'Roots' in mixotrophic algae

A polyphasic method for the characterization of epiphytic diatoms growing on Gelidium corneum

Epiphytic diatoms associated with marine macroalgae play vital ecological roles in nutrient cycling and primary production, yet their study remains limited due to the lack of standardized methodologies. This study focuses on diatom communities growing on Gelidium corneum, a key red alga in the Cantabrian coast (Spain). Samples were collected from two depths along the northern coast of Spain and processed using both morphological and molecular approaches. Morphological analysis involved diatom frustule preparation using hydrogen peroxide digestion, acid treatments, and permanent slide mounting, enabling identification through light microscopy. Molecular analysis employed DNA extraction and rbcL marker-based metabarcoding, allowing detailed taxonomic characterization. Results highlight the efficacy of combining morphological and molecular techniques to overcome the limitations of either approach individually. By standardizing procedures, we enhance the reproducibility and comparability of studies focused on diatom epiphytes. Our results highlight the ecological significance of diatom-macroalgal interactions and provide a framework for future investigations into these essential but underexplored communities.•A polyphasic method was developed for studying epiphytic diatoms on Gelidium corneum, combining morphological and molecular tools.•The approach overcomes challenges in diatom characterization, including intricate host morphology and cryptic species identification.•Standardized protocols enhance reproducibility and offer insights into diatom-macroalgal ecological interactions.

Herbivore functions in the hot-seat: Resilience of Acanthurus triostegus to marine heatwaves

Herbivorous fishes play a crucial role in the conservation of coral reefs threatened by thermal stress (e.g., marine heatwaves and long-term ocean warming) by helping to maintain reefs in a coral-dominated state via the removal of algae. However, as thermally sensitive ectotherms, rising thermal stress may also pose a serious threat to these fishes and the critical ecosystem functions they deliver. Here we evaluate the consequences of thermal stress on the capacity of a common herbivorous coral reef fish (Acanthurus triostegus) to control finely filamentous matrices of Caulerpa sertularioides and C. verticillata algae in Hawai'i, by characterizing in-vivo changes in metabolic demands, diurnal foraging rates, activity patterns and individual condition in a laboratory setting during winter (24.0±0.1°C), summer (27.5±0.1°C), and at the peak of a representative marine heatwave, (31.0±0.1°C). Rising temperatures caused significant increases in standard metabolic rate (from ~135 O2 kg-1 h-1 in winter to 224 O2 kg-1 h-1 at the peak of a marine heatwave), but not in the proportion of time spent active (~83-96%) or foraging (~2.4 bites min-1). Consequently, A. triostegus gained body mass during summer and winter, but lost ~0.8% body mass per day during the marine heatwave. Given marine heatwaves can last for weeks to months, these results indicate that while herbivorous coral reef fishes may continue to remove algae during periods of thermal stress, their ability to control many macroalga may be limited due to precipitous reductions in individual performance. Therefore, in addition to algal types, the thermal sensitivity in herbivorous reef fishes will need to be considered for the successful implementation of coral-algal management strategies in a warmer world.

Bacterial communities on giant kelp in the Magellan Strait: Geographical and intra-thallus patterns

The giant kelp Macrocystis pyrifera is categorized as a keystone species, forming highly productive forests that provide ecosystem services and host a remarkable marine biodiversity of macro and microorganisms. The association of microorganisms with the algae is close and can be functionally interdependent. The Magellan Strait, a natural marine passage between the Atlantic and Pacific oceans, harbours extensive giant kelp forests. However, information related to the diversity of bacterial communities in this region is still scarce. In this study, 16S rRNA gene metabarcoding was used to characterize the diversity and composition of bacterial communities associated with apical blades and sporophylls of M. pyrifera from different sites (Bahía Buzo, San Gregorio, and Buque Quemado). Additionally, data from satellites and reanalysis, as well as tide data, were used to characterize the environmental variability. The findings revealed discernible local variations in bacterial taxa across sampling sites, with consistent dominance of Proteobacteria, Verrucomicrobia, Bacteroidetes, and Planctomycetes. Furthermore, a distinctive bacterial community structure was identified between apical and sporophyll blades of M. pyrifera. This research marks the inaugural characterization of bacterial community diversity and composition associated with M. pyrifera in the remote and understudied sub-Antarctic region of the Magellan Strait.

A comprehensive review on microbial diversity and anticancer compounds derived from seaweed endophytes: a pharmacokinetic and pharmacodynamic approach

Seaweed endophytes are a rich source of microbial diversity and bioactive compounds. This review provides a comprehensive analysis of the microbial diversity associated with seaweeds and their interaction between them. These diverse bacteria and fungi have distinct metabolic pathways, which result in the synthesis of bioactive compounds with potential applications in a variety of health fields. We examine many types of seaweed-associated microorganisms, their bioactive metabolites, and their potential role in cancer treatment using a comprehensive literature review. By incorporating recent findings, we hope to highlight the importance of seaweed endophytes as a prospective source of novel anticancer drugs and promote additional studies in this area. We also investigate the pharmacokinetic and pharmacodynamic profiles of these bioactive compounds because understanding their absorption, distribution, metabolism, excretion (ADMET), and toxicity profiles is critical for developing bioactive compounds with anticancer potential into effective cancer drugs. This knowledge ensures the safety and efficacy of proposed medications prior to clinical trials. This study not only provides promise for novel and more effective treatments for cancer with fewer side effects, but it also emphasizes the necessity of sustainable harvesting procedures and ethical considerations for protecting the delicate marine ecology during bioprospecting activities.

