Ulva growth, development and applications

The green marine macroalgae Ulva (Ulvophyceae) are common algae distributed worldwide, which play a key role in aquatic ecosystems. Ulva species are a potentially valuable resource for food, feed, fertiliser and fuel but can also cause massive nuisance blooms if they grow unchecked. For correct growth and development, Ulva requires the presence of a combination of regulatory morphogenetic compounds released by associated epiphytic bacteria in addition to nutritional parameters. The first results chapter examines the extent of specificity or generality of bacteria-induced morphogenesis in Ulva, by cross-testing bacteria isolated from several Ulva species on Ulva mutabilis and Ulva intestinalis. We show that pairs of bacterial strains isolated from Ulva species can fully rescue U. mutabilis or U. intestinalis morphology. In the second results chapter, activity of algal growth- and morphogenesis-promoting factors (AGPFs) derived from bacteria were estimated in a land-based integrated multitrophic aquaculture system (IMTA) of fish and macroalgae (located at the coastal lagoon Ria de Aveiro, Portugal), using a standardised bioassay with axenic cultures of Ulva. Nutrient availability was also assessed in this IMTA system. The study thus informs aspects of the various potential aquaculture-environment interactions. It was observed that both the water from the lagoon (external to the farm system) and the water from the fish pond (input for algae cultures) could completely restore the normal growth and morphology of the macroalga under axenic conditions. The results highlight the presence of a sufficient chemical cocktail of AGPFs in this IMTA system required for growth and morphogenesis of Ulva. In addition, the water of fish farming increases the nutrient availability (nitrate and ammonium) needed for macroalgae production. The conclusion of this chapter is that Ulva´s sustainable growth and development can benefit from multitrophic aquaculture systems and shallow water systems, due to the naturally enriched AGPFs and their in-situ production by bacteria in intensive algal aquacultures. In the final results chapter, the effects of U. intestinalis extracts on germination and root development in the model land plant Arabidopsis thaliana were examined. Ulva extract concentrations above 0.1% inhibited Arabidopsis germination and root growth. Ulva extract <0.1% stimulated root growth. All concentrations of Ulva extract inhibited lateral root formation. An abscisic-acid insensitive mutant showed altered sensitivity to germination- and root growth-inhibition. Ethylene- and cytokinin-insensitive mutants were partly insensitive to germination-inhibition. This suggests that different mechanisms mediate each effect of Ulva extract on early Arabidopsis development and that multiple hormones contribute to germination-inhibition. Taken together, the results of this thesis highlight: (i) Specific Ulva-associated bacterial functions (promoting cell division, or cell differentiation) that cannot be assigned to a specific genus/taxonomic group of bacteria, (ii) an IMTA system ensuring an adequate supply of nutrients and a sufficient chemical mixture of AGPFs for reliable Ulva cultivation and (iii) the first-characterised mechanisms to date by which Ulva extract can impact germination and growth in Arabidopsis

Microbiome-Dependent Adaptation of Seaweeds Under Environmental Stresses: A Perspective

The microbiome of macroalgae facilitates their adaptation to environmental stress. As bacteria release algal growth and morphogenesis promoting factors (AGMPFs), which are necessary for the healthy development of macroalgae, bacteria play a crucial role in stress adaptation of bacterial-algal interactions. To better understand the level of macroalgal dependence on the microbiome under various stress factors such as light, temperature, salt, or micropollutants, we propose a reductionist analysis of a tripartite model system consisting of the axenic green alga Ulva (Chlorophyta) re-infected with two essential bacteria. This analysis will allow us to decipher the stress response of each symbiont within this cross-kingdom interaction. The paper highlights studies on possible survival strategies embedded in cross-kingdom interactions that govern the stress adaptation, including general features of metabolic pathways in the macroalgal host or more specific features such as alterations in the composition and/or diversity of bacterial assemblages within the microbiome community. Additionally, we present some preliminary results regarding the effect of recently isolated bacteria from the Potter Cove, King George Island (Isla 25 de Mayo) in Antarctica, on the model system Ulva mutabilis Føyn purified gametes. The results indicate that cold-adapted bacteria release AGMPFs, inducing cell differentiation, and cell division in purified cultures. We propose that microbiome engineering can be used to increase the adaptability of macroalgae to stressful situations with implications for, e.g., the sustainable management of (land-based) aquaculture systems

