Algal Host Research Articles

Novel chemical weapon of an exotic macroalga inhibits recruitment of native competitors in the invaded range

Summary Allelopathy is an important non‐resource interaction in terrestrial plant communities that may affect invasions by non‐indigenous plants. The ‘novel weapons hypothesis’ (NWH) predicts that non‐indigenous plants will become invasive if they have allelopathic compounds that assemblages in the new range are not adapted to. Recently, the non‐indigenous, chemically rich macroalga Bonnemaisonia hamifera (Hariot) has become one of the most abundant filamentous red algae in Scandinavian waters. We used B. hamifera to specifically test the aspect of the NWH that concerns invasion success based on novel allelochemicals in the invaded range. Allelopathic interactions were tested through effects on the growth rate of adult native macroalgae in co‐cultures with B. hamifera and through the settlement success of native macroalgal propagules and microalgae on surfaces coated with 1,1,3,3‐tetrabromo‐2‐heptanone. We also investigated whether 1,1,3,3‐tetrabromo‐2‐heptanone can be transferred from B. hamifera to its native host algae, as a means of pre‐emptive competition. The settlement of native macroalgal propagules and microalgae was strongly inhibited on surfaces coated with 1,1,3,3‐tetrabromo‐2‐heptanone at ecologically relevant concentrations, but there were no effects of adult B. hamifera on growth rates of adults of the six native naturally co‐occurring species. The compound was shown to be transferred from B. hamifera to the surface of its native host algae at inhibitory concentrations in both laboratory and field experiments. By inhibiting the settlement of propagules on its thallus and on surrounding surfaces, B. hamifera achieves a competitive advantage over native macroalgae, a finding that parallels previous reports on soil‐ and litter‐mediated allelopathic interactions among vascular plants. Because competition for available substrata in marine benthic systems is intense, the ability to reserve space may be vital for B. hamifera's successful invasion. This is the first example of an allelopathic compound that can be transferred by direct contact from an exotic to a native species, with an active and unaltered function. Synthesis. Our results clearly show that the main secondary metabolite of the invasive red alga B. hamifera has strong allelopathic effects towards native competitors, suggesting that its novel chemical weapon is important for the highly successful invasion of new ranges.

Host selection by larvae of a marine insectHalocladius variabilis: nutritional dependency or escape from predation?

Larvae of the Holarctic marine chironomid,Halocladius variabilis(Staeger), have strong fidelity to the tuft-forming brown alga,Elachista fucicola(Velley) Areschoug, an abundant epiphyte on intertidal fucoids of the North Atlantic. We show that larvae are sufficiently motile to select an algal host in a Petri dish within 3–4 cm, and that larvae show differential behaviour with respect to host selection in the presence or absence of a predator. In the absence of predators 53% of larvae found an algal host within 1 hour; however, after 24 hours, there was no significant difference in host selection. When an isopod predator (Idoteasp.) was present, more larvae found a host within 1 hour (81%) andElachistawas chosen over three of the four other hosts. Furthermore, when larvae were present inElachista, predator (Carcinus maenus) success was significantly reduced relative to two other algal hosts. The adaptive significance ofElachistaas a refuge from predation was confirmed by experiments demonstrating that larval growth with other algal hosts was greater than withElachista. These experiments suggest that microhabitat selection by larvae ofH. variabilisreveals important tradeoffs for growth and predator avoidance.

Ectobiotic and Endobiotic Bacteria Associated with Eutreptiella sp. Isolated from Long Island Sound

Diversity and functional association of bacteria with Eutreptiella sp. was investigated. 16S rDNA analysis of ectobiotic bacteria revealed various lineages of Alphaproteobacteria and abundant Gammaproteobacteria, specifically Marinobacter. Antibiotic treatment yielded axenic cultures, and experiments based on them indicated that ectobiotic bacteria likely provide vitamin B(12) and other growth-enhancing factors for the alga. Further, DAPI staining and transmission electron microscopy revealed endobiotic bacteria in the cytoplasm of algal cells. 16S rDNA analysis showed that the bacteria belonged to one species that was most closely related to Rickettsiales endosymbionts of other organisms and phylogenetically affiliated with a new group of aquatic Rickettsiales. Observations from a diel experiment indicated that the endobiotic bacteria reproduced asynchronously with Eutreptiella sp. and had no adverse effects on lipid production (bioenergetics) or growth of the host alga. Our study reveals a diverse microbiome associated with this euglenoid alga, offering a system for studying the roles of algae-bacteria associations.