Effects of Hyphomonas Strains on the Growth of Red Algae Pyropia Species by Attaching Specifically to Their Rhizoids

Bacteria and marine macroalgae form close associations, while various bacteria affect the morphogenesis and growth of macroalgae. Hyphomonas strains exhibit normal morphogenetic activity in protoplasts of the red alga Pyropia yezoensis (nori). However, the effects of the bacteria on the growth of Pyropia from protoplast cells to regenerated thalli remain unknown. Here, we assessed the growth of P. yezoensis and Pyropia tenera using combined cultures of three Hyphomonas strains (LNM10-16, SCM-2, and LNM-9) and three algal media (artificial seawater with vitamins, artificial seawater, and natural seawater) over 7 weeks. Third week after culture, the three Hyphomonas strains showed almost similar levels of normal growth activity for both Pyropia species. However, at 7 weeks, significant differences were observed among the three Hyphomonas strains in terms of length, length-to-width ratio, and normal morphology of Pyropia thalli. LNM10-16 significantly promoted the thalli length and length-to-width ratios of both Pyropia species in artificial seawater without vitamins and natural seawater, compared with the other two Hyphomonas strains. P. yezoensis cultured in artificial seawater with vitamins showed a much higher demand for LNM10-16 in development of the thalli length than P. tenera. These results may be explained by differences in the growth activities of Hyphomonas strains and the nutrient requirements of Pyropia species. Furthermore, the bacteria were more specifically attached to the rhizoid surfaces of both species. This study is the first to reveal that Hyphomonas strains affect the growth of Pyropia species by attaching to their rhizoids.

No bacterial-mediated alleviation of thermal stress in a brown seaweed suggests the absence of ecological bacterial rescue effects

While microbiome alterations are increasingly proposed as a rapid mechanism to buffer organisms under changing environmental conditions, studies of these processes in the marine realm are lagging far behind their terrestrial counterparts. Here, we used a controlled laboratory experiment to examine whether the thermal tolerance of the brown seaweed Dictyota dichotoma, a common species in European coastal ecosystems, could be enhanced by the repeated addition of bacteria from its natural environment. Juvenile algae from three genotypes were subjected for two weeks to a temperature gradient, spanning almost the entire thermal range that can be tolerated by the species (11-30 °C). At the start of the experiment and again in the middle of the experiment, the algae were inoculated with bacteria from their natural environment or left untouched as a control. Relative growth rate was measured over the two-week period, and we assessed bacterial community composition prior to and at the end of the experiment. Since the growth of D. dichotoma over the full thermal gradient was not affected by supplementing bacteria, our results indicate no scope for bacterial-mediated stress alleviation. The minimal changes in the bacterial communities linked to bacterial addition, particularly at temperatures above the thermal optimum (22-23 °C), suggest the existence of a barrier to bacterial recruitment. These findings indicate that ecological bacterial rescue is unlikely to play a role in mitigating the effects of ocean warming on this brown seaweed.

A pathway to improve seaweed aquaculture through microbiota manipulation

Eukaryotic hosts are associated with microbial communities that are critical to their function. Microbiota manipulation using beneficial microorganisms, for example, in the form of animal probiotics or plant growth-promoting microorganisms (PGPMs), can enhance host performance and health. Recently, seaweed beneficial microorganisms (SBMs) have been identified that promote the growth and development and/or improve disease resistance of seaweeds. This knowledge coincides with global initiatives seeking to expand and intensify seaweed aquaculture. Here, we provide a pathway with the potential to improve commercial cultivation of seaweeds through microbiota manipulation, highlighting that seaweed restoration practices can also benefit from further understanding SBMs and their modes of action. The challenges and opportunities of different approaches to identify and apply SBMs to seaweed aquaculture are discussed.

Effect of UV-radiation on the physiology of the invasive green seaweed Codium fragile and its associated bacteria

Invasive species such as seaweeds often have a broad tolerance, allowing them to colonize novel habitats. During invasion, also new epibacteria can be formed on seaweeds, which have important chemo-ecological effects. Since UV-radiation (UVR) is one of the main factors affecting seaweeds and their epibacteria, we tested its effect on intertidal and subtidal thalli of the invasive seaweed Codium fragile from three sites and monitored photosynthesis, antioxidant activity and epibacteria. Exposure to UV-radiation resulted in photoinhibition with a subsequent low recovery in subtidal thalli from 23°S compared to 27°S and 30°S, which both showed a higher and almost complete recovery. However, a high antioxidant activity was present in all thalli, permitting to explain its relatively high tolerance to new environments. UV-radiation modified the composition of the epibacteria community by reducing its diversity and evenness. Our results showed that C. fragile responds plastic to variable UV-radiation (depending on site and water depth), which contributes to its high invasion potential.

The seaweed holobiont: from microecology to biotechnological applications

In the ocean, seaweed and microorganisms have coexisted since the earliest stages of evolution and formed an inextricable relationship. Recently, seaweed has attracted extensive attention worldwide for ecological and industrial purposes, but the function of its closely related microbes is often ignored. Microbes play an indispensable role in different stages of seaweed growth, development and maturity. A very diverse group of seaweed‐associated microbes have important functions and are dynamically reconstructed as the marine environment fluctuates, forming an inseparable ‘holobiont’ with their host. To further understand the function and significance of holobionts, this review first reports on recent advances in revealing seaweed‐associated microbe spatial and temporal distribution. Then, this review discusses the microbe and seaweed interactions and their ecological significance, and summarizes the current applications of the seaweed–microbe relationship in various environmental and biological technologies. Sustainable industries based on seaweed holobionts could become an integral part of the future bioeconomy because they can provide more resource‐efficient food, high‐value chemicals and medical materials. Moreover, holobionts may provide a new approach to marine environment restoration.

