Cryptochromes are flavin-binding proteins that act as blue light receptors in bacteria, fungi, plants, and insects and are components of the circadian oscillator in mammals. Animal and plant cryptochromes are evolutionarily divergent, although the unicellular alga Chlamydomonas reinhardtii (Chlamydomonas throughout) has both an animal-like cryptochrome and a plant cryptochrome (pCRY; formerly designated CPH1). Here, we show that the pCRY protein accumulates at night as part of a complex. Functional characterization of pCRY was performed based on an insertional mutant that expresses only 11% of the wild-type pCRY level. The pcry mutant is defective for central properties of the circadian clock. In the mutant, the period is lengthened significantly, ultimately resulting in arrhythmicity, while blue light-based phase shifts show large deviations from what is observed in wild-type cells. We also show that pCRY is involved in gametogenesis in Chlamydomonas. pCRY is down-regulated in pregametes and gametes, and in the pcry mutant, there is altered transcript accumulation under blue light of the strictly light-dependent, gamete-specific gene GAS28. pCRY acts as a negative regulator for the induction of mating ability in the light and for the loss of mating ability in the dark. Moreover, pCRY is necessary for light-dependent germination, during which the zygote undergoes meiosis that gives rise to four vegetative cells. In sum, our data demonstrate that pCRY is a key blue light receptor in Chlamydomonas that is involved in both circadian timing and life cycle progression.

Diatoms and related algae, in contrast to higher plants, have a xanthophyll-dominated light harvesting complex and an endoplasmic reticulum (ER) network surrounding the plastid. We have previously demonstrated that polypeptide constituents of the light harvesting complex from the diatom Phaeodactylum tricornutum are nuclear encoded and synthesized as higher molecular weight precursors in the cytoplasm. The amino-termini of the precursor proteins, as deduced from their gene sequences, have features of a signal peptide. Here, we show that the precursor polypeptides can be cotranslationally imported and processed by an in vitro microsomal membrane system, suggesting that cytoplasmically synthesized proteins require a signal peptide to traverse an ER before entering the plastid. These results are discussed in the context of plastid evolution.

Algae are (mostly) photosynthetic eukaryotes that occupy multiple branches of the tree of life, and are vital for planet function and health. In this review, we highlight a transformative period in studies of the evolution and functioning of this extraordinary group of organisms and their potential for novel applications, wrought by high-throughput 'omic' and reverse genetic methods. We cover the origin and diversification of algal groups, explore advances in understanding the link between phenotype and genotype, consider algal sex determination, and review progress in understanding the roots of algal multicellularity. Experimental evolution studies to determine how algae evolve in changing environments are highlighted, as is their potential as production platforms for compounds of commercial interest, such as biofuel precursors, nutraceuticals, or therapeutics.

We are studying the multigene family encoding the fucoxanthin-chlorophyll binding proteins (fcp genes) that constitute the major component of the photosystem II-associated light harvesting complex in diatoms and brown algae. The characteristics of clusters of fcp genes on the genome of the diatom Phaeodactylum tricornutum are described. Sequence analysis of two genomic clones, PT5 and PT4, has demonstrated the presence of four fcp genes (fcpA, fcpB, fcpC, fcpD) on the former and two fcp genes (fcpE, fcpF) on the latter. The proteins encoded by the six characterized fcp genes range in similarity from 86% to 99%. The genes within each cluster are separated by short intergenic sequences (between 0.5 to 1.1 kb). None of these genes contain introns and all appear to be transcribed with short 5' transcribed, untranslated leader sequences; the transcription initiation sites were mapped 26 to 48 bases upstream of the ATG translation start site. Small conserved motifs are found among all of the genes just upstream of both the translation and the transcription start sites. The codon bias is similar in all of the fcp genes, with a predominance of pyrimidines in the third positions of codons of the four codon families. The two fcp genes that are most similar are fcpC and fcpD, and might represent a recent gene duplication. Southern analyses using fcp cDNAs as hybridization probes suggest that there may be additional sequences on the P. tricornutum genome that resemble the characterized fcp sequences.

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.