The epiphytic microbiota of the globally widespread macroalga Cladophora glomerata (Chlorophyta, Cladophorales)

The filamentous chlorophyte Cladophora produces abundant nearshore populations in marine and freshwaters worldwide, often dominating periphyton communities and producing nuisance growths under eutrophic conditions. High surface area and environmental persistence foster such high functional and taxonomic diversity of epiphytic microfauna and microalgae that Cladophora has been labeled an ecological engineer. We tested the hypotheses that (1) Cladophora supports a structurally and functionally diverse epiphytic prokaryotic microbiota that influences materials cycling and (2) mutualistic host-microbe interactions occur. Because previous molecular sequencing-based analyses of the microbiota of C. glomerata found as western Lake Michigan beach drift had identified pathogenic associates such as Escherichia coli, we also asked if actively growing lentic C. glomerata harbors known pathogens. We used 16S rRNA gene amplicon pyrosequencing to examine the microbiota of C. glomerata of Lake Mendota, Dane, Wisconsin, United States, during the growing season of 2011, at the genus- or species-level to infer functional phenotypes. We used correlative scanning electron and fluorescence microscopy to describe major prokaryotic morphotypes. We found microscopic evidence for diverse bacterial morphotypes, and molecular evidence for ca. 100 distinct sequence types classifiable to genus at the 80% confidence level or species at the 96-97% level within nine bacterial phyla, but not E. coli or related human pathogens. We inferred that bacterial epiphytes of lentic C. glomerata have diverse functions in materials cycling, with traits that indicate the occurrence of mutualistic interactions with the algal host.

A MOLECULAR PHYLOGENY OF ACROCHAETE AND OTHER ENDOPHYTIC GREEN ALGAE (ULVALES, CHLOROPHYTA)(1).

A molecular phylogeny was reconstructed from a culture collection of >150 isolates of epi-endophytic and endophytic green algae, based on nucleotide sequences of the plastid tufA and nuclear ITS2 loci. The cultures were isolated from a variety of algal hosts, notably the red algae Chondrus crispus, Mastocarpus stellatus, and Osmundea species, and the brown algae Chorda filum and Fucus serratus. The phylogeny revealed that in the Ulvales the majority of isolates fell into Acrochaete (Ulvellaceae), Ulva (Ulvaceae), Bolbocoleon (Bolbocoleaceae), and at least two unknown genera provisionally assigned to the Kornmanniaceae. Acrochaete was monophyletic. The genus was also more specious than previously described with 12 species, including up to six new species awaiting formal description. Isolates identified as Acrochaete repens, the type species of the genus, were polyphyletic. The remainder of the isolates were placed in the Ulotrichales. The results confirm that the endophytic habit supports a broad diversity of algal taxa and suggest that blade formation is a relatively recent innovation within the green algae.

Algicidal lactones from the marine Roseobacter clade bacterium Ruegeria pomeroyi

Volatiles released by the marine Roseobacter clade bacterium Rugeria pomeroyi were collected by use of a closed-loop stripping headspace apparatus (CLSA) and analysed by GC–MS. Several lactones were found for which structural proposals were derived from their mass spectra and unambiguously verified by the synthesis of reference compounds. An enantioselective synthesis of two exemplary lactones was performed to establish the enantiomeric compositions of the natural products by enantioselective GC–MS analyses. The lactones were subjected to biotests to investigate their activity against several bacteria, fungi, and algae. A specific algicidal activity was observed that may be important in the interaction between the bacteria and their algal hosts in fading algal blooms.