Insights into the bacterial community composition of farmed Caulerpa lentillifera: A comparison between contrasting health states

The bacterial communities of Caulerpa lentillifera were studied during an outbreak of an unknown disease in a sea grape farm from Vietnam. Clear differences between healthy and diseased cases were observed at the order, genus, and Operational Taxonomic Unit (OTU) level. A richer diversity was detected in the diseased thalli of C. lentillifera, as well as the dominance of the orders Flavobacteriales (phylum Bacteroidetes) and Phycisphaerales (Planctomycetes). Aquibacter, Winogradskyella, and other OTUs of the family Flavobacteriaceae were hypothesized as detrimental bacteria, this family comprises some well-known seaweed pathogens. Phycisphaera together with other Planctomycetes and Woeseia were probably saprophytes of C. lentillifera. The Rhodobacteraceae and Rhodovulum dominated the bacterial community composition of healthy C. lentillifera. The likely beneficial role of Bradyrhizobium, Paracoccus, and Brevundimonas strains on nutrient cycling and phytohormone production was discussed. The bleaching of diseased C. lentillifera might not only be associated with pathogens but also with an oxidative response. This study offers pioneering insights on the co-occurrence of C. lentillifera-attached bacteria, potential detrimental or beneficial microbes, and a baseline for understanding the C. lentillifera holobiont. Further applied and basic research is urgently needed on C. lentillifera microbiome, shotgun metagenomic, metatranscriptomic, and metabolomic studies as well as bioactivity assays are recommended.

Microbiota-Macroalgal Relationships at a Hawaiian Intertidal Bench Are Influenced by Macroalgal Phyla and Associated Thallus Complexity

The ocean represents the largest biome on earth; however, we have only begun to understand the diversity and function of the marine microbial inhabitants and their interactions with macroalgal species. Macroalgae play an integral role in overall ocean biome health and serve both as major primary producers and foundation species in the ecosystem. Previous studies have been limited, focusing on the microbiome of a single algal species or its interaction with selected microbes. This project aimed to understand overall biodiversity of microbial communities associated with five common macroalgal species and to determine the drivers of these communities at 'Ewa Beach, O'ahu, HI. Representative species of Chlorophyta (green), Ochrophyta (brown), and Rhodophyta (red) algae, each species having various levels of calcification, thallus complexity, and status as native or invasive species, were collected from an intertidal bench in May 2019. A portion of the V3-V4 variable region of the small-subunit rRNA gene was amplified for high-throughput sequencing using universal bacterial primers to elucidate the core and variable algal microbiome. Significant differences in bacterial community composition were only partially explained by host species, whether the host was native or invasive, and thallus complexity. Macroalgal phylum explained the most variation in associated microbial communities at 'Ewa Beach. This study advances our understanding of microbial-macroalgal interactions and their connectivity by producing insight into factors that influence the community structure of macroalga-associated microbiota. IMPORTANCE Generally, most eukaryotic organisms form relationships with microbes that are important in mediating host organismal health. Macroalgae are a diverse group of photosynthetic eukaryotic organisms that serve as primary producers and foundational species in many ecosystems. However, little is known about their microbial counterparts across a wide range of macroalgal morphologies, phylogenies, and calcification levels. Thus, to further understand the factors involved in bacterial community composition associated with macroalgal species at one point in time, representative samples were collected across phyla. Here, we show that both host macroalga phyla and morphology influenced the associated microbial community. Additionally, we show that the invasive species Avrainvillea lacerata does not have a unique microbial community on this intertidal bench, further supporting the idea that host phylum strongly influences microbial community composition.

Green Algal Models for Multicellularity

The repeated evolution of multicellularity across the tree of life has profoundly affected the ecology and evolution of nearly all life on Earth. Many of these origins were in different groups of photosynthetic eukaryotes, or algae. Here, we review the evolution and genetics of multicellularity in several groups of green algae, which include the closest relatives of land plants. These include millimeter-scale, motile spheroids of up to 50,000 cells in the volvocine algae; decimeter-scale seaweeds in the genus Ulva (sea lettuce); and very plantlike, meter-scale freshwater algae in the genus Chara (stoneworts). We also describe algae in the genus Caulerpa, which are giant, multinucleate, morphologically complex single cells. In each case, we review the life cycle, phylogeny, and genetics of traits relevant to the evolution of multicellularity, and genetic and genomic resources available for the group in question. Finally, we suggest routes toward developing these groups as model organisms for the evolution of multicellularity. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The surface bacterial community of an Australian kelp shows cross-continental variation and relative stability within regions

Epiphytic microbial communities often have a close relationship with their eukaryotic host, assisting with defence, health, disease prevention and nutrient transfer. Shifts in the structure of microbial communities could therefore have negative effects on the individual host and indirectly impact the surrounding ecosystem, particularly for major habitat-forming hosts, such as kelps in temperate rocky shores. Thus, an understanding of the structure and dynamics of host-associated microbial communities is essential for monitoring and assessing ecosystem changes. Here, samples were taken from the ecologically important kelp, Ecklonia radiata, over a 17-month period, from six different sites in two distinct geographic regions (East and West coasts of Australia), separated by ∼3,300 kms, to understand variation in the kelp bacterial community and its potential environmental drivers. Differences were observed between kelp bacterial communities between the largely disconnected geographical regions. In contrast, within each region and over time the bacterial communities were considerably more stable, despite substantial seasonal changes in environmental conditions.

Plastics and sedimentation foster the spread of a non-native macroalga in seagrass meadows

Plastics are found in marine environments worldwide, and their effects on macrophytes (seagrasses and macroalgae) colonizing sandy bottoms are still poorly known. Seagrass meadows are valuable but declining ecosystems due to local and global-change related stressors, including sediment disturbance and introduced macroalgae. Understanding whether plastics pose a further threat to seagrasses is critically important. In two simultaneous additive experiments performed in an aquaculture tank, we examined the individual and combined effects of macroplastics (non-biodegradable high-density polyethylene and biodegradable starch-based) and sedimentation (no and repeated sedimentation) on the performance (in terms of biomass and architectural variables) of a native Mediterranean seagrass (Cymodocea nodosa) and an introduced macroalga (Caulerpa cylindracea), and on the intensity of their interactions. Macroplastics were still present in sediments after 18 months. Cymodocea nodosa produced a greater biomass and longer horizontal rhizome internodes forming clones with more spaced shoots probably to escape from plastics. Plastics prevented C. nodosa to react to sedimentation by increasing vertical rhizome growth. Under C. cylindracea invasion, C. nodosa allocated more biomass to roots, particularly to fine roots. In the presence of C. nodosa, C. cylindracea performance was reduced. High-density polyethylene (HDPE) plastic and sedimentation shifted species interactions from competitive to neutral. These results suggest that both HDPE and biodegradable starch-based macroplastics, if deposited on marine bottoms, could make seagrasses vulnerable to sedimentation and reduce plant cover within meadows. HDPE plastic and sedimentation could contribute to the decline of seagrass habitats by facilitating the spread of non-native macroalgae within meadows. Overall, the study highlights the urgent need to implement more effective post-marketing management actions to prevent a further entering of plastics in natural environments in the future, as well as to establish to conservation measures specifically tailored to protect seagrass habitats from plastic pollution.

