IMMUNOCYTOCHEMICAL CHARACTERIZATION OF THE INTRAPYRENOID THYLAKOIDS OF CRYPTOMONADS1

Thylakoid lamellae extend into the pyrenoids of only two genera of cryptomonad algae, Chroomonas and Hemiselmis, We used immunoelectron microscopy to assess the photosynthetic competency of cryptomonad intrapyrenoid thylakoids. Intrapyrenoid thylakoids possess phycobiliproteins and the chlorophyll a/c₂ light-harvesting complex, both of which are associated with photosystem (PS) II in a light-harvesting capacity. In addition, thylakoids that extend into the pyrenoid of Hemiselmis brunnescens were immunolabelled by anti-PSI. These results indicate that cryptomonad intrapyrenoid thylakoids likely function in a manner analogous to thylakoids of the chloroplast stroma. Moreover, our observation that the Calvin cycle enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is pyrenoid-localized in these two cryptophytes indicates that the processes of photosynthetic O₂-evolution and ribulose 1,5-bisphosphate (RuBP) carboxylation/oxygenation are not spatially separated in these algae.     

Nuclear and Nucleomorph SSU rDNA Phylogeny in the Cryptophyta and the Evolution of Cryptophyte Diversity

The plastid-bearing members of the Cryptophyta contain two functional eukaryotic genomes of different phylogenetic origin, residing in the nucleus and in the nucleomorph, respectively. These widespread and diverse protists thus offer a unique opportunity to study the coevolution of two different eukaryotic genomes within one group of organisms. In this study, the SSU rRNA genes of both genomes were PCR-amplified with specific primers and phylogenetic analyses were performed on different data sets using different evolutionary models. The results show that the composition of the principal clades obtained from the phylogenetic analyses of both genes was largely congruent, but striking differences in evolutionary rates were observed. These affected the topologies of the nuclear and nucleomorph phylogenies differently, resulting in long-branch attraction artifacts when simple evolutionary models were applied. Deletion of long-branch taxa stabilized the internal branching order in both phylogenies and resulted in a completely resolved topology in the nucleomorph phylogeny. A comparison of the tree topologies derived from SSU rDNA sequences with characters previously used in cryptophyte systematics revealed that the biliprotein type was congruent, but the type of inner periplast component incongruent, with the molecular trees. The latter is indicative of a hidden cellular dimorphism (cells with two periplast types present in a single clonal strain) of presumably widespread occurrence throughout cryptophyte diversity, which, in consequence, has far-reaching implications for cryptophyte systematics as it is practiced today.     

The RAB Family GTPase Rab1A from Plasmodium falciparum Defines a Unique Paralog Shared by Chromalveolates and Rhizaria

ABSTRACT. The RAB GTPases, which are involved in regulation of endomembrane trafficking, exhibit a complex but incompletely understood evolutionary history. We elucidated the evolution of the RAB1 subfamily ancestrally implicated in the endoplasmic reticulum-to-Golgi traffic. We found that RAB1 paralogs have been generated over the course of eukaryotic evolution, with some duplications coinciding with the advent of major eukaryotic lineages (e.g. Metazoa, haptophytes). We also identified a unique, derived RAB1 paralog, orthologous to the Plasmodium Rab1A, that occurs in stramenopiles, alveolates, and Rhizaria, represented by the chlorarachniophyte Gymnochlora stellata . This finding is consistent with the recently documented existence of a major eukaryotic clade ("SAR") comprising these three lineages. We further found a Rab1A-like protein in the cryptophyte Guillardia theta , but it exhibits unusual features among RAB proteins: absence of a C-terminal prenylation motif and an N-terminal extension with two MSP domains; and its phylogenetic relationships could not be established convincingly due to its divergent nature. Our results nevertheless point to a unique membrane trafficking pathway shared by at least some lineages of chromalveolates and Rhizaria, an insight that has implications towards interpreting the early evolution of eukaryotes and the endomembrane system.     

