Laura L. Govers

Marine Phytophthora species can hamper conservation and restoration of vegetated coastal ecosystems

Phytophthora species are potent pathogens that can devastate terrestrial plants, causing billions of dollars of damage yearly to agricultural crops and harming fragile ecosystems worldwide. Yet, virtually nothing is known about the distribution and pathogenicity of their marine relatives. This is surprising, as marine plants form vital habitats in coastal zones worldwide (i.e. mangrove forests, salt marshes, seagrass beds), and disease may be an important bottleneck for the conservation and restoration of these rapidly declining ecosystems. We are the first to report on widespread infection of Phytophthora and Halophytophthora species on a common seagrass species, Zostera marina (eelgrass), across the northern Atlantic and Mediterranean. In addition, we tested the effects of Halophytophthora sp. Zostera and Phytophthora gemini on Z. marina seed germination in a full-factorial laboratory experiment under various environmental conditions. Results suggest that Phytophthora species are widespread as we found these oomycetes in eelgrass beds in six countries across the North Atlantic and Mediterranean. Infection by Halophytophthora sp. Zostera, P. gemini, or both, strongly affected sexual reproduction by reducing seed germination sixfold. Our findings have important implications for seagrass ecology, because these putative pathogens probably negatively affect ecosystem functioning, as well as current restoration and conservation efforts.

Review: Host-pathogen dynamics of seagrass diseases under future global change

Human-induced global change is expected to amplify the disease risk for marine biota. However, the role of disease in the rapid global decline of seagrass is largely unknown. Global change may enhance seagrass susceptibility to disease through enhanced physiological stress, while simultaneously promoting pathogen development. This review outlines the characteristics of disease-forming organisms and potential impacts of global change on three groups of known seagrass pathogens: labyrinthulids, oomycetes and Phytomyxea. We propose that hypersalinity, climate warming and eutrophication pose the greatest risk for increasing frequency of disease outbreaks in seagrasses by increasing seagrass stress and lowering seagrass resilience. In some instances, global change may also promote pathogen development. However, there is currently a paucity of information on these seagrass pathosystems. We emphasise the need to expand current research to better understand the seagrass-pathogen relationships, serving to inform predicative modelling and management of seagrass disease under future global change scenarios.

Multiple Halophytophthora spp. and Phytophthora spp. including P. gemini , P. inundata and P. chesapeakensis sp. nov. isolated from the seagrass Zostera marina in the Northern hemisphere

A plethora of oomycetes was isolated mostly from Zostera marina but also from other halophilic plants originating from several locations including the Wadden Sea, Limfjord (Denmark), the Dutch Delta area (the Netherlands), Thau lagoon (France), Lindholmen (Sweden) and Chesapeake Bay (Virginia, U.S.). Based on ITS sequences, seven different groups could be distinguished. The largest group was assigned to Phytophthora gemini (Germany, Sweden, the Netherlands, U.S.). The CoxI sequences of all P. gemini strains were identical indicating that P. gemini is probably an invasive species in the Wadden Sea. A second group was identified as P. inundata (the Netherlands, Denmark), that was also isolated from the halophilic plants Aster tripolium and Salicornia europaea. Four strains, originating from Chesapeake Bay clustered in a monophyletic clade with high bootstrap support at the ITS as well as the CoxI loci. They are phylogenetically closely related to P. gemini and are considered to represent a new species described here as Phytophthora chesapeakensis sp. nov. In addition, Salisapilea sapeloensis was isolated from Zostera noltii. Eleven other strains belonging to three unidentified taxa, originating from the Wadden Sea, the Dutch Delta area and Thau lagoon, clustered each in a monophyletic clade with high bootstrap support at the ITS locus, including Halophytophthora vesicula, the type species of the genus Halophytophthora. Hence, these strains were considered to belong to the Halophytophthora sensu stricto group and probably represent three new Halophytophthora species, informally designated here as Halophytophthora sp-1, Halophytophthora sp-3 and Halophytophthora sp-4 sensu Nigrelli and Thines. Halophytophthora sp-2 was not detected in this study. In addition, P. gemini and Halophytophthora sp-3 were obtained by baiting from locations in the Wadden Sea and Halophytophthora sp-1 was obtained by baiting from the Delta area.

A facultative mutualistic feedback enhances the stability of tropical intertidal seagrass beds

Marine foundation species such as corals, seagrasses, salt marsh plants, and mangrove trees are increasingly found to engage in mutualistic interactions. Because mutualisms by their very nature generate a positive feedback between the species, subtle environmental impacts on one of the species involved may trigger mutualism breakdown, potentially leading to ecosystem regime shifts. Using an empirically parameterized model, we investigate a facultative mutualism between seagrass and lucinid bivalves with endosymbiotic sulfide-oxidizing gill bacteria in a tropical intertidal ecosystem. Model predictions for our system show that, by alleviating the build-up of toxic sulfide, this mutualism maintains an otherwise intrinsically unstable seagrass ecosystem. However, an increase in seagrass mortality above natural levels, due to e.g. desiccation stress, triggers mutualism breakdown. This pushes the system in collapse-and-recovery dynamics (‘slow-fast cycles’) characterized by long-term persistent states of bare and seagrass-dominated, with rapid transitions in between. Model results were consistent with remote sensing analyses that suggest feedback-mediated state shifts induced by desiccation. Overall, our combined theoretical and empirical results illustrate the potential of mutualistic feedbacks to stabilize ecosystems, but also reveal an important drawback as small environmental changes may trigger shifts. We therefore suggest that mutualisms should be considered for marine conservation and restoration of seagrass beds.

Seagrass leaf element content: A global overview

Knowledge on the role of seagrass leaf elements and in particular micronutrients and their ranges is limited. We present a global database, consisting of 1126 unique leaf values for ten elements, obtained from literature and unpublished data, spanning 25 different seagrass species from 28 countries. The overall order of average element values in seagrass leaves was Na>K>Ca>Mg>S>Fe>Al>Si>Mn>Zn. Although we observed differences in leaf element content between seagrass families, high intraspecific variation indicated that leaf element content was more strongly determined by environmental factors than by evolutionary history. Early successional species had high leaf Al and Fe content. In addition, seagrass leaf element content also showed correlations with macronutrients (N and P), indicating that productivity also depends on other elements. Expected genomes of additional seagrass species in combination with experiments manipulating (micro)nutrients and environmental drivers might enable us to unravel the importance of various elements to sustain productive and flourishing meadows.

Banc d'Arguin - Food webs with trophic effects by seagrass removed

This file contains matrices from which we we removed seagrass (Zostra noltii) and any species exclusively feeding on it.