High Nitrogen and Phosphorous Acquisition by Belowground Parts of Caulerpa prolifera (Chlorophyta) Contribute to the Species' Rapid Spread in Ria Formosa Lagoon, Southern Portugal

Despite worldwide proliferation of the genus Caulerpa and subsequent effects on benthic communities, little is known about the nutritional physiology of the Caulerpales. Here, we investigated the uptake rates of ammonium, nitrate, amino acids, and phosphate through the fronds and rhizoids + stolon, the internal translocation of nitrogen, and developed a nitrogen budget for the rapidly spreading Caulerpa prolifera in Ria Formosa lagoon, southern Portugal. Caulerpa prolifera acquired nutrients by both aboveground and belowground parts at similar rates, except nitrate, for which fronds showed 2-fold higher uptake rates. Ammonium was the preferential nitrogen source (81% of the total nitrogen acquisition), and amino acids, which accounted for a significant fraction of total N acquisition (19%), were taken up at faster rates than nitrate. Basipetal translocation of ¹⁵ N incorporated as ammonium was nearly 3-fold higher than acropetal translocation, whereas ¹⁵ N translocation as nitrate and amino acids was smaller but equal in either direction. The estimated total nitrogen acquisition by C. prolifera was 689 μmol · m^-2 · h^-1 , whereas the total nitrogen requirement for growth was 672 μmol · m^-2  · h^-1 . The uptake of ammonium and amino acids by belowground parts accounted for the larger fraction of the total nitrogen acquisition of C. prolifera and is sufficient to satisfy the species nitrogen requirements for growth. This may be one reason explaining the fast spreading of the seaweed in the bare sediments of Ria Formosa where it does not have any macrophyte competitors and the concentration of nutrients is high.

Incubation with macroalgae induces large shifts in water column microbiota, but minor changes to the epibiota of co-occurring macroalgae

Macroalgae variably promote and deter microbial growth through release of organic carbon and antimicrobial compounds into the water column. Consequently, macroalgae influence the microbial composition of the surrounding water column and biofilms on nearby surfaces. Here, we use manipulative experiments to test the hypotheses that (i) Nereocystis luetkeana and Mastocarpus sp. macroalgae alter the water column microbiota in species-specific manner, that (ii) neighbouring macroalgae alter the bacterial communities on the surface (epibiota) of actively growing Nereocystis luetkeana meristem fragments (NMFs), and that (iii) neighbours alter NMF growth rate. We also assess the impact of laboratory incubation on macroalgal epibiota by comparing each species to wild counterparts. We find strong differences between the Nereocystis and Mastocarpus epibiota that are maintained in the laboratory. Nereocystis and Mastocarpus alter water column bacterial community composition and richness in a species specific manner, but cause only small compositional shifts on NMF surfaces that do not differ by species, and do not change richness. Co-incubation with macroalgae results in significant change in abundance of fivefold more genera in the water column compared to NMF surfaces, although the direction (i.e., enrichment or reduction) of shift is generally consistent between the water and NMF surfaces. Finally, NMFs grew during the experiment, but growth did not depend on the presence or identity of neighbouring macroalgae. Thus, macroalgae exhibit a strong and species-specific influence on the water column microbiota, but a much weaker influence on the epibiota of neighbouring macroalgae. Overall, these results support the idea that macroalgae surfaces are highly selective and demonstrate that modulations of macroalgal microbiota operate within an overarching paradigm of host species specificity.

Entangled fates of holobiont genomes during invasion: nested bacterial and host diversities in Caulerpa taxifolia

Successful prevention and mitigation of biological invasions requires retracing the initial steps of introduction, as well as understanding key elements enhancing the adaptability of invasive species. We studied the genetic diversity of the green alga Caulerpa taxifolia and its associated bacterial communities in several areas around the world. The striking congruence of α and β diversity of the algal genome and endophytic communities reveals a tight association, supporting the holobiont concept as best describing the unit of spreading and invasion. Both genomic compartments support the hypotheses of a unique accidental introduction in the Mediterranean and of multiple invasion events in southern Australia. In addition to helping with tracing the origin of invasion, bacterial communities exhibit metabolic functions that can potentially enhance adaptability and competitiveness of the consortium they form with their host. We thus hypothesize that low genetic diversities of both host and symbiont communities may contribute to the recent regression in the Mediterranean, in contrast with the persistence of highly diverse assemblages in southern Australia. This study supports the importance of scaling up from the host to the holobiont for a comprehensive understanding of invasions.

Host and Environmental Specificity in Bacterial Communities Associated to Two Highly Invasive Marine Species (Genus Asparagopsis)

