ALBERT

Description: BACKGROUND: Global change will affect patterns of nutrient upwelling in marine environments, potentially becoming even stricter regulators of phytoplankton primary productivity. To better understand phytoplankton nutrient utilization on the subcellular basis, we assessed the transcriptomic responses of the life-cycle stages of the biogeochemically important microalgae Emiliania huxleyi to nitrogen limitation. Cells grown in batch cultures were harvested at 'early' and 'full' nitrogen limitation and were compared with non-limited cells. We applied microarray-based transcriptome profilings, covering ~10.000 known E. huxleyi gene models, and screened for expression patterns that indicate the subcellular responses. RESULTS: The diploid life-cycle stage scavenges nitrogen from external organic sources and -like diatoms- uses the ornithine-urea cycle to rapidly turn over cellular nitrogen. The haploid stage reacts similarly, although nitrogen scavenging is less pronounced and lipid oxidation is more prominent. Generally, polyamines and proline appear to constitute major organic pools that back up cellular nitrogen. Both stages induce a malate:quinone-oxidoreductase that efficiently feeds electrons into the respiratory chain and drives ATP generation with reduced respiratory carbon throughput. CONCLUSIONS: The use of the ornithine-urea cycle to budget the cellular nitrogen in situations of limitation resembles the responses observed earlier in diatoms. This suggests that underlying biochemical mechanisms are conserved among distant clades of marine phototrophic protists. The ornithine-urea cycle and proline oxidation appear to constitute a sensory-regulatory system that monitors and controls cellular nitrogen budgets under limitation. The similarity between the responses of the life-cycle stages, despite the usage of different genes, also indicates a strong functional consistency in the responses to nitrogen-limitation that appears to be owed to biochemical requirements. The malate:quinone-oxidoreductase is a genomic feature that appears to be absent from diatom genomes, and it is likely to strongly contribute to the uniquely high endurance of E. huxleyi under nutrient limitation.

Description: The acquisition of dissolved inorganic carbon by aquatic primary producers became increasingly challenging with higher structural complexity of algae, and with simultaneously declining atmospheric CO2 partial pressure. The seemingly easy diffusive supply of CO2 to RubisCO turned into a bottleneck for photosynthesis, which consequently required alternative inorganic carbon acquisition processes and pathways to evolve. In order to ensure sufficient CO2 supply to RubisCO, aquatic photosynthesizing organisms started to employ facilitated CO2 uptake, active HCO3- trafficking across multiple membranes as well as carbonic anhydrases, located at the outer cell membrane and in several cellular compartments. The modes of these so-called CO2-concentrating mechanisms (CCMs) are very diverse, non-canonical even within phylogenetic groups, and possess differently efficient CO2 accumulation capacities, depending on the requirements of RubisCO, the physico-chemical conditions in the boundary layer, membrane properties and cellular architecture. However, different independently evolved CCMs also exhibit a high degree of functional similarity, owing to the functional similarity of the photosynthetic process. To introduce the topic to the reader, this chapter starts with a brief outline of RubisCO´s properties and the reasons why CCMs are required (4.2). Then, the principle chemical nature of dissolved inorganic carbon in water is described (4.3): Its speciation and kinetic behavior and relevant co-determinants of carbonate chemistry. We furthermore touch upon the physico-chemical basis of carbon availability in aquatic environments (4.4.), and subsequently elaborate on the known transport modes of different inorganic carbon species. Subsequently, the current state of knowledge on existing strategies in main algal groups is presented (4.5-4.9). Finally, we consider the operation of CCMs in the context of co-occurring cellular processes (4.10), such as calcification and N2 fixation, which rely on the provision of ample inorganic carbon and/or energy and, in the case of calcification, can have important consequences for compartmental pH homeostasis.