge the available data and our ability to analyze and interpret complicated models that generate complex behavior. End-to-end modeling is in its early developmental stages and thus presents an opportunity to establish an open-access, community-based approach supported by a suite of true interdisciplinary efforts.

Growth dynamics and toxicity of Alexandrium fundyense (Dinophyceae): the effect of changing N:P supply ratios on internal toxin and nutrient levels

The effect of N- and P-limitation on the growth and paralytic shellfish toxin content of Alexandrium fundyense was studied in a series of batch cultures in which the N:P supply mass ratio was varied from less than 1 up to 160 (cf. Redfield ≡ 7.2). N- and P-limitation of growth was observed at the lower and higher ratios respectively, with dual limitation between. Cellular parameters were similar during the exponential phase of growth, regardless of external N:P ratio. Upon nutrient exhaustion cell yields and internal pools were affected, although these also varied according to the N-source (nitrate vs. ammonium). Generally N-limitation alone led to a higher C:N ratio, lower C:P and N:P ratios and a decreased toxin content in stationary-phase cells. P-limitation led to increased cell size and cell quotas, lower C:N ratios, and higher C:P and N:P ratios (to maxima of 350 and 35 respectively). Complex internal P-fractionation schemes were found unnecessary to determine the P-status of cells and extraction using potassium persulphate, cold water and cold HCl alone were adequate. Severe P-limitation led to increased toxin content but only when N was also limiting, suggesting a synergistic effect of N and P availability on toxin synthesis and turnover. A positive relationship was found between toxin content and the intracellular concentration of arginine, but this varied with the nutrient status (in particular P-status) of the cells.

Promotion of harmful algal blooms by zooplankton predatory activity.

The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator–prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator–prey model, it is demonstrated that these blooms can develop through the self-propagating failure of normal predator–prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.

Modelling multi-nutrient interactions in phytoplankton; balancing simplicity and realism

Abstract Modelling multi-nutrient interactions in phytoplankton growth is considered in the context of balancing the level of complexity with the adequacy of output for ammonium, nitrate, P, Si, Fe, temperature and light limitations. Phytoplankton submodels for placement in ecosystem simulators should be capable of producing a believable output. This requires not only a realistic growth rate response to limiting nutrients but also a realistic consumption of non’ or lesser limiting nutrients. The Monod structure, and allies, fails these requirements. The complexity of the Droop version of the quota model is not matched by flexibility, especially for the description of Si assimilation. A normalised version of the Caperon-Meyer quota model is a better (and no more expensive) structure that, by the addition of a feedback controlled uptake equation and a Monod-type Si assimilation control, gives considerable flexibility at reasonable computational cost. Once the step is taken to include one complex mechanistic component there is considerable advantage to be gained from adding further mechanistic structures with little more cost in integration time. This is especially so for light-iron interactions. Overly simplistic models should not be used just because they offer advantages in computational costs at the expense of realism, even if they give satisfactory fits to a particular data set, as output is more likely to be erroneous in ‘what-if’ scenarios and also during simulation of data-poor periods of extant data series.

The relationship between the dissolved inorganic carbon concentration and growth rate in marine phytoplankton

A range of marine phytoplankton was grown in closed systems in order to investigate the kinetics of dissolved inorganic carbon (DIC) use and the influence of the nitrogen source under conditions of constant pH. The kinetics of DIC use could be described by a rectangular hyperbolic curve, yielding estimations of KG(DIG) (the half saturation constant for carbon–specific growth, i.e. Cμ) and μmax (the theoretical maximum Cμ). All species attained a KG(DIC) within the range of 30–750μM DIC. For most species, NH+4 use enabled growth with a lower KG(DIC) and/or, for two species, an increase in μmax. At DIC concentrations of > 1.6 mM, Cμ was > 90% saturated for all species relative to the rate at the natural seawater DIC concentration of 2.0 mM. The results suggest that neither the rate nor the extent of primary productivity will be significantly limited by the DIC in the quasi–steady–state conditions associated with oligotrophic oceans. The method needs to be applied in the conditions associated with dynamic coastal (eutrophic) systems for clarification of a potential DIC rate limitation where cells may grow to higher densities and under variable pH and nitrogen supply.