As habitats change due to global and local pressures, population resilience, and adaptive processes depend not only on their gene pools but also on their associated bacteria communities. The hologenome can play a determinant role in adaptive evolution of higher organisms that rely on their bacterial associates for vital processes. In this study, we focus on the associated bacteria of the two most invasive seaweeds in southwest Iberia (coastal mainland) and nearby offshore Atlantic islands, Asparagopsis taxiformis and Asparagopsis armata. Bacterial communities were characterized using 16S rRNA barcoding through 454 next generation sequencing and exploratory shotgun metagenomics to provide functional insights and a backbone for future functional studies. The bacterial community composition was clearly different between the two species A. taxiformis and A. armata and between continental and island habitats. The latter was mainly due to higher abundances of Acidimicrobiales, Sphingomonadales, Xanthomonadales, Myxococcales, and Alteromonadales on the continent. Metabolic assignments for these groups contained a higher number of reads in functions related to oxidative stress and resistance to toxic compounds, more precisely heavy metals. These results are in agreement with their usual association with hydrocarbon degradation and heavy-metals detoxification. In contrast, A. taxiformis from islands contained more bacteria related to oligotrophic environments which might putatively play a role in mineralization of dissolved organic matter. The higher number of functional assignments found in the metagenomes of A. taxiformis collected from Cape Verde Islands suggest a higher contribution of bacteria to compensate nutrient limitation in oligotrophic environments. Our results show that Asparagopsis-associated bacterial communities have host-specificity and are modulated by environmental conditions. Whether this environmental effect reflects the host's selective requirements or the locally available bacteria remains to be addressed. However, the known functional capacities of these bacterial communities indicate their potential for eco-physiological functions that could be valuable for the host fitness.

Life at the boundary: photosynthesis at the soil-fluid interface. A synthesis focusing on mosses

Mosses are among the earliest branching embryophytes and probably originated not later than the early Ordovician when atmospheric CO2 was higher and O2 was lower than today. The C3 biochemistry and physiology of their photosynthesis suggests, by analogy with tracheophytes, that growth of extant bryophytes in high CO2 approximating Ordovician values would increase the growth rate. This occurs for many mosses, including Physcomitrella patens in suspension culture, although recently published transcriptomic data on this species at high CO2 and present-day CO2 show down-regulation of the transcription of several genes related to photosynthesis. It would be useful if transcriptomic (and proteomic) data comparing growth conditions are linked to measurements of growth and physiology on the same, or parallel, cultures. Mosses (like later-originating embryophytes) have been subject to changes in bulk atmospheric CO2 and O2 throughout their existence, with evidence, albeit limited, for positive selection of moss Rubisco. Extant mosses are subject to a large range of CO2 and O2 concentrations in their immediate environments, especially aquatic mosses, and mosses are particularly influenced by CO2 generated by, and O2 consumed by, soil chemoorganotrophy from organic C produced by tracheophytes (if present) and bryophytes.

Quorum sensing is a language of chemical signals and plays an ecological role in algal-bacterial interactions

Algae are ubiquitous in the marine environment, and the ways in which they interact with bacteria are of particular interest in marine ecology field. The interactions between primary producers and bacteria impact the physiology of both partners, alter the chemistry of their environment, and shape microbial diversity. Although algal-bacterial interactions are well known and studied, information regarding the chemical-ecological role of this relationship remains limited, particularly with respect to quorum sensing (QS), which is a system of stimuli and response correlated to population density. In the microbial biosphere, QS is pivotal in driving community structure and regulating behavioral ecology, including biofilm formation, virulence, antibiotic resistance, swarming motility, and secondary metabolite production. Many marine habitats, such as the phycosphere, harbour diverse populations of microorganisms and various signal languages (such as QS-based autoinducers). QS-mediated interactions widely influence algal-bacterial symbiotic relationships, which in turn determine community organization, population structure, and ecosystem functioning. Understanding infochemicals-mediated ecological processes may shed light on the symbiotic interactions between algae host and associated microbes. In this review, we summarize current achievements about how QS modulates microbial behavior, affects symbiotic relationships, and regulates phytoplankton chemical ecological processes. Additionally, we present an overview of QS-modulated co-evolutionary relationships between algae and bacterioplankton, and consider the potential applications and future perspectives of QS.

Host-microbe interactions as a driver of acclimation to salinity gradients in brown algal cultures

Like most eukaryotes, brown algae live in association with bacterial communities that frequently have beneficial effects on their development. Ectocarpus is a genus of small filamentous brown algae, which comprises a strain that has recently colonized freshwater, a rare transition in this lineage. We generated an inventory of bacteria in Ectocarpus cultures and examined the effect they have on acclimation to an environmental change, that is, the transition from seawater to freshwater medium. Our results demonstrate that Ectocarpus depends on bacteria for this transition: cultures that have been deprived of their associated microbiome do not survive a transfer to freshwater, but restoring their microflora also restores the capacity to acclimate to this change. Furthermore, the transition between the two culture media strongly affects the bacterial community composition. Examining a range of other closely related algal strains, we observed that the presence of two bacterial operational taxonomic units correlated significantly with an increase in low salinity tolerance of the algal culture. Despite differences in the community composition, no indications were found for functional differences in the bacterial metagenomes predicted to be associated with algae in the salinities tested, suggesting functional redundancy in the associated bacterial community. Our study provides an example of how microbial communities may impact the acclimation and physiological response of algae to different environments, and thus possibly act as facilitators of speciation. It paves the way for functional examinations of the underlying host-microbe interactions, both in controlled laboratory and natural conditions.

Hologenome theory supported by cooccurrence networks of species-specific bacterial communities in siphonous algae (Caulerpa)

The siphonous algae of the Caulerpa genus harbor internal microbial communities hypothesized to play important roles in development, defense and metabolic activities of the host. Here, we characterize the endophytic bacterial community of four Caulerpa taxa in the Mediterranean Sea, through 16S rRNA amplicon sequencing. Results reveal a striking alpha diversity of the bacterial communities, similar to levels found in sponges and coral holobionts. These comprise (1) a very small core community shared across all hosts (< 1% of the total community), (2) a variable portion (ca. 25%) shared by some Caulerpa taxa but not by all, which might represent environmentally acquired bacteria and (3) a large (>70%) species-specific fraction of the community, forming very specific clusters revealed by modularity in networks of cooccurrence, even in areas where distinct Caulerpa taxa occurred in sympatry. Indirect inferences based on sequence homology suggest that these communities may play an important role in the metabolism of their host, in particular on their ability to grow on anoxic sediment. These findings support the hologenome theory and the need for a holistic framework in ecological and evolutionary studies of these holobionts that frequently become invasive.