具尾蓝隐藻(Chroomonas caudata Geitler)研究──Ⅱ.藻蓝蛋白(Phycocyanin)的初步分析

初步分析了具尾蓝隐藻的藻蓝蛋白,其吸收光谱为一双峰曲线,两个吸收峰分别为５９０ｎｍ放６４０ｎｍ,按Ａ．Ｎ．Ｇｌａｚｅｒ等关于隐藻藻蓝蛋白分型的意见具尾蓝隐菏的藻蓝蛋白属于Ⅱ型ＰＣ－６４５。     

THE atpA GENE CLUSTER OF GUILLARDIA THETA (CRYPTOPHYTA): A PIECE IN THE PUZZLE OF CHLOROPLAST GENOME EVOLUTION

We cloned and sequenced the genes of the atp A ( atp 1) cluster, together with its upstream and downstream sequences, of the chloroplast genome of the cryptomonad Guillardia theta Hill et Wetherbee and used gene signatures, predicted amino acid sequences, and gene arrangements for phylogenetic inferences. The Guillardia atp A cluster contains the genes atp I,H,G,F,D,A in the given order, thus reflecting the cyanobacterial/red algal/chromophyte gene arrangement. In addition, the order of both the upstream sequences rpo B,C₁,C₂, rps 2, tsf and the downstream sequences ycf 16/24, which is found exclusively in red algal chloroplast genomes, is conserved in Guillardia. This gene order, which presumably is the result of the reduction process of the cyanobacterial genome following primary endosymbiosis, provides strong evidence for a red algal ancestry of the Guillardia chloroplast and supports the hypothesis of secondary endosymbioses giving rise to chl a + c -containing algae. The close evolutionary relationship of the chloroplasts of Guillardia and red algae is furthermore supported by the lack of introns, high degrees of sequence similarities, and additional gene signatures, including spacers, gene overlaps, and inverted repeats. Gene cluster analysis, including the ATPase genes together with their upstream and downstream genes, is consistent with a single primary photosynthetic eukaryote that gave rise to all extant algal lineages and land plants by either direct filiation or secondary endosymbioses.     

Phytoplankton sterol contents vary with temperature,phosphorus and silicate supply: a study on three freshwater species

The understanding of environmentally induced changes in the biochemical composition of phytoplankton species is of great importance in both physiological studies and ecological food web research. In extensive laboratory experiments we tested the influence of two different temperatures (10°C and 25°C) and a phosphorus supply gradient on the sterol concentrations of the three freshwater phytoplankton species Scenedesmus quadricauda, Cryptomonas ovata and Cyclotella meneghiniana. The diatom C. meneghiniana was additionally exposed to a silicate gradient. In two separate experiments we analysed (1) possible interactive effects of temperature and phosphorus supply and (2) the effect of four phosphorus levels and three silicate levels on algal sterol concentrations. We observed that sterol concentrations were higher at 25°C than at 10°C in S. quadricauda and C. meneghiniana, but were not affected by temperature in C. ovata. Interactive effects of temperature and phosphorus supply on sterol concentrations were found in C. meneghiniana. This presumably was due to the bioconversion of one sterol (24-methylenecholesterol) into another (22-dihydrobrassicasterol). Increasing phosphorus supply resulted in species-specific effects on sterol concentrations, viz. an optimum curve response in S. quadricauda, a saturation curve response in C. meneghiniana and no change in sterol concentration in C. ovata. Effects of silicate supply on the sterols of C. meneghiniana equalled the effects of phosphorus supply. Albeit we did not observe a general trend in the three phytoplankton species tested, we conclude that sterol concentrations of phytoplankton are strongly affected by temperature and nutrient supply. Interactive effects point out the importance of taking into account more than just one environmental factor when assessing the effects of environmentally induced changes on phytoplankton sterol concentrations.     

