The pyrenoid of the unicellular green alga Chlamydomonas reinhardtii is a microcompartment situated in the centre of the cup-shaped chloroplast, containing up to 90% of cellular Rubisco. Traversed by a network of dense, knotted thylakoid tubules, the pyrenoid has been proposed to influence thylakoid biogenesis and ultrastructure. Mutants that are unable to assemble a pyrenoid matrix, due to expressing a vascular plant version of the Rubisco small subunit, exhibit severe growth and photosynthetic defects and have an ineffective carbon-concentrating mechanism (CCM). The present study set out to determine the cause of photosynthetic limitation in these pyrenoid- less lines. We tested whether electron transport and light use were compromised as a direct structural consequence of pyrenoid loss or as a metabolic effect downstream of lower CCM activity and resulting CO2 limitation. Thylakoid organization was unchanged in the mutants, including the retention of intrapyrenoid-type thylakoid tubules, and photosynthetic limitations associated with the absence of the pyrenoid were rescued by exposing cells to elevated CO2 levels. These results demonstrate that Rubisco aggregation in the pyrenoid functions as an essential element for CO2 delivery as part of the CCM, and does not play other roles in maintenance of photosynthetic membrane energetics.

In chromophytic algae the major light-harvesting complex is the fucoxanthin chlorophyll alc protein complex. Recently, we have cloned several highly related cDNA and genomic sequences encoding the fucoxanthin chlorophyll a/c proteins from the diatom Phaeodactylum tricornutum. These genes are clustered on the nuclear genome. The sequences of the fucoxanthin chlorophyll a/c proteins as deduced from the gene sequences have some similarity to the chlorophyll a/b proteins associated with light-harvesting complexes of higher plants and green algae. Like the chlorophyll a/b proteins of higher plants, the fucoxanthin chlorophyll a/c proteins are synthesized as higher-molecular weight precursors in the cytoplasm of the cell and are transported into the plastids. However, the mode of transport into diatom plastids is very different from the mechanism involved in transporting proteins into the chloroplasts of higher plants and green algae. We focus here on the characteristics of the fucoxanthin chlorophyll alc proteins, the mode of transport of these proteins into plastids, the arrangement of the genes encoding these proteins, and efforts to utilize these genes to develop a DNA transformation system for diatoms.

Photosynthetic organisms have evolved to modulate their metabolism to accommodate the highly dynamic light and nutrient conditions in nature. In this review we discuss ways in which the green alga Chlamydomonas reinhardtii acclimates to nitrogen and sulfur deprivation, conditions that would limit the anabolic use of excitation energy because of a markedly reduced capacity for cell growth and division. Major aspects of this acclimation process are stringently regulated and involve scavenging the limited nutrient from internal and external sources, and the redirection of fixed carbon toward energy storage (e.g. starch, oil). However, photosynthetic organisms have also evolved mechanisms to dissipate excess absorbed light energy, and to eliminate potentially dangerous energetic electrons through the reduction of O-2 and H+ to H2O; this reduction can occur both through photosynthetic electron transport (e.g. Mehler reaction, chlororespiration) and mitochondrial respiration. Furthermore, algal cells likely exploit other energy management pathways that are currently not linked to nutrient limitation responses or that remain to be identified.

This article focuses on light-harvesting complexes (LHCs) in oxygen evolving photosynthetic organisms. These organisms include cyanobacteria, red algae, plants, green algae, brown algae, diatoms, chrysophytes, and dinoflagellates. We highlight the diversity of pigment-protein complexes that fuel the conversion of radiant energy to chemical bond energy in land plants and the diverse groups of the algae, detail the ways in which environmental parameters (i.e. light quantity and quality, nutrients) modulate the synthesis of these complexes, and discuss the evolutionary relationships among the LHC structural polypeptides.

We have isolated, from the prokaryotic cyanobacterium Synechococcus sp, strain PCC 7942, a gene encoding a protein of 72 amino acids [designated high light inducible protein (HLIP)] with similarity to the extended family of eukaryotic chlorophyll a/b binding proteins (CABs). HLIP has a single membrane-spanning alpha-helix, whereas both the CABs and the related early light inducible proteins have three membrane spanning helices, Hence, HLIP may represent an evolutionary progenitor of the eukaryotic members of the CAB extended family. We also show that the gene encoding HLIP is induced by high light and blue/UV-A radiation, The evolution, regulation, and potential function of HLIP are discussed.