The green seaweed Ulva: a model system to study morphogenesis

Green macroalgae, mostly represented by the Ulvophyceae, the main multicellular branch of the Chlorophyceae, constitute important primary producers of marine and brackish coastal ecosystems. Ulva or sea lettuce species are some of the most abundant representatives, being ubiquitous in coastal benthic communities around the world. Nonetheless the genus also remains largely understudied. This review highlights Ulva as an exciting novel model organism for studies of algal growth, development and morphogenesis as well as mutualistic interactions. The key reasons that Ulva is potentially such a good model system are: (i) patterns of Ulva development can drive ecologically important events, such as the increasing number of green tides observed worldwide as a result of eutrophication of coastal waters, (ii) Ulva growth is symbiotic, with proper development requiring close association with bacterial epiphytes, (iii) Ulva is extremely developmentally plastic, which can shed light on the transition from simple to complex multicellularity and (iv) Ulva will provide additional information about the evolution of the green lineage.

An intracellular transcriptomic atlas of the giant coenocyte Caulerpa taxifolia

Convergent morphologies have arisen in plants multiple times. In non-vascular and vascular land plants, convergent morphology in the form of roots, stems, and leaves arose. The morphology of some green algae includes an anchoring holdfast, stipe, and leaf-like fronds. Such morphology occurs in the absence of multicellularity in the siphonous algae, which are single cells. Morphogenesis is separate from cellular division in the land plants, which although are multicellular, have been argued to exhibit properties similar to single celled organisms. Within the single, macroscopic cell of a siphonous alga, how are transcripts partitioned, and what can this tell us about the development of similar convergent structures in land plants? Here, we present a de novo assembled, intracellular transcriptomic atlas for the giant coenocyte Caulerpa taxifolia. Transcripts show a global, basal-apical pattern of distribution from the holdfast to the frond apex in which transcript identities roughly follow the flow of genetic information in the cell, transcription-to-translation. The analysis of the intersection of transcriptomic atlases of a land plant and Caulerpa suggests the recurrent recruitment of transcript accumulation patterns to organs over large evolutionary distances. Our results not only provide an intracellular atlas of transcript localization, but also demonstrate the contribution of transcript partitioning to morphology, independent from multicellularity, in plants.

Plants include both the green algae and land plants. Multiple times, root, stem, and leaf-like structures arose independently in plant lineages. In some instances, such as the siphonous algae, these structures arose in the absence of multicellularity. It has been argued by some that the morphology of multicellular land plant organs similarly arises independently of cell division patterns. Here, we explore the partitioning of gene transcripts within what is debatably the largest single-celled organism in the world, the siphonous alga Caulerpa taxifolia. We find that within this giant cell specific transcripts localize within pseudo-organs (morphological structures that are not comprised of cells or tissue). The overall pattern of transcript accumulation follows an apical-basal pattern within the cell. Moreover, transcripts related to different cellular processes, such as transcription and translation, localize to specific regions. Analyzing the signatures of transcript accumulation in land plant organs and the pseudo-organs of Caulerpa, we find that groups of transcripts accumulate together in morphological structures across evolution at rates higher than expected by chance. Together, our results demonstrate a relationship between transcript partitioning and organism morphology, independent from multicellularity, throughout diverse plant lineages.

Isolation of hyphomonas strains that induce normal morphogenesis in protoplasts of the marine red alga Pyropia yezoensis

Marine macroalgae cannot develop normal morphology under axenic conditions although normal morphogenesis can be sustained when certain bacteria are present. In this study, bacteria that induced normal morphogenesis in the red alga Pyropia yezoensis (Nori) were identified. The bacteria were isolated from algal media, thalli, tissue debris, and purified protoplasts during protoplast isolation from P. yezoensis laboratory cultures. 16S rRNA gene sequence analysis showed these bacterial isolates belonged to α-Proteobacteria (12 groups), γ-Proteobacteria (3 groups), and Flavobacteria (2 groups). Axenic protoplasts of P. yezoensis generated by removing epiphytic bacteria were co-cultured along with the bacterial isolates. Most axenic protoplasts showed irregular morphogenetic and anaplastic cells; cells with normal morphology were scarce. However, inoculation with 11 strains of Hyphomonas (α-Proteobacteria) led to significantly higher normal morphogenetic rates (4.5-7.3 %, P < 0.01 or 0.05) compared to axenic protoplasts (0.06 %). These Hyphomonas strains were recovered from all experiments; thus, certain Hyphomonas strains can induce normal morphogenesis in P. yezoensis protoplasts. Direct inoculation of the Hyphomonas strain exhibited higher morphogenetic activity than inoculation of its extracellular and intracellular products. This is the first study demonstrating the influence of specific bacteria on protoplast morphology in marine macroalgae.

Cells inside cells: symbiosis and continuing phagotrophy

Plastids evolved after a cell entered a genetically different cell, followed by integration of the two genomes. Many photosynthetic cells can ingest organic particles as food, as now demonstrated in an ancestral green alga.

Permanent residents or temporary lodgers: characterizing intracellular bacterial communities in the siphonous green alga Bryopsis

The ecological success of giant celled, siphonous green algae in coastal habitats has repeatedly been linked to endophytic bacteria living within the cytoplasm of the hosts. Yet, very little is known about the relative importance of evolutionary and ecological factors controlling the intracellular bacterial flora of these seaweeds. Using the marine alga Bryopsis (Bryopsidales, Chlorophyta) as a model, we explore the diversity of the intracellular bacterial communities and investigate whether their composition is controlled by ecological and biogeographic factors rather than the evolutionary history of the host. Using a combination of 16S rDNA clone libraries and denaturing gradient gel electrophoresis analyses, we show that Bryopsis harbours a mixture of relatively few but phylogenetically diverse bacterial species. Variation partitioning analyses show a strong impact of local environmental factors on the presence of Rickettsia and Mycoplasma in their association with Bryopsis. The presence of Flavobacteriaceae and Bacteroidetes, on the other hand, reflects a predominant imprint of host evolutionary history, suggesting that these bacteria are more specialized in their association. The results highlight the importance of interpreting the presence of individual bacterial phylotypes in the light of ecological and evolutionary principles such as phylogenetic niche conservatism to understand complex endobiotic communities and the parameters shaping them.

