Macroalgal–bacterial interactions: identification and role of thallusin in morphogenesis of the seaweed Ulva (Chlorophyta)

Macroalgal microbiomes have core functions related to biofilm formation, growth, and morphogenesis of seaweeds. In particular, the growth and development of the sea lettuce Ulva spp. (Chlorophyta) depend on bacteria releasing morphogenetic compounds. Under axenic conditions, the macroalga Ulva mutabilis develops a callus-like phenotype with cell wall protrusions. However, co-culturing with Roseovarius sp. (MS2) and Maribacter sp. (MS6), which produce various stimulatory chemical mediators, completely recovers morphogenesis. This ecological reconstruction forms a tripartite community which can be further studied for its role in cross-kingdom interactions. Hence, our study sought to identify algal growth- and morphogenesis-promoting factors (AGMPFs) capable of phenocopying the activity of Maribacter spp. We performed bioassay-guided solid-phase extraction in water samples collected from U. mutabilis aquaculture systems. We uncovered novel ecophysiological functions of thallusin, a sesquiterpenoid morphogen, identified for the first time in algal aquaculture. Thallusin, released by Maribacter sp., induced rhizoid and cell wall formation at a concentration of 11 pmol l-1. We demonstrated that gametes acquired the iron complex of thallusin, thereby linking morphogenetic processes with intracellular iron homeostasis. Understanding macroalgae-bacteria interactions permits further elucidation of the evolution of multicellularity and cellular differentiation, and development of new applications in microbiome-mediated aquaculture systems.FCT: UID/Multi/04326/2019; UIDB/04326/2020; CCMAR/ID/16/2018; CEECINST/00114/2018. German Research Foundation (DFG) CRC1127; European Union (EU) 642575; ASSEMBLE 227799; COST Action 'Phycomorph' FA1406.info:eu-repo/semantics/publishedVersio

Macroalgal morphogenesis induced by waterborne compounds and bacteria in coastal seawater

Axenic gametes of the marine green macroalga Ulva mutabilis Foyn (Ria Formosa, locus typicus) exhibit abnormal development into slow-growing callus-like colonies with aberrant cell walls. Under laboratory conditions, it was previously demonstrated that all defects in growth and thallus development can be completely abolished when axenic gametes are inoculated with a combination of two specific bacterial strains originally identified as Roseo-bacter sp. strain MS2 and Cytophaga sp. strain MS6. These bacteria release diffusible morphogenetic compounds (= morphogens), which act similar to cytokinin and auxin. To investigate the ecological relevance of the waterborne bacterial morphogens, seawater samples were collected in the Ria Formosa lagoon (Algarve, Southern Portugal) at 20 sampling sites and tidal pools to assess their morphogenetic effects on the axenic gametes of U. mutabilis. Specifically the survey revealed that sterile-filtered seawater samples can completely recover growth and morphogenesis of U. mutabilis under axenic conditions. Morphogenetic activities of free-living and epiphytic bacteria isolated from the locally very abundant Ulva species (i.e., U. rigida) were screened using a multiwell-based testing system. The most represented genera isolated from U. rigida were Alteromonas, Pseudoalteromonas and Sulfitobacter followed by Psychrobacter and Polaribacter. Several naturally occurring bacterial species could emulate MS2 activity (= induction of cell divisions) regardless of taxonomic affiliation, whereas the MS6 activity (= induction of cell differentiation and cell wall formation) was species-specific and is probably a feature of difficult-to-culture bacteria. Interestingly, isolated bacteroidetes such as Algoriphagus sp. and Polaribacter sp. could individually trigger complete Ulva morphogenesis and thus provide a novel mode of action for bacterial-induced algal development. This study also highlights that the accumulation of algal growth factors in a shallow water body separated from the open ocean by barrier islands might have strong implications to, for example, the wide usage of natural coastal seawater in algal (land based) aquacultures of Ulva

Categorization of the algal morphology of <i>U</i>. <i>mutabilis</i>.