Phycodnavirus potassium ion channel proteins question the virus molecular piracy hypothesis.

Phycodnaviruses are large dsDNA, algal-infecting viruses that encode many genes with homologs in prokaryotes and eukaryotes. Among the viral gene products are the smallest proteins known to form functional K+ channels. To determine if these viral K+ channels are the product of molecular piracy from their hosts, we compared the sequences of the K+ channel pore modules from seven phycodnaviruses to the K+ channels from Chlorella variabilis and Ectocarpus siliculosus, whose genomes have recently been sequenced. C. variabilis is the host for two of the viruses PBCV-1 and NY-2A and E. siliculosus is the host for the virus EsV-1. Systematic phylogenetic analyses consistently indicate that the viral K+ channels are not related to any lineage of the host channel homologs and that they are more closely related to each other than to their host homologs. A consensus sequence of the viral channels resembles a protein of unknown function from a proteobacterium. However, the bacterial protein lacks the consensus motif of all K+ channels and it does not form a functional channel in yeast, suggesting that the viral channels did not come from a proteobacterium. Collectively, our results indicate that the viruses did not acquire their K+ channel-encoding genes from their current algal hosts by gene transfer; thus alternative explanations are required. One possibility is that the viral genes arose from ancient organisms, which served as their hosts before the viruses developed their current host specificity. Alternatively the viral proteins could be the origin of K+ channels in algae and perhaps even all cellular organisms.

Genomes and Virulence Factors of Novel Bacterial Pathogens Causing Bleaching Disease in the Marine Red Alga Delisea pulchra

Nautella sp. R11, a member of the marine Roseobacter clade, causes a bleaching disease in the temperate-marine red macroalga, Delisea pulchra. To begin to elucidate the molecular mechanisms underpinning the ability of Nautella sp. R11 to colonize, invade and induce bleaching of D. pulchra, we sequenced and analyzed its genome. The genome encodes several factors such as adhesion mechanisms, systems for the transport of algal metabolites, enzymes that confer resistance to oxidative stress, cytolysins, and global regulatory mechanisms that may allow for the switch of Nautella sp. R11 to a pathogenic lifestyle. Many virulence effectors common in phytopathogenic bacteria are also found in the R11 genome, such as the plant hormone indole acetic acid, cellulose fibrils, succinoglycan and nodulation protein L. Comparative genomics with non-pathogenic Roseobacter strains and a newly identified pathogen, Phaeobacter sp. LSS9, revealed a patchy distribution of putative virulence factors in all genomes, but also led to the identification of a quorum sensing (QS) dependent transcriptional regulator that was unique to pathogenic Roseobacter strains. This observation supports the model that a combination of virulence factors and QS-dependent regulatory mechanisms enables indigenous members of the host alga's epiphytic microbial community to switch to a pathogenic lifestyle, especially under environmental conditions when innate host defence mechanisms are compromised.

Who Is in There? Exploration of Endophytic Bacteria within the Siphonous Green Seaweed Bryopsis (Bryopsidales, Chlorophyta)

Associations between marine seaweeds and bacteria are widespread, with endobiotic bacterial-algal interactions being described for over 40 years. Also within the siphonous marine green alga Bryopsis, intracellular bacteria have been visualized by electron microscopy in the early ‘70s, but were up to now never molecularly analyzed. To study this partnership, we examined the presence and phylogenetic diversity of microbial communities within the cytoplasm of two Bryopsis species by combining fluorescence in situ hybridization (FISH), denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene clone libraries. Sequencing results revealed the presence of Arcobacter, Bacteroidetes, Flavobacteriaceae, Mycoplasma, Labrenzia, Phyllobacteriaceae and Xanthomonadaceae species. Although the total diversity of the endobiotic communities was unique to each Bryopsis culture, Bacteroidetes, Mycoplasma, Phyllobacteriaceae, and in particular Flavobacteriaceae bacteria, were detected in several Bryopsis samples collected hundreds of kilometres apart. This suggests that Bryopsis closely associates with well-defined endophytic bacterial communities of which some members possibly maintain an endosymbiotic relationship with the algal host.