What we can learn from sushi: a review on seaweed-bacterial associations

Many eukaryotes are closely associated with bacteria which enable them to expand their physiological capacities. Associations between algae (photosynthetic eukaryotes) and bacteria have been described for over a hundred years. A wide range of beneficial and detrimental interactions exists between macroalgae (seaweeds) and epi- and endosymbiotic bacteria that reside either on the surface or within the algal cells. While it has been shown that these chemically mediated interactions are based on the exchange of nutrients, minerals, and secondary metabolites, the diversity and specificity of macroalgal-bacterial relationships have not been thoroughly investigated. Some of these alliances have been found to be algal or bacterial species-specific, whereas others are widespread among different symbiotic partners. Reviewing 161 macroalgal-bacterial studies from the last 55 years, a definite bacterial core community, consisting of Gammaproteobacteria, CFB group, Alphaproteobacteria, Firmicutes, and Actinobacteria species, seems to exist which is specifically (functionally) adapted to an algal host-associated lifestyle. Because seaweed-bacterial associations are appealing from evolutionary and applied perspectives, future studies should integrate the aspects of diverse biological fields.

SELECTIVE ELIMINATION OF CHLOROPLASTIDIAL DNA FOR METAGENOMICS OF BACTERIA ASSOCIATED WITH THE GREEN ALGA CAULERPA TAXIFOLIA (BRYOPSIDOPHYCEAE)(1)

Molecular analyses of bacteria associated with photosynthetic organisms are often confounded by coamplification of the chloroplastidial 16S rDNA with the targeted bacterial 16S rDNA. This major problem has hampered progress in the characterization of bacterial communities associated to photosynthetic organisms and has limited the full realization of the potential offered by the last generation of metagenomics approaches. A simple and inexpensive method is presented, based on ethanol and bleach treatments prior to extraction, to efficiently discard a great part of chloroplastidial DNA without affecting the characterization of bacterial communities through pyrosequencing. Its effectiveness for the description of bacterial lineages associated to the green alga Caulerpa taxifolia (M. Vahl) C. Agardh was much higher than that of the preexisting enrichment protocols proposed for plants. Furthermore, this new technique requires a very small amount of biological material compared to the other current protocols, making it more realistic for systematic use in ecological and phylogenetic studies and opening promising prospects for metagenomics of green algae, as shown by our data.

BIOADHESION IN CAULERPA MEXICANA (CHLOROPHYTA): RHIZOID-SUBSTRATE ADHESION(1)

The attachment of the psammophytic alga Caulerpa mexicana Sond. ex Kütz., a coenocytic green alga, to crushed CaCO3 particles was examined utilizing the scanning electron microscope and fluorescently tagged antivitronectin antibodies. Plants attached to the substrate through morphologically variable tubular rhizoidal extensions that grew from the stolon. In this study, we describe two means of attachment: (i) the rhizoid attachment to limestone gravel by thigmoconstriction, where tubular extensions of the rhizoid wrapped tightly around the substrate and changed morphology to fit tightly into crevices in the limestone, and (ii) through adhesion pads that formed in contact with the limestone granules. Flattened rhizoidal pads were observed to secrete a fibrillar material that contained vitronectin-like proteins identified through immunolocialization and that facilitated binding of the rhizoid to the substrate.

Evolution and cytological diversification of the green seaweeds (Ulvophyceae)

The Ulvophyceae, one of the four classes of the Chlorophyta, is of particular evolutionary interest because it features an unrivaled morphological and cytological diversity. Morphological types range from unicells and simple multicellular filaments to sheet-like and complex corticated thalli. Cytological layouts range from typical small cells containing a single nucleus and chloroplast to giant cells containing millions of nuclei and chloroplasts. In order to understand the evolution of these morphological and cytological types, the present paper aims to assess whether the Ulvophyceae are monophyletic and elucidate the ancient relationships among its orders. Our approach consists of phylogenetic analyses (maximum likelihood and Bayesian inference) of seven nuclear genes, small subunit nuclear ribosomal DNA and two plastid markers with carefully chosen partitioning strategies, and models of sequence evolution. We introduce a procedure for fast site removal (site stripping) targeted at improving phylogenetic signal in a particular epoch of interest and evaluate the specificity of fast site removal to retain signal about ancient relationships. From our phylogenetic analyses, we conclude that the ancestral ulvophyte likely was a unicellular uninucleate organism and that macroscopic growth was achieved independently in various lineages involving radically different mechanisms: either by evolving multicellularity with coupled mitosis and cytokinesis (Ulvales-Ulotrichales and Trentepohliales), by obtaining a multinucleate siphonocladous organization where every nucleus provides for its own cytoplasmic domain (Cladophorales and Blastophysa), or by developing a siphonous organization characterized by either one macronucleus or millions of small nuclei and cytoplasmic streaming (Bryopsidales and Dasycladales). We compare different evolutionary scenarios giving rise to siphonous and siphonocladous cytologies and argue that these did not necessarily evolve from a multicellular or even multinucleate state but instead could have evolved independently from a unicellular ancestor.