Carotenogenesis diversification in phylogenetic lineages of Rhodophyta

Carotenoid composition is very diverse in Rhodophyta. In this study, we investigated whether this variation is related to the phylogeny of this group. Rhodophyta consists of seven classes, and they can be divided into two groups on the basis of their morphology. The unicellular group (Cyanidiophyceae, Porphyridiophyceae, Rhodellophyceae, and Stylonematophyceae) contained only β‐carotene and zeaxanthin, “ZEA‐type carotenoids.” In contrast, within the macrophytic group (Bangiophyceae, Compsopogonophyceae, and Florideophyceae), Compsopogonophyceae contained antheraxanthin in addition to ZEA‐type carotenoids, “ANT‐type carotenoids,” whereas Bangiophyceae contained α‐carotene and lutein along with ZEA‐type carotenoids, “LUT‐type carotenoids.” Florideophyceae is divided into five subclasses. Ahnfeltiophycidae, Hildenbrandiophycidae, and Nemaliophycidae contained LUT‐type carotenoids. In Corallinophycidae, Hapalidiales and Lithophylloideae in Corallinales contained LUT‐type carotenoids, whereas Corallinoideae in Corallinales contained ANT‐type carotenoids. In Rhodymeniophycidae, most orders contained LUT‐type carotenoids; however, only Gracilariales contained ANT‐type carotenoids. There is a clear relationship between carotenoid composition and phylogenetics in Rhodophyta. Furthermore, we searched open genome databases of several red algae for references to the synthetic enzymes of the carotenoid types detected in this study. β‐Carotene and zeaxanthin might be synthesized from lycopene, as in land plants. Antheraxanthin might require zeaxanthin epoxydase, whereas α‐carotene and lutein might require two additional enzymes, as in land plants. Furthermore, Glaucophyta contained ZEA‐type carotenoids, and Cryptophyta contained β‐carotene, α‐carotene, and alloxanthin, whose acetylenic group might be synthesized from zeaxanthin by an unknown enzyme. Therefore, we conclude that the presence or absence of the four enzymes is related to diversification of carotenoid composition in these three phyla.     

An investigation into the possible light-shielding role of gas vacuoles in a planktonic blue-green alga

When the gas vacuoles of Anabaena flos-aquae Bréb. ex Born. et Flah. are collapsed, the optical properties of the alga change. While this may suggest a light-shielding role, photosynthetic measurements indicate that intact gas vacuoles reduce the light falling on the thylakoids by only 4%, or less. Intact gas vacuoles offer no protection against the lethal effects of ultraviolet light. When the alga is grown at high light intensity the gas vacuoles are fewer in number but are oriented peripherally in the cells. However, this does not markedly affect their light shielding efficiency. Spectrophotometric measurements carried out by others indicate a light shielding role by gas vacuoles in a non-planktonic blue-green alga, Nostoc muscorum Kütz., but do not give a quantitative estimate of this effect. In Anabaena no definite evidence of light-shielding is obtained by such a method. All of the experiments described were conducted with dilute algal suspensions to investigate shielding effects in individual cells. Possible self-shading effects in dense suspensions and surface water blooms require further investigation.     

STRUCTURE AND SIGNIFICANCE OF THE CRYPTOMONAD FLAGELLAR APPARATUS. I. CRYPTOMONAS OVATA (CRYPTOPHYTA)1

The absolute configuration of the flagellar apparatus in Cryptomonas ovata has been elucidated and found to be similar to that reported for Chilomonas paramecium. Variations apparent in the flagellar apparatus of Cryptomonas ovata include the presence of striations in the mitochondrion associated lamella, a rhizostyle which does not bear wing-like extensions from the microtubules and does not lie close to the nucleus, and a striated fibrous anchoring structure associated with one basal body which has not hitherto been described. The flagellar apparatus also includes a four stranded microtubular root which traverses into the anterior dorsal lobe of the cell, a striated fibrous root which is associated with a five stranded microtubular root, and a two stranded Cr root. The homologous nature of these roots to those in the larger cryptomonads is discussed in relation to the apparent reduction in flagellar apparatus size and complexity among the smaller cryptomonads. A diagrammatic reconstruction of the flagellar apparatus of Cryptomonas ovata is also presented.