Stacey M. Trevathan-Tackett

Quantifying and modelling the carbon sequestration capacity of seagrass meadows – A critical assessment

Seagrasses are among the planets most effective natural ecosystems for sequestering (capturing and storing) carbon (C); but if degraded, they could leak stored C into the atmosphere and accelerate global warming. Quantifying and modelling the C sequestration capacity is therefore critical for successfully managing seagrass ecosystems to maintain their substantial abatement potential. At present, there is no mechanism to support carbon financing linked to seagrass. For seagrasses to be recognised by the IPCC and the voluntary C market, standard stock assessment methodologies and inventories of seagrass C stocks are required. Developing accurate C budgets for seagrass meadows is indeed complex; we discuss these complexities, and, in addition, we review techniques and methodologies that will aid development of C budgets. We also consider a simple process-based data assimilation model for predicting how seagrasses will respond to future change, accompanied by a practical list of research priorities.

Losses and recovery of organic carbon from a seagrass ecosystem following disturbance

Seagrasses are among the Earths most efficient and long-term carbon sinks, but coastal development threatens this capacity. We report new evidence that disturbance to seagrass ecosystems causes release of ancient carbon. In a seagrass ecosystem that had been disturbed 50 years ago, we found that soil carbon stocks declined by 72%, which, according to radiocarbon dating, had taken hundreds to thousands of years to accumulate. Disturbed soils harboured different benthic bacterial communities (according to 16S rRNA sequence analysis), with higher proportions of aerobic heterotrophs compared with undisturbed. Fingerprinting of the carbon (via stable isotopes) suggested that the contribution of autochthonous carbon (carbon produced through plant primary production) to the soil carbon pool was less in disturbed areas compared with seagrass and recovered areas. Seagrass areas that had recovered from disturbance had slightly lower (35%) carbon levels than undisturbed, but more than twice as much as the disturbed areas, which is encouraging for restoration efforts. Slow rates of seagrass recovery imply the need to transplant seagrass, rather than waiting for recovery via natural processes. This study empirically demonstrates that disturbance to seagrass ecosystems can cause release of ancient carbon, with potentially major global warming consequences.

Sediment anoxia limits microbial-driven seagrass carbon remineralization under warming conditions

ABSTRACT Seagrass ecosystems are significant carbon sinks, and their resident microbial communities ultimately determine the quantity and quality of carbon sequestered. However, environmental perturbations have been predicted to affect microbial‐driven seagrass decomposition and subsequent carbon sequestration. Utilizing techniques including 16S‐rDNA sequencing, solid‐state NMR and microsensor profiling, we tested the hypothesis that elevated seawater temperatures and eutrophication enhance the microbial decomposition of seagrass leaf detritus and rhizome/root tissues. Nutrient additions had a negligible effect on seagrass decomposition, indicating an absence of nutrient limitation. Elevated temperatures caused a 19% higher biomass loss for aerobically decaying leaf detritus, coinciding with changes in bacterial community structure and enhanced lignocellulose degradation. Although, community shifts and lignocellulose degradation were also observed for rhizome/root decomposition, anaerobic decay was unaffected by temperature. These observations