Primary structure and carbohydrate binding specificity of a potent anti-HIV lectin isolated from the filamentous cyanobacterium Oscillatoria agardhii

The primary structure of a lectin, designated Oscillatoria agardhii agglutinin (OAA), isolated from the freshwater cyanobacterium O. agardhii NIES-204 was determined by the combination of Edman degradation and electron spray ionization-mass spectrometry. OAA is a polypeptide (Mr 13,925) consisting of two tandem repeats. Interestingly, each repeat sequence of OAA showed a high degree of similarity to those of a myxobacterium, Myxococcus xanthus hemagglutinin, and a marine red alga Eucheuma serra lectin. A systematic binding assay with pyridylaminated oligosaccharides revealed that OAA exclusively binds to high mannose (HM)-type N-glycans but not to other N-glycans, including complex types, hybrid types, and the pentasaccharide core or oligosaccharides from glycolipids. OAA did not interact with any of free mono- and oligomannoses that are constituents of the branched oligomannosides. These results suggest that the core disaccharide, GlcNAc-GlcNAc, is also essential for binding to OAA. The binding activity of OAA to HM type N-glycans was dramatically decreased when alpha1-2 Man was attached to alpha1-3 Man branched from the alpha1-6 Man of the pentasaccharide core. This specificity of OAA for HM-type oligosaccharides is distinct from other HM-binding lectins. Kinetic analysis with an HM heptasaccharide revealed that OAA possesses two carbohydrate binding sites per molecule, with an association constant of 2.41x10(8) m-1. Furthermore, OAA potently inhibits human immunodeficiency virus replication in MT-4 cells (EC50=44.5 nm). Thus, we have found a novel lectin family sharing similar structure and carbohydrate binding specificity among bacteria, cyanobacteria, and marine algae.

Intracellular Compartmentation in the Biosynthesis of Caulerpenyne: Study on Intact Macroalgae Using Stable-Isotope-Labeled Precursors

The biosynthesis of caulerpenyne 1 was studied in the invasive green alga Caulerpa taxifolia. The investigation was performed on intact algae with stable-isotope-labeled precursors administered under mixotrophic growth conditions. According to the labeling pattern, after incorporation of 1-(13)C-acetate and (13)CO(2), respectively, the biosynthesis of the sesquiterpene backbone occurs in the chloroplast and follows the methyl-erythritol-4-phosphate (MEP) pathway. In contrast, the acetyl residues of caulerpenyne 1 are derived from a cytosolic resource. [structure: see text]

Using genetic techniques to investigate the sources of the invasive alga Caulerpa taxifolia in three new locations in Australia

The invasive green alga Caulerpa taxifolia has gained a high profile due to 'outbreaks' in the Mediterranean and California. During the year 2000 three new discrete locations colonised by abundant C. taxifolia were discovered in New South Wales (NSW), Australia. Sequencing of the internal transcribed spacer (ITS) region of the ribosomal DNA was used to explore the source(s) of these new records, which is an important prerequisite for subsequent environmental management responses. Our results indicate that the NSW C. taxifolia originated from several sources and, hence, through different invasion events. For two of the new records (Port Hacking, Careel Bay) it can be excluded that they are derived from the so-called "aquarium strain" of C. taxifolia, closely related to the invasive Mediterranean populations. Port Hacking is likely to have originated from tropical native populations. However, samples from Lake Conjola cannot be sufficiently distinguished with the applied technique from native C. taxifolia in Moreton Bay and the Mediterranean/"aquarium strain".

Phylogenetic analyses of Caulerpa taxifolia (Chlorophyta) and of its associated bacterial microflora provide clues to the origin of the Mediterranean introduction

The accidental introduction of Caulerpa taxifolia into the Mediterranean is no longer under dispute. What has eluded researchers until now, is definitive evidence for the original, biogeographical source population. Here we present two independent lines of evidence that support an Australian origin for the Mediterranean populations of C. taxifolia. First, we reanalysed algal rDNA-internal transcribed spacer (rDNA-ITS) sequences, combining previously published sequences from different studies with 22 new sequences. The ITS sequence comparison showed that the Australian sample is the sister group of the Mediterranean-aquarium clade. Second, cloned bacterial 16S rDNA gene sequences were analysed from the associated microflora of C. taxifolia collected from Australia, Tahiti, the Philippines and the Mediterranean. Five bacterial lineages were identified, of which three were dominant. Alpha Proteobacteria were the most abundant and were found in all samples. In contrast, members of the beta Proteobacterial line and Cytophaga-Flexibacter-Bacteroides line (CFB) were mainly associated with Mediterranean and Australian samples. Frequency distributions of the five bacterial lineages were significantly different among biogeographical locations. Phylogenetic analyses of the 54 bacterial sequences derived from the four C. taxifolia individuals resulted in a well-resolved tree with high bootstrap support. The topologies of the beta Proteobacteria and CFB mirror the geographical sources of their algal hosts. Bacterial-algal associations provide an identification tool that may have wide application for the detection of marine invasions.

Land plant biochemistry

Biochemical studies have complemented ultrastructural and, subsequently molecular genetic evidence consistent with the Charophyceae being the closest extant algal relatives of the embryophytes. Among the genes used in such molecular phylogenetic studies is that rbcL) for the large subunit of ribulose bisphosphate carboxylase-oxygenase (RUBISCO). The RUBISCO of the embryophytes is derived, via the Chlorophyta. from that of the cyanobacteria. This clade of the molecular phylogeny of RUBISCO shows a range of kinetic characteristics, especially of CO2 affinities and of CO2/O2 selectivities. The range of these kinetic values within the bryophytes is no greater than in the rest of the embryophytes; this has implications for the evolution of the embryophytes in the high atmospheric CO2 environment of the late Lower Palaeozoic. The differences in biochemistry between charophycean algae and embryophytes can to some extent be related functionally to the structure and physiology of embryophytes. Examples of components of embryophytes, which are qualitatively or quantitatively different from those of charophytes, are the water repellent/water resistant extracellular lipids, the rigid phenolic polymers functional in water-conducting elements and mechanical support in air, and in UV-B absorption, flavonoid phenolics involved in UV-B absorption and in interactions with other organisms, and the greater emphasis on low Mr organic acids. retained in the plant as free acids or salts, or secreted to the rhizosphere. The roles of these components are discussed in relation to the environmental conditions at the time of evolution of the terrestrial embryophytes. A significant point about embryophytes is the predominance of nitrogen-free extracellular structural material (a trait shared by most algae) and UV-B screening components, by contrast with analogous components in many other organisms. An important question, which has thus far been incompletely addressed, is the extent to which the absence from bryophytes of the biochemical pathways which produce components found only in tracheophytes is the result of evolutionary loss of these functions.