Marine Macroalgae: Defence against bacterial pathogens

Macroalgae supports a wide range of organisms, most importantly in temperate marine ecosystems. They act as a source of food; provide substrata to settle on and protective environments for early life history stages for many invertebrates. As well as being habitat formers and primary producers. Recently, there has been a massive decline in overall biodiversity and loss of algal species in coastal marine environments. Evidence has suggested that macroalgae are under threat from diseases caused by bacteria, viruses and fungi, could be a major cause of the decline. An increase in disease is attributed to opportunistic pathogens that can take advantage of an already weakened host. Therefore, scientists have said that with the impacts of global climate change could result in a greater increase in the rate and severity of bacterial disease of marine macroalgae.

Disease phenotypes often include rotting, abnormal tissue development or changes in pigmentation that appear as blotches, spots, rusts or bleaching. Over the past decade, there has been a gradual increase in the reporting of these symptoms. As there is increasing demand for macroalage as food sources and potential biofuels, it is important that they are well studied.

Macroalgae lack a cell-based, adaptive immune response but do have defence capabilities. Bioactive secondary metabolites are used to regulate colonization of bacteria and other epibionts. These algal metabolites can alter the bacterial community by selecting for beneficial bacterial populations. They can also interfere with bacterial communication networks and gene regulation, in particular quorum sensing systems. Additionally, macroalgal associated bacteria shows signs of inhibitory activities against other surface colonizers, allowing bacteria to outcompete and displace other bacteria.

Pathogen-induced defences include specific recognition of bacteria, followed by a series of immune signalling pathways involving oxidised polyunsaturated fatty acids or oxylipins. However, contrasting data suggests that the signalling pathways are different depending on the species. Oxylipin signalling in red algae increases the expression of stress-related genes and triggers oxidative burst activity in kelp.

Virulence traits in bacteria are highly varied which include toxins, adhesion factors, and mechanisms for nutrition acquisition from the host and to evade host immune responses. Another important virulence determinant of bacteria is the ability to detoxify reactive oxygen species released during oxidative burst of algal cells. Marine bacteria commonly harbour antioxidants and are essential for the progressive virulence, especially in Vibrio species.

Host-bacterial interactions are often highly complex and depend on multiple factors including the state of the host, the pathogenic potential of the bacterium and whether there are any environmental stressors affecting the macroalgae. Tolerances of environmental parameters will affect the overall performance of the algae, making it potentially susceptible to microbial pathogens. Therefore, with environmental changes happening as a result of global climate change, the roles in which bacteria play in structuring the future oceans ecosystems needs to be extensively investigated.

This review included many examples of these processes, however mostly done on terrestrial plants which has then been applied to marine algae, which suggests that more research is needed on this topic. As they contribute to primary production in the oceans and therefore are at the bottom of the food web, all organisms will be affected by the decline in macroalgae populations and this is why I think this is an important area of research.

Egan, S., Fernandes, N.D., Kumar, V., Gardiner, M. and Thomas, T. (2014) Bacterial pathogens, virulence mechanism and host defence in marine macroalgae. Environmental Microbiology. 16: 925-938