Herbivorous amphipods inhabit protective microhabitats within thalli of giant kelp Macrocystis pyrifera

Many small marine herbivores utilize specific algal hosts, but the ultimate factors that shape host selection are not well understood. For example, the use of particular microhabitats within algal hosts and the functional role of these microhabitats have received little attention, especially in large algae such as kelps. We studied microhabitat use of the herbivorous amphipod Peramphithoe femorata that inhabits nest-like domiciles on the blades of giant kelp Macrocystis pyrifera. The vertical position of nest-bearing blades along the stipe of the algal thallus and the position of the nests within the lateral blades of M. pyrifera were surveyed in two kelp forests in northern-central Chile. Additionally, we conducted laboratory and field experiments to unravel the mechanisms driving the observed distributions. Peramphithoe femorata nests were predominantly built on the distal blade tips in apical sections of the stipes. Within-blade and within-stipe feeding preferences of P. femorata did not explain the amphipod distribution. Amphipods did not consistently select distal over proximal blade sections in habitat choice experiments. Mortality of tethered amphipods without nests was higher at the seafloor than at the sea surface in the field. Nests mitigated mortality of tethered amphipods, especially at the seafloor. Thus, protective microhabitats within thalli of large kelp species can substantially enhance survival of small marine herbivores. Our results suggest that differential survival from predation might be more important than food preferences in determining the microhabitat distribution of these herbivores.

Grazing damage and encrustation by an invasive bryozoan reduce the ability of kelps to withstand breakage by waves

Increased breakage of macroalgal fronds during large wave events can significantly reduce canopy cover and biomass. We examined the effects of encrustation by the invasive bryozoan Membranipora membranacea and damage by the snail Lacuna vincta on the ability of kelp blades (Saccharina longicruris, Laminaria digitata, and Laminaria complanata) to withstand wave forces. Using standard materials testing procedures, we documented significant reductions in the maximum stress before breakage, toughness, and extensibility of blade material following bryozoan encrustation. Histological sections of blade tissue indicated a significant degradation of the outer layers of cells following prolonged encrustation by M. membranacea as a likely cause of weakening. Full-thickness perforations and partial-thickness grazing scars also reduced blade strength, suggesting that grazing damage can initiate cracks that lead to blade breakage. Our findings provide a mechanistic link between the damaging effects of mesograzers and encrusting bryozoa on their algal hosts and the export of detrital material from subtidal kelp beds.

The enigmatic genome of Chara australis virus

Most of the genomic sequence of Chara australis virus (CAV), previously called Chara corallina virus, has been determined. It is a ssRNA molecule of 9065 nt with at least four ORFs. At its 5' end is an ORF encoding a protein of 227 kDa, distantly homologous to the multifunctional replicases of benyviruses and rubiviruses. Next is an ORF encoding a protein of 44 kDa, homologous to the helicases of pestiviruses. The third ORF encodes an unmatched protein of 38 kDa that is probably a movement protein. The fourth and 3'-terminal ORF encodes a protein of 17.7 kDa homologous to the coat proteins of tobamoviruses. The short methyltransferase region of the CAV replicase matches only the C-terminal motif of benyvirus methyltransferases. This and other clues indicate that approximately 11% and 2% of the 5' and 3' termini of the complete CAV genome, respectively, are missing from the sequence. The aligned amino acid sequences of the CAV proteins and their nearest homologues contain many gaps but relationships inferred from them were little affected by removal of these gaps. Sequence comparisons show that three of the CAV genes may have diverged from the most closely related genes of other viruses 250-450 million years ago, and the sister relationship between the genes of CAV and those of benyviruses and tobamoviruses, mirroring the ancient sister relationship between charophytes (i.e. the algal host of CAV) and embryophytes (i.e. the plant hosts of tobamoviruses and benyviruses), is congruent with this possibility.

