The relatedness of Prochloron sp. Isolated from different didemnid ascidian hosts

Copper coordination chemistry of the patellamides - cyanobactins in the ascidian-Prochloron symbiosis

Prochloron didemni, an obligate symbiont of certain ascidians (sea squirts found in tropical areas), produces various cyclic pseudo-octapeptides in large quantities. These secondary metabolites have attracted the attention of medicinal chemists and, due to their four azol(in)e and four amide donor groups, coordination chemists have become interested in these molecules. The structures of the metal-free macrocycles and their dinuclear copper(II) complexes are known, and solution equilibria, spectroscopic properties and a range of biologically relevant reactions have been studied in detail. However, until recently, the properties of the patellamides and structures of the copper(II) complexes in living systems have not been known unambiguously. These are reviewed in the present Perspective and, as a result, it now is possible to discuss possible biological functions of these species.

Microenvironment and phylogenetic diversity of Prochloron inhabiting the surface of crustose didemnid ascidians

The cyanobacterium Prochloron didemni is primarily found in symbiotic relationships with various marine hosts such as ascidians and sponges. Prochloron remains to be successfully cultivated outside of its host, which reflects a lack of knowledge of its unique ecophysiological requirements. We investigated the microenvironment and diversity of Prochloron inhabiting the upper, exposed surface of didemnid ascidians, providing the first insights into this microhabitat. The pH and O2 concentration in this Prochloron biofilm changes dynamically with irradiance, where photosynthetic activity measurements showed low light adaptation (Ek ∼ 80 ± 7 μmol photons m(-2) s(-1)) but high light tolerance. Surface Prochloron cells exhibited a different fine structure to Prochloron cells from cloacal cavities in other ascidians, the principle difference being a central area of many vacuoles dissected by single thylakoids in the surface Prochloron. Cyanobacterial 16S rDNA pyro-sequencing of the biofilm community on four ascidians resulted in 433 operational taxonomic units (OTUs) where on average -85% (65-99%) of all sequence reads, represented by 136 OTUs, were identified as Prochloron via blast search. All of the major Prochloron-OTUs clustered into independent, highly supported phylotypes separate from sequences reported for internal Prochloron, suggesting a hitherto unexplored genetic variability among Prochloron colonizing the outer surface of didemnids.

Host specificity and phylogeography of the prochlorophyte Prochloron sp., an obligate symbiont in didemnid ascidians

Prochloron is an oxygenic photosynthetic bacterium that lives in obligate symbiosis with didemnid ascidians, such as Diplosoma spp., Lissoclinum spp. and Trididemnum spp. This study investigated the genetic diversity of the genus Prochloron by constructing a phylogenetic tree based on the 16S rRNA gene sequences of 27 isolates from 11 species of didemnid ascidians collected from Japan, Australia and the USA. The 27 isolates formed three phylogenetic groups: 22 of the samples were identified to be closely related members of Prochloron. Two samples, isolated from Trididemnum nubilum and Trididemnum clinides, were found to belong to the species Synechocystis trididemni, the closest relative of Prochloron. Three isolates formed a separate group from both Prochloron sp. and S. trididemni, potentially indicating a new symbiotic phylotype. Genomic polymorphism analysis, employing cyanobacterium-specific highly iterative palindrome 1 repeats, could not delineate the isolates further. For the Prochloron sp. isolates, the phylogenetic outcome was independent of host species and geographic origin of the sample indicating a low level of host specificity, low genetic variation within the taxon and possibly a lack of a host-symbiont relationship during reproductive dispersal. This study contributes significantly to the understanding of Prochloron diversity and phylogeny, and implications for the evolutionary relationship of prochlorophytes, cyanobacteria and chloroplasts are also discussed.

The relationship of a prochlorophyte Prochlorothrix hollandica to green chloroplasts

It is generally accepted that chloroplasts arose from one or more endosymbiotic events between an ancestral cyanobacterium and a eukaryote. Such an origin fits well in the case of the chloroplasts of rhodophytes that, like cyanobacteria, contain chlorophyll a and phycobilin pigments. The green chloroplasts from higher plants, green algae, and euglenoids however, contain chlorophyll b as well as chlorophyll a, and lack phycobilins. Consequently, it has been suggested that they arose independently of the rhodophyte chloroplasts, from an ancestral prokaryote containing that complement of pigments. The 'prochlorophytes' Prochloron didemni (an exosymbiont on didemnid ascidians) and Prochlorothrix hollandica (a recently discovered, free-living, filamentous form) have been suggested to be modern counterparts of the ancestor of the green chloroplasts because they are prokaryotes that also contain both chlorophylls a and b, and lack phycobilins. We report here a 16S rRNA-based phylogenetic analysis of P. hollandica. The organism is found to fall within the cyanobacterial line of descent, as do the green chloroplasts, but it is not a specific relative of green chloroplasts. Thus, similar pigment compositions do not necessarily reflect close evolutionary relationships.

Peptidoglycan-polysaccharide complex in the cell wall of the filamentous prochlorophyte Prochlorothrix hollandica

A peptidoglycan-polysaccharide complex composed of N-acetylglucosamine, N-acetylmuramic acid, muramic acid 6-phosphate, L-alanine, D-alanine, D-glutamic acid, meso-diaminopimelic acid, N-acetylmannosamine, mannose, galactose, glucose, and phosphate was isolated from cell walls of the filamentous prochlorophyte Prochlorothrix hollandica; this complex was similar in chemical composition and structure to that found in cyanobacteria. Peptide patterns of partial acid hydrolysates of the isolated peptidoglycan revealed an A1 gamma structure with direct cross-linkage (m-diaminopimelic acid-D-alanine) of the peptide side chains. The degree of cross-linkage (63%) was found to be in the range of values obtained for gram-positive bacteria and cyanobacteria.

The problems of Prochloron

Prokaryotic green algae (prochlorophytes), which contain chlorophylls a and b but no bilin pigments, may be phylogenetically related to ancestral chloroplasts if symbiogenesis occurred. They may be otherwise related to eukaryotic chlorophytes. They could have evolved from cyanophytes by loss of phycobilin and gain of chlorophyll b synthesis. These possibilities are briefly discussed. Relevant evidence from biochemical studies in many collaborative laboratories is now becoming available for the resolution of such questions.