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The importance of algae-derived biofuels has been highlighted by the current problems associated with fossil fuels. Considerable past research has shown that limiting nutrients such as nitrogen and phosphorus increases the cellular lipid content in microalgae. However, limiting the supply of nutrients results in decreased biomass, which in turn decreases the overall lipid productivity of cultures. Therefore, nutrient limitation has been a subject of dispute as to whether it will benefit biofuel production on an industrial scale. Our research explores the physiological changes a cell undergoes when exposed to nitrogen and phosphorus limitations, both individually and in combination, and also examines the biotechnological aspects of manipulating N and P in order to increase cellular lipids, by analyzing the lipid production. We show that nitrogen starvation and also nitrogen plus phosphorus starvation combined have a more profound effect on the physiology and macromolecular pools of Chlamydomonas reinhardtii than does phosphorus starvation alone. The photosynthetic performance of C. reinhardtii underwent drastic changes under nitrogen starvation, but remained relatively unaffected under phosphorus starvation. The neutral lipid concentration per cell was at least 2.4-fold higher in all the nutrient-starved groups than the nutrient-replete controls, but the protein level per cell was lower in the nitrogen-starved groups. Overall, nitrogen starvation has a more dramatic effect on the physiology and neutral lipids and protein levels of C. reinhardtii than phosphorus starvation. However, the level of total lipids per volume of culture obtained was similar among nutrient-replete and all of the nutrient-starved groups. We conclude that combined nitrogen and phosphorus starvation does not likely benefit biofuel production in terms of enhanced lipid or biomass production.

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Culture experiments were conducted on ten phytoplankton species to examine their biological and physiological responses during exposure to oil and a combination of oil and dispersant. The species tested included a range of taxa typically found in the Gulf of Mexico such as cyanobacteria, chlorophytes, and diatoms. Cultures were exposed to Macondo surrogate oil using the water accommodated fraction (WAF), and dispersed oil using a chemically enhanced WAF (CEWAF) and diluted CEWAF, to replicate conditions following the Deepwater Horizon spill in the Gulf of Mexico. A range of responses were observed, that could broadly class the algae as either "robust" or "sensitive" to oil and/or dispersant exposure. Robust algae were identified as Synechococcus elongatus, Dunaliella tertiolecta, two pennate diatoms Phaeodactylum tricornutum and Navicula sp., and Skeletonema grethae CCMP775, and were largely unaffected by any of the treatments (no changes to growth rate or time spent in lag phase relative to controls). The rest of the phytoplankton, all centric diatoms, exhibited at least some combination of reduced growth rates or increased lag time in response to oil and/or dispersant exposure. Photophysiology did not have a strong treatment effect, with significant inhibition of photosynthetic efficiency (F /F ) only observed in the CEWAF, if at all. We found that the effects of oil and dispersants on phytoplankton physiology were species-dependent, and not always detrimental. This has significant implications on how oil spills might impact phytoplankton community structure and bloom dynamics in the Gulf of Mexico, which in turn impacts higher trophic levels.

Permeable sediments are common across continental shelves and are critical contributors to marine biogeochemical cycling. Organic matter in permeable sediments is dominated by microalgae, which as eukaryotes have different anaerobic metabolic pathways to prokaryotes such as bacteria and archaea. Here we present analyses of flow-through reactor experiments showing that dissolved inorganic carbon is produced predominantly as a result of anaerobic eukaryotic metabolic activity. In our experiments, anaerobic production of dissolved inorganic carbon was consistently accompanied by large dissolved H2 production rates, suggesting the presence of fermentation. The production of both dissolved inorganic carbon and H2 persisted following administration of broad spectrum bactericidal antibiotics, but ceased following treatment with metronidazole. Metronidazole inhibits the ferredoxin/hydrogenase pathway of fermentative eukaryotic H2 production, suggesting that pathway as the source of H2 and dissolved inorganic carbon production. Metabolomic analysis showed large increases in lipid production at the onset of anoxia, consistent with documented pathways of anoxic dark fermentation in microalgae. Cell counts revealed a predominance of microalgae in the sediments. H2 production was observed in dark anoxic cultures of diatoms (Fragilariopsis sp.) and a chlorophyte (Pyramimonas) isolated from the study site, substantiating the hypothesis that microalgae undertake fermentation. We conclude that microalgal dark fermentation could be an important energy-conserving pathway in permeable sediments.