Exploring bacteria-induced growth and morphogenesis in the green macroalga order Ulvales (Chlorophyta)

Abstract

Green macroalgae, such as Ulvales, lose their typical morphology completely when grown under axenic conditions or in the absence of the appropriate microbiome. As a result, slow growing aberrant phenotypes or even callus-like morphotypes are observed in Ulvales. The cross-kingdom interactions between marine algae and microorganisms are hence not only restricted by the exchange of macronutrients, including vitamins and nutrients, but also by infochemicals such as bacterial morphogenetic compounds. The latter are a fundamental trait mediating the mutualism within the chemosphere where the organisms interact with each other via compounds in their surroundings. Approximately 60 years ago, pilot studies demonstrated that certain bacteria promote growth, whereas other bacteria induce morphogenesis; this is particularly true for the order of Ulvales. However, only slow progress was made towards the underlying mechanism due to the complexity of, for example, algal cultivation techniques, and the lack of standardized experiments in the laboratory. A breakthrough in this research was the discovery of the morphogenetic compound thallusin, which was isolated from an epiphytic bacterium and induces normal germination restoring the foliaceous morphotypes of Monostroma. Owing to the low concentration, the purification and structure elucidation of highly biologically active morphogenetic compounds are still challenging. Recently, it was found that only the combination of two specific bacteria from the Rhodobacteraceae and Flavobacteriaceae can completely recover the growth and morphogenesis of axenic Ulva mutabilis cultures forming a symbiotic tripartite community by chemical communication. This review combines literature detailing evidences of bacteria-induced morphogenesis in Ulvales. A set of standardized experimental approaches is further proposed for the preparation of axenic algal tissues, bacteria isolation, co-cultivation experiments, and the analysis of the chemosphere.

Phytohormones in red seaweeds: a technical review of methods for analysis and a consideration of genomic data

“…However, P. yezoensis and B. fuscopurpurea have no OPDA (Mori, Matsuura and Mikami, unpublished), which is consistent with the lack of JA in these Bangiophyceae. Moreover, in P. patens, despite the absence of bioactive GA 1 and GA 4 , the precursors ent-kaurene and ent-kaurenoic acid and also their metabolites other than GAs regulate red light-dependent cellular differentiation and blue light-dependent negative phototropism (Hayashi et al 2010, Miyazaki et al 2014, 2015. However, little is known so far about the presence of these molecules in red seaweeds, and the confirmation of the presence of GA precursors in red seaweeds is needed to understand the evolutional diversity of GA biosynthesis in plants.…”

“…Based on the most extensive studies of interactions between bacteria and marine green seaweeds to date (Joint et al 2007, Wichard 2015, the association of seaweeds with epiphytic bacteria is commonly thought to be essential for their normal growth, morphogenesis, and reproduction (Egan et al 2013, Singh and Reddy 2014, Wichard 2015, Liu et al 2017). In addition to exchanges of nutrient chemicals and morphological regulators (Egan et al 2013, Singh and Reddy 2014, Wichard 2015, bacterium-derived IAA is involved in bud formation in the Florideophyte Gracilaria dura (Singh et al 2011a). With regard to phytohormones, it was recently reported that diatoms receive IAA from their symbiotic bacteria, and these are required for efficient growth (Amin et al 2015).…”

Morphogenesis of Ulva mutabilis (Chlorophyta) induced by Maribacter species (Bacteroidetes, Flavobacteriaceae)

“…Instead, the alga forms callus-like, slow growing structures with colourless protrusions from the exterior cell wall (Spoerner et al 2012, Wichard 2015. Early experiments of Provasoli (1958) have already pointed out that treatment of Ulva with antibiotics results in abnormal growth.…”

“…Further experiments examined the role of isolated bacteria in activating developmental and growth promoting traits in Ulva and revealed species-specific interactions, as no combination of bacteria showed the complete recovery of the normal morphotypes (Provasoli and Pintner 1980, Marshall et al 2006, Spoerner et al 2012, Wichard 2015. Two bacterial isolates from non-axenic laboratory cultures of U. mutabilis were later found to induce the complete morphogenesis of U. mutabilis, forming a tripartite community (Spoerner et al 2012, Wichard 2015. The bacterial strains were originally described as Roseobacter sp.…”

“…To have morphogenesis-inducing strains available for Gametophytes were propagated for gamete production and release under laboratory conditions (Wichard and Oertel 2010). Afterwards axenic gametes were prepared according to Wichard (2015). Purified gametes [mating type ( + )] were inoculated with selected bacteria (final concentration OD 600 = 1 × 10 − 6 ) in 10 ml Ulva culture medium and kept in the dark for 24 h to let gametes settle on culture tissue flask.…”