Evidence for the involvement of surface carbohydrates in the recognition of Haematococcus pluvialis by the parasitic blastoclad Paraphysoderma sedebokerensis

The unicellular green alga Haematococcus pluvialis (Chlorophyta, Volvocales) is currently the best commercial source of the natural red ketocarotenoid astaxanthin. Paraphysoderma sedebokerensis (Blastocladiomycota), a parasitic blastoclad that is specific for this microalga, was recently isolated and identified in our laboratory. In this study, we investigated the recognition process between the parasite and H. pluvialis. Obligatory requirements for recognition were identified as an ion concentration in the medium of 20 mM, the presence of calcium ions, and neutral to basic conditions; these requirements imply that a protein is involved in the process. In a search for potential lectin–sugar interactions as a major event in the recognition process, we screened for exposed glycosidic moieties on the cell wall of the alga and on the parasite zoospore surface. Competition experiments with the appropriate lectins and monosugars identified Ricinus communis agglutinin (RCA 120) as the lectin that recognizes Gal- N-acetyl- d-glucosamine, an oligosaccharide located on the host. We propose that an RCA 120-like lectin–sugar interaction mediates the highly specific interaction between the blastocladian parasite and its algal host.

An examination of the epiphytic nature of Gambierdiscus toxicus, a dinoflagellate involved in ciguatera fish poisoning

Twenty-four specimen of macroalgae were collected in nearshore waters of the island of Hawaii, identified, and maintained to examine how the epiphytic relationship between Gambierdiscus toxicus (isolate BIG12) varied among the macroalgal species. Gambierdiscus cells were introduced to Petri dishes containing 100 g samples of each macroalgal host, which were examined at two, 16, 24, and every 24–72 h thereafter, over a 29-day period. Gambierdiscus proliferated in the presence of some host species (e.g., Galaxaura marginata and Jania sp.), but grew little in the presence of other species (e.g., Portieria hornemannii). Gambierdiscus exhibited high survival rates (>99%) in the presence of Chaetomorpha sp., but died before the end of the experiment (after 21 days) with other host species (e.g., Dictyota and Microdictyon spp.). Gambierdiscus avoided contact with P. hornemannii, but averaged up to 30% attachment with other host species. The numbers of Gambierdiscus cells belonging to one of three classes (alive and attached; alive and unattached; and dead) were determined for each time point. The 24 algal hosts were grouped according to their commonalities relative to these three classes using a Bray-Curtis similarity index, similarity profile (SIMPROF) permutation tests, and Multi-Dimensional Scaling (MDS) analysis (PRIMER 6). The resultant six groupings were used to construct different Gambierdiscus growth profiles for the different algal hosts. Group A is characterized by a preponderance of unattached cells and high mortality rates. Groups B, C, E, and F also displayed high proportions of unattached cells, but mortality either occurred later (Groups B and C) or rates were lower (Groups E and F). Group D had the highest proportion of attached cells. Group E contained three out of the four chlorophyte species, while Group F contained the majority of the rhodophytes. Over 50% of the species in Group F are considered to be palatable, whereas Groups A, B, and C are composed of species that exhibit chemical defenses against herbivory. The results of this study coupled with previous findings indicate that Gambierdiscus is not an obligate epiphyte; it can be free-swimming and found in the plankton. The conditions that lead to changes between epiphytic and planktonic stages need to be better studied in order to determine how they affect Gambierdiscus growth and physiology, connectivity and dispersion mechanisms, and toxin movement up into the foodweb.

The Jekyll-and-Hyde chemistry of Phaeobacter gallaeciensis

Emiliania huxleyi, an environmentally important marine microalga, has a bloom-and-bust lifestyle in which massive algal blooms appear and fade. Phaeobacter gallaeciensis belongs to the roseobacter clade of α-Proteobacteria, the populations of which wax and wane with that of E. huxleyi. Roseobacter are thought to promote algal growth by biosynthesizing and secreting antibiotics and growth stimulants (auxins). Here we show that P. gallaeciensis switches its secreted small molecule metabolism to the production of potent and selective algaecides, the roseobacticides, in response to p-coumaric acid, an algal lignin breakdown product that is symptomatic of aging algae. This switch converts P. gallaeciensis into an opportunistic pathogen of its algal host.