Plastid evolution has been attributed to a single primary endosymbiotic event that occurred about 1.6 billion years ago (BYA) in which a cyanobacterium was engulfed and retained by a eukaryotic cell, although early steps in plastid integration are poorly understood. The photosynthetic amoeba Paulinella chromatophora represents a unique model for the study of plastid evolution because it contains cyanobacterium-derived photosynthetic organelles termed 'chromatophores' that originated relatively recently (0.09-0.14 BYA). The chromatophore genome is about a third the size of the genome of closely related cyanobacteria, but 10-fold larger than most plastid genomes. Several genes have been transferred from the chromatophore genome to the host nuclear genome through endosymbiotic gene transfer (EGT). Some EGT-derived proteins could be imported into chromatophores for function. Two photosynthesis-related genes ( psaI and csos4A) are encoded by both the nuclear and chromatophore genomes, suggesting that EGT in Paulinella chromatophora is ongoing. Many EGT-derived genes encode proteins that function in photosynthesis and photoprotection, including an expanded family of high-light-inducible (ncHLI) proteins. Cyanobacterial hli genes are high-light induced and required for cell viability under excess light. We examined the impact of light on Paulinella chromatophora and found that this organism is light sensitive and lacks light-induced transcriptional regulation of chromatophore genes and most EGTderived nuclear genes. However, several ncHLI genes have reestablished light-dependent regulation, which appears analogous to what is observed in cyanobacteria. We postulate that expansion of the ncHLI gene family and its regulation may reflect the light/oxidative stress experienced by Paulinella chromatophora as a consequence of the as yet incomplete integration of host and chromatophore metabolisms.

Cyanobacteria can survive in many environments that are often nutrient-limited. They have developed a suite of responses to deal with both biotic and abiotic stresses. In this review the responses of cyanobacteria to sulphur and phosphorus starvation are discussed. Both these macronutrients, as well as nitrogen, are often limiting either in freshwater or in marine environments. Although other macro- and micro-nutrients may be limiting in the environment but they are least studied. The alterations that occur in the cell, at both the physiological and the molecular level, are described. These range from specific responses to cope with a particular nutrient limitation or general responses to nutrient starvation, The understanding of how cyanobacteria cope with nutrient stress can lead to several alternative strategies is also discussed.

Biotic interactions underlie life's diversity and are the lynchpin to understanding its complexity and resilience within an ecological niche. Algal biologists have embraced this paradigm, and studies building on the explosive growth in omics and cell biology methods have facilitated the in-depth analysis of nonmodel organisms and communities from a variety of ecosystems. In turn, these advances have enabled a major revision of our understanding of the origin and evolution of photosynthesis in eukaryotes, bacterial-algal interactions, control of massive algal blooms in the ocean, and the maintenance and degradation of coral reefs. Here, we review some of the most exciting developments in the field of algal biotic interactions and identify challenges for scientists in the coming years. We foresee the development of an algal knowledgebase that integrates ecosystem-wide omics data and the development of molecular tools/resources to perform functional analyses of individuals in isolation and in populations. These assets will allow us to move beyond mechanistic studies of a single species towards understanding the interactions amongst algae and other organisms in both the laboratory and the field.

Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently utilize available resources. We have developed screens to isolate mutants unable to degrade their light-harvesting complex (phycobilisome) in response to nutrient deprivation. Complementation of these nonbleaching mutants has allowed us to uncover several genes involved in the pathway of sensing and responding to nutrient deprivation. One of these non-bleaching mutants has a lesion in a gene that is designated nblA encoding a small polypeptide that has homology to putative polypeptides in other cyanobacteria and red algae. No catalytic function for this polypeptide has been established although it is absolutely required for phycobilisome degradation A second mutant is complemented by the nblB gene, which encodes a polypeptide with homology to the family of proteins involved in attaching chromophores to apophycobiliprotein subunits. This protein may be required for removing bilin chromophores from phycobiliprotein subunits prior to their degradation. A third mutant was complemented by the nblR gene. NblR is a response regulator that appears to control at least some of the 'general' responses that occur during any of a number of different stress conditions. It is required Mr degradation of phycobilisomes and also appears to be necessary for controlling changes in photosynthetic activity that occur during exposure of cells to either nutrient limiting conditions or high light; this control is critical for allowing the cells to survive adverse environmental conditions.