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"Ulva mutabilis allows fascinating studies of both chemical communication and biological developmental processes," says Dr. Thomas Wichard, head of the research group “Chemical Ecology of Ulva” at the University of Jena. He studies the communication between Ulva mutabilis and its bacterial companions. "The bacteria are responsible for the adhesion of Ulva to its substratum, for its growth, and the development of its blade morphology" explains Wichard, member of the Phycology Section of the German Botanical Society (DBG) of which the algal researchers are members.
Trick to Separate Bacteria and Algae
Wichard is particularly interested in the chemicals produced by the bacteria as these affect the development of the alga from its tiny reproductive cell into the adult ‘sea lettuce’ morphology. In order to study the communication between the two very different life forms, Ulva cultures free from bacteria, so-called axenic cultures, had to be produced. As this is very difficult, a trick was applied: during reproduction Ulva produces motile reproductive cells (gametes) which always swim towards a light source – a phenomenon called phototaxis. As the motile gametes are faster than their associated bacteria, they accumulate at the illuminated site of a cultivation dish. Here, the bacteria-free reproductive cells are collected and used to establish algal cultures free from any other organisms.
Juvenile Algae ”on Demand”
The discovery of several chemicals that regulate the formation, and release, of the reproduction cells (gametes) of Ulva has increased considerably the understanding of the chemical communication between Ulva and their associated bacteria. These chemicals allow the seaweed to synchronously develop large numbers of reproductive cells and induce their simultaneous release into the sea water. This control of reproductive timing and their phototaxis after release significantly increases the chance of reproductive cells to find a mating partner in the vast ocean. In the laboratory, researchers have deliberately removed these substances to allow the formation and release of reproductive cells at any time, and thus the establishment of a new bacteria-free algal generation. "These controlled culture conditions are essential to our understanding of how bacteria live in a symbiotic relationship with Ulva and ultimately how they affect the growth and development of the alga," Wichard explains.
Only the Right Bacteria Allow the Algae to Thrive
Juvenile Ulva cultures without associated bacteria develop very slowly and do not form the expected morphology known from nature but instead a mass of undifferentiated cells. Two specific types of bacteria, belonging to the strains of Roseobacter and the Cytophaga, have been shown to be associated with Ulva and shown to influence the development of the alga. The Roseobacter induce the blade cell division of the alga so that the typical ‘lettuce-shaped’ blade is formed, thereby acting similarly to the plant hormone cytokine. The Cytophaga trigger a differentiation of basal cells into a so-called rhizoid which enables the fixation of the alga to the substratum; thereby they act similarly to the plant hormone auxin. The Cytophaga additionally ensures the correct structure of the algal cell walls. Only the combined presence of these two types of bacteria initiates and mediates the typical ‘sea lettuce’ shape.
Searching for the Needle in Sea Water
The bacteria are not the only organisms unleashing chemical substances; Ulva also releases a complex cocktail of metabolites which the researchers would like to decipher in to the water. "The chemical compounds involved in the communication between Ulva and the bacteria are produced in such small quantities that identifying the substances is an enormous challenge," biochemist Wichard comments on his daily lab routine. "It's like looking for the proverbial needle in a haystack - only here the medium is sea water". Meanwhile, his team have isolated the first candidates which probably contribute to the normal development of the alga Ulva mutabilis.
European Network of Researchers Studies the Morphogenesis of Macro-Algae
Other macroalgae closely or even remotely related to Ulva are now being examined to determine which factors and bacterial partners influence the development of their shape. More than 30 research teams from across Europe have now joined forces in a network called "Phycomorph" to explore the growth and development of macroalgae. Since the end of November 2014 they have received research funding for four years from the European Community (European Cooperation in Science and Technology Association, COST) to combine their expertise in joint research projects. Wichard is delighted to be part of this network with his European colleagues and work towards establishing sea lettuce as a model organism.
Settlement, Algal Blooms and Biofilters
The bacterial partners of Ulva also support the alga in selecting a suitable habitat for attachment. Bacterial signalling molecules induce motile zoospores, another type of Ulva reproductive cells, to germinate only in appropriate locations. The juvenile seaweed and the bacteria then form a layer, for example over a rock, where the seaweed eventually grows to its final size. Ulva species are found not only on natural coastal hard bottom surfaces, but also as fouling organisms on ship hulls where they impede the boats’ rapid movement through the water.
In nature many Ulva species grow rapidly under certain conditions, for example where untreated, nutrient-rich wastewater from agriculture and densely populated areas is flushed into the sea. They then produce green algal blooms (‘Green Tides’) producing a large amount of algal biomass – a phenomenon which can be observed worldwide in many coastal regions and almost endangered the Olympic sailing regatta 2008 in China. In addition, Ulva deposited on shorelines form algal mats can deprive underlying sediments and their associated fauna of air. However, on the other hand, Ulva is capable of rapidly absorbing excess nutrients from wastewater. Thus researchers are already working on using the sea lettuce as a reliable bio-filter for future coastal wastewater treatment.
Initial Genome Analyses
Algal researchers are enthusiastic about Ulva because of its many unexplored secrets. In contrast to other more established model systems however, there is still much pioneering work ahead in Ulva research. The scientific community hopes the first Ulva Genome Project recently launched in the UK will boost their research activities and improve access to molecular biological techniques. The teams are now waiting in suspense for genomic data to provide an insight into the biological programming of algal development. "This ambitious project will also help unravel the deeper mysteries of the multi-layered communication between Ulva and its surrounding genetic basis” says Wichard, who is looking forward to the results.