<p><b>(A)</b> The tripartite community of <i>U</i>. <i>mutabilis</i> with the essential interactions investigated in this study using a standardized experimental set-up. One of the strains MS1, MS2 or MS3 along with MS6 recovers completely growth, development and morphogenesis of <i>U</i>. <i>mutabilis</i>: Strains MS1 (<i>Halomonas</i> sp.), MS2 (originally classified as <i>Roseobacter</i> sp.) and MS3 (<i>Sulfitobacter</i> sp.) induce cell division and growth, whereas MS6 (originally classified as <i>Cytophaga</i> sp.) promotes rhizoid and cell wall formation [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146307#pone.0146307.ref011" target="_blank">11</a>]. <b>(B)</b> “<i>Ulva</i> bioassay array” for morphogenesis assessment. The multiwell based survey of the morphogenetic activity using axenic gametes of <i>U</i>. <i>mutabilis</i> allows the fast determination of the various morphotypes categorized by a color code. Representative morphotypes <b>I:</b> callus like morphology of an axenic culture (yellow); <b>II:</b> cell divisions and growth of blade cells with malformed cell walls (black arrow) in the presence of only MS2 (pink); <b>III:</b> Rhizoid formation (white arrow) and normal cell wall formation in the presence of only MS6 (red) <b>IV:</b> Complete morphogenesis in the presence of both strains MS2 and MS6 (green). Images (I-IV) were taken from Wichard (2015) <i>Front</i>. <i>Plant Sci</i>. 6:86 (black bar = 100 μm).</p

Ria formosa lagoon.

<p>(<b>A</b>) Scheme and purpose of sampling. (<b>B</b>) The morphogenetic activity of sterile-filtered water collected from several sampling sites within the lagoon [(1–3: Ramalhete Marine Station, blue dot) in 2010, (1–11, red dots) in 2011 and (1–6, green dots) in 2012] and two control samples outside the lagoon was tested on axenic <i>Ulva mutabilis</i> gametes sl mt(+). The numbers of the sites represent the chronological order of the sampling.</p

Phylogenetic tree of the isolated bacteria.

<p>Maximum likelihood phylogram of isolated and cultivatable bacteria inferred from the 16S rRNA gene. Bacteria were collected through swapping from the surface of <i>U</i>. <i>rigida</i> collected in tidal pools in 2011. <i>Halomonas</i> sp. MS1, <i>Roseobacter</i> sp. MS2, <i>Sulfitobacter</i> sp. MS3 and <i>Cytophaga</i> sp. MS6 were used as reference strains isolated from <i>U</i>. <i>mutabilis</i> [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146307#pone.0146307.ref011" target="_blank">11</a>]. The algal morphotypes induced by the bacteria are annotated.</p

Bioassay of the morphogenetic activity with doubly sterile-filtered water collected from various sampling sites across the lagoon.

<p>(<b>A</b>) A dilution series of the seawater collected in 2011 with UCM was tested to estimate the potential morphogenetic activity on axenic <i>Ulva</i> gametes. The control of axenic growth is shown for comparison. To estimate the MS2-like activity, the cell numbers of the germlings were counted 14 days after inoculation. (<b>B</b>) To estimate the MS6-like activity, the percentage of algae with a normal cell wall was evaluated 7 days after the first cell wall deformation was observed in axenic control cultures. Error bars represent (<b>A</b>) confidential intervals (P = 0.95; n = 60 individual algae) or (<b>B</b>) standard deviations (n = 60 individual algae). The dotted line indicates the maximum growth and development under axenic conditions.</p

Effects of once-weekly exenatide on cardiovascular outcomes in type 2 diabetes

BACKGROUND: The cardiovascular effects of adding once-weekly treatment with exenatide to usual care in patients with type 2 diabetes are unknown. METHODS: We randomly assigned patients with type 2 diabetes, with or without previous cardiovascular disease, to receive subcutaneous injections of extended-release exenatide at a dose of 2 mg or matching placebo once weekly. The primary composite outcome was the first occurrence of death from cardiovascular causes, nonfatal myocardial infarction, or nonfatal stroke. The coprimary hypotheses were that exenatide, administered once weekly, would be noninferior to placebo with respect to safety and superior to placebo with respect to efficacy. RESULTS: In all, 14,752 patients (of whom 10,782 [73.1%] had previous cardiovascular disease) were followed for a median of 3.2 years (interquartile range, 2.2 to 4.4). A primary composite outcome event occurred in 839 of 7356 patients (11.4%; 3.7 events per 100 person-years) in the exenatide group and in 905 of 7396 patients (12.2%; 4.0 events per 100 person-years) in the placebo group (hazard ratio, 0.91; 95% confidence interval [CI], 0.83 to 1.00), with the intention-to-treat analysis indicating that exenatide, administered once weekly, was noninferior to placebo with respect to safety (P<0.001 for noninferiority) but was not superior to placebo with respect to efficacy (P=0.06 for superiority). The rates of death from cardiovascular causes, fatal or nonfatal myocardial infarction, fatal or nonfatal stroke, hospitalization for heart failure, and hospitalization for acute coronary syndrome, and the incidence of acute pancreatitis, pancreatic cancer, medullary thyroid carcinoma, and serious adverse events did not differ significantly between the two groups. CONCLUSIONS: Among patients with type 2 diabetes with or without previous cardiovascular disease, the incidence of major adverse cardiovascular events did not differ significantly between patients who received exenatide and those who received placebo