Acquisition and assimilation of carbon in non-bleached and bleached corals

Reef-forming corals cycle carbon (C) between the coral host, their endosymbiotic algae, and their skeleton. At elevated sea-surface temperatures this relationship breaks down and the corals bleach by expelling their endosymbiotic algae or these algae lose their photosynthetic pigments. The effect of thermally induced bleaching on the C cycling of 2 ecologically important coral species was investigated. The acquisition and assimilation of photoautotrophically and heterotrophically acquired C was examined via pulse-chase labeling experiments in thermally bleached and non- bleached Montipora capitata and Porites compressa corals. In non-bleached corals photoautotrophic and heterotrophic C were acquired and assimilated very differently. Namely, photoautotrophically acquired C was used to meet short-term metabolic demands and calcification, whereas heterotroph- ically acquired C was retained in both the coral host and endosymbiotic algae. In bleached corals there was a dramatic reduction in the assimilation of photoautotrophically acquired C by the endosymbiotic algae, in the translocation of C from the algae to the coral host, and in the C assimi- lated in the skeleton. The level of heterotrophically acquired C assimilated into bleached corals was similar to that in non-bleached corals, and was a direct source of organic C to the endosymbiotic algae. This host-to-endosymbiotic algal supply of heterotrophic C may stimulate endosymbiotic algal recovery. These findings show the importance of both photoautotrophic and heterotrophic C to coral function and demonstrate that both play a crucial role in the recovery from bleaching.

Epibacterial community patterns on marine macroalgae are host‐specific but temporally variable

Marine macroalgae are constantly exposed to epibacterial colonizers. The epiphytic bacterial patterns and their temporal and spatial variability on host algae are poorly understood. To investigate the interaction between marine macroalgae and epiphytic bacteria, this study tested if the composition of epibacterial communities on different macroalgae was specific and persisted under varying biotic and abiotic environmental conditions over a 2-year observation time frame. Epibacterial communities on the co-occurring macroalgae Fucus vesiculosus, Gracilaria vermiculophylla and Ulva intestinalis were repeatedly sampled in summer and winter of 2007 and 2008. The epibacterial community composition was analysed by denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene libraries. Epibacterial community profiles did not only differ significantly at each sampling interval among algal species, but also showed consistent seasonal differences on each algal species at a bacterial phylum level. These compositional patterns re-occurred at the same season of two consecutive years. Within replicates of the same algal species, the composition of bacterial phyla was subject to shifts at the bacterial species level, both within the same season but at different years and between different seasons. However, 7-16% of sequences were identified as species specific to the host alga. These findings demonstrate that marine macroalgae harbour species-specific and temporally adapted epiphytic bacterial biofilms on their surfaces. Since several algal host-specific bacteria were highly similar to other bacteria known to either avoid subsequent colonization by eukaryotic larvae or to exhibit potent antibacterial activities, algal host-specific bacterial associations are expected to play an important role for marine macroalgae.

Gene expression in the scleractinian Acropora microphthalma exposed to high solar irradiance reveals elements of photoprotection and coral bleaching.