Effects of green seaweed extract on Arabidopsis early development suggest roles for hormone signalling in plant responses to algal fertilisers

The growing population requires sustainable, environmentally-friendly crops. The plant growth enhancing properties of algal extracts have suggested their use as biofertilisers. The mechanism(s) by which algal extracts affect plant growth are unknown. We examined the effects of extracts from the common green seaweed Ulva intestinalis on germination and root development in the model land plant Arabidopsis thaliana. Ulva extract concentrations above 0.1% inhibited Arabidopsis germination and root growth. Ulva extract less than 0.1% stimulated root growth. All concentrations of Ulva extract inhibited lateral root formation.An abscisic-acid-insensitive mutant, abi1, showed altered sensitivity to germination- and root growth-inhibition. Ethylene- and cytokinin-insensitive mutants were partly insensitive to germination-inhibition. This suggests that different mechanisms mediate each effect of Ulva extract on early Arabidopsis development and that multiple hormones contribute to germination inhibition. Elemental analysis showed that Ulva contains high levels of Aluminium ions (Al3+). Ethylene and cytokinin have been suggested to function in Al3+-mediated root growth inhibition: our data suggest that if Ulva Al3+ levels inhibit root growth, this is via a novel mechanism. We suggest algal extracts should be used cautiously as fertilisers, as the inhibitory effects on early development may outweigh any benefits if the concentration of extract is too high

Cross-kingdom signalling regulates spore germination in the moss <i>Physcomitrella patens</i>

Plants live in close association with microorganisms that can have beneficial or detrimental effects. The activity of bacteria in association with flowering plants has been extensively analysed. Bacteria use quorum-sensing as a way of monitoring their population density and interacting with their environment. A key group of quorum sensing molecules in Gram-negative bacteria are the N-acylhomoserine lactones (AHLs), which are known to affect the growth and development of both flowering plants, including crops, and marine algae. Thus, AHLs have potentially important roles in agriculture and aquaculture. Nothing is known about the effects of AHLs on the earliest-diverging land plants, thus the evolution of AHL-mediated bacterial-plant/algal interactions is unknown. In this paper, we show that AHLs can affect spore germination in a representative of the earliest plants on land, the Bryophyte moss Physcomitrella patens. Furthermore, we demonstrate that sporophytes of some wild isolates of Physcomitrella patens are associated with AHL-producing bacteria

Bacteria-induced morphogenesis of Ulva intestinalis and Ulva mutabilis (Chlorophyta): a contribution to the lottery theory

The green marine macroalgae of the class Ulvophyceae (Ulvophytes) are common algae distributed worldwide particularly in intertidal areas, which play a key role in aquatic ecosystems. They are potentially valuable resources for food, animal feed and fuel but can also cause massive nuisance blooms. Members of Ulvaceae, like many other seaweeds, harbour a rich diversity of epiphytic bacteria with functions related to host growth and morphological development. In the absence of appropriate bacterially derived signals, germ cells of the genus Ulva develop into 'atypical' colonies consisting of undifferentiated cells with abnormal cell walls. This paper examines the specificity of bacteria-induced morphogenesis in Ulva, by cross-testing bacteria isolated from several Ulva species on two Ulva species, the emerging model system Ulva mutabilis and the prominent biofouler species Ulva intestinalis. We show that pairs of bacterial strains isolated from species other than U. mutabilis and U. intestinalis can fully rescue axenic plantlets generated either from U. mutabilis or U. intestinalis gametes. This laboratory-based study demonstrates that different compositions of microbial communities with similar functional characteristics can enable complete algal morphogenesis and thus supports the 'competitive lottery' theory for how symbiotic bacteria drive algal development.</p