Allometry and stoichiometry of unicellular, colonial and multicellular phytoplankton

Phytoplankton life forms, including unicells, colonies, pseudocolonies, and multicellular organisms, span a huge size range. The smallest unicells are less than 1 microm3 (e.g. cyanobacteria), while large unicellular diatoms may attain 10(9) microm3, being visible to the naked eye. Phytoplankton includes chemo-organotrophic unicells, colonies and multicellular organisms that depend on symbionts or kleptoplastids for their capacity to photosynthesize. Analyses of physical (transport within cells, diffusion boundary layers, package effect, turgor, and vertical movements) and biotic (grazing, viruses and other parasitoids) factors indicate potential ecological constraints and opportunities that differ among the life forms. There are also variations among life forms in elemental stoichiometry and in allometric relations between biovolume and specific growth. While many of these factors probably have ecological and evolutionary significance, work is needed to establish those that are most important, warranting explicit description in models. Other factors setting limitations on growth rate (selecting slow-growing species) await elucidation.

IS THE GROWTH RATE HYPOTHESIS APPLICABLE TO MICROALGAE?1

The growth rate hypothesis (GRH) asserts, from known biochemistry, that maintaining high growth rates requires high concentrations of ribosomes. Since ribosomes are rich in phosphorus (P), the GRH predicts a positive correlation between growth rate and P content; this correlation is observed in some organisms. We consider the application of the GRH to phytoplankton and identify several key problems that require further research before the hypothesis can be accepted for these organisms. There are severe methodological problems that confound interpretation of data for testing the GRH. These problems include the measurement of protein and nucleic acids (such that ratio of these components carries a high level of uncertainty), studies of steady‐state versus dynamic systems, and the presentation of data per cell (especially as cell size varies with growth rate limitations) and the calculation of growth rates. In addition, because of the short generation times and rapid responses of these organisms to perturbations, ribosome and RNA content is expected to vary in response to (de)repression of various systems; content may increase on application of growth‐limiting stress. Finally, that most phytoplankton accumulate P when not P stressed conflicts with the GRH. In consequence, the value of the GRH for any sort of predictive role in nature appears to be severely limited. We conclude that the GRH cannot be assumed to apply to phytoplankton taxa without first performing experimental tests under transient conditions.

Prey selection and rejection by a microflagellate; implications for the study and operation of microbial food webs

Abstract In a series of experiments, measurements were made of both numbers and biovolumes of the phototrophs Dunaliella primolecta (Butcher) (7.6 μm diameter), Isochrysis galbana (Parke) (4.5 μm), and Micromonas pusilla (Butcher) (1.5 μm), together with the phagotrophic dinoflagellate Oxyrrhis marina (Dujardin) (typically 16–20 μm). This enabled the calculation of the ‘equivalent encounter distance’ (l eq ), which gives a measure of the distance an average sized predator would have to swim in order to encounter a biovolume of prey equal to its own cell volume. If predation of a given prey type continues when its l eq is greater than that of an alternative prey item, then the predator is deemed to be demonstrating a preference for the former item. When confronted with all three phototrophs, Oxyrrhis selected Dunaliella first but, despite the 25-fold difference in cell volume, showed no preference for Isochrysis over Micromonas. Oxyrrhis may also reject Isochrysis on occasion, an event which seems to be associated particularly with elevated C:N ratios in the phototroph. Oxyrrhis has been seen to exhibit cannibalism when in the presence of Isochrysis biovolumes (biomass concentrations) an order of magnitude above that of Oxyrrhis . Such plasticity in prey selection makes it very difficult to predict the outcome of predator-prey interactions, especially where (as between Dunaliella and Isochrysis ) there are also growth interactions between the prey species. It also suggests that results obtained from short term studies of predation-kinetics, or in studies conducted under conditions such as in steady-state cultures and in continuous darkness, should not be generalised to more realistic environmental conditions.

Importance of Interactions between Food Quality, Quantity, and Gut Transit Time on Consumer Feeding, Growth, and Trophic Dynamics

Ingestion kinetics of animals are controlled by both external food availability and feedback from the quantity of material already within the gut. The latter varies with gut transit time (GTT) and digestion of the food. Ingestion, assimilation efficiency, and thus, growth dynamics are not related in a simple fashion. For the first time, the important linkage between these processes and GTT is demonstrated; this is achieved using a biomass‐based, mechanistic multinutrient model fitted to experimental data for zooplankton growth dynamics when presented with food items of varying quality (stoichiometric composition) or quantity. The results show that trophic transfer dynamics will vary greatly between the extremes of feeding on low‐quantity/high‐quality versus high‐quantity/low‐quality food; these conditions are likely to occur in nature. Descriptions of consumer behavior that assume a constant relationship between the kinetics of grazing and growth irrespective of food quality and/or quantity, with little or no recognition of the combined importance of these factors on consumer behavior, may seriously misrepresent consumer activity in dynamic situations.