BackgroundThe success of tropical reef-building corals depends on the metabolic co-operation between the animal host and the photosynthetic performance of endosymbiotic algae residing within its cells. To examine the molecular response of the coral Acropora microphthalma to high levels of solar irradiance, a cDNA library was constructed by PCR-based suppression subtractive hybridisation (PCR-SSH) from mRNA obtained by transplantation of a colony from a depth of 12.7 m to near-surface solar irradiance, during which the coral became noticeably paler from loss of endosymbionts in sun-exposed tissues.Methodology/Principal FindingsA novel approach to sequence annotation of the cDNA library gave genetic evidence for a hypothetical biosynthetic pathway branching from the shikimic acid pathway that leads to the formation of 4-deoxygadusol. This metabolite is a potent antioxidant and expected precursor of the UV-protective mycosporine-like amino acids (MAAs), which serve as sunscreens in coral phototrophic symbiosis. Empirical PCR based evidence further upholds the contention that the biosynthesis of these MAA sunscreens is a ‘shared metabolic adaptation’ between the symbiotic partners. Additionally, gene expression induced by enhanced solar irradiance reveals a cellular mechanism of light-induced coral bleaching that invokes a Ca2+-binding synaptotagmin-like regulator of SNARE protein assembly of phagosomal exocytosis, whereby algal partners are lost from the symbiosis.Conclusions/SignificanceBioinformatics analyses of DNA sequences obtained by differential gene expression of a coral exposed to high solar irradiance has revealed the identification of putative genes encoding key steps of the MAA biosynthetic pathway. Revealed also by this treatment are genes that implicate exocytosis as a cellular process contributing to a breakdown in the metabolically essential partnership between the coral host and endosymbiotic algae, which manifests as coral bleaching.

Algae as hosts for epifaunal bryozoans: Role of functional groups and taxonomic relatedness

Macroalgae build biogenic habitats which give shelter and provide a suitable physical environment for a great variety of organisms. Structural complexity of algal substrates may influence the composition of their attached epifauna. The aim of this study is to test whether the taxonomic relatedness of the algal hosts and the functional groups to which they belong influence the species richness and composition of their epifaunal bryozoans. We analysed 36 algal genera from the Atlantic coast of South America between 42°S and Cape Horn. Changes in bryozoan species richness (number of species) among different algal functional groups (filamentous algae, foliose algae, corticated foliose algae, corticated macrophytes) were non-significant. The composition of the epifaunal assemblages differed significantly only between filamentous and foliose algae. Sheet-like bryozoans (i.e. encrusting, pluriserial colonies) were more frequent on foliose than on filamentous algae, while runner-like species (i.e. uniserial stolons) were characteristic epibionts on filamentous thallii. Similarity of bryozoan assemblages increased with increasing taxonomic relatedness of their hosts. As most filamentous seaweeds analysed in this study are members of the Order Ceramiales, the influence of algal taxonomic relatedness and functional groups on the composition of their bryozoan assemblages can be viewed as two different aspects of the same phenomenon.

Identification of an l -Rhamnose Synthetic Pathway in Two Nucleocytoplasmic Large DNA Viruses

Nucleocytoplasmic large DNA viruses (NCLDVs) are characterized by large genomes that often encode proteins not commonly found in viruses. Two species in this group are Acanthocystis turfacea chlorella virus 1 (ATCV-1) (family Phycodnaviridae, genus Chlorovirus) and Acanthamoeba polyphaga mimivirus (family Mimiviridae), commonly known as mimivirus. ATCV-1 and other chlorovirus members encode enzymes involved in the synthesis and glycosylation of their structural proteins. In this study, we identified and characterized three enzymes responsible for the synthesis of the sugar L-rhamnose: two UDP-D-glucose 4,6-dehydratases (UGDs) encoded by ATCV-1 and mimivirus and a bifunctional UDP-4-keto-6-deoxy-D-glucose epimerase/reductase (UGER) from mimivirus. Phylogenetic analysis indicated that ATCV-1 probably acquired its UGD gene via a recent horizontal gene transfer (HGT) from a green algal host, while an earlier HGT event involving the complete pathway (UGD and UGER) probably occurred between a protozoan ancestor and mimivirus. While ATCV-1 lacks an epimerase/reductase gene, its Chlorella host may encode this enzyme. Both UGDs and UGER are expressed as late genes, which is consistent with their role in posttranslational modification of capsid proteins. The data in this study provide additional support for the hypothesis that chloroviruses, and maybe mimivirus, encode most, if not all, of the glycosylation machinery involved in the synthesis of specific glycan structures essential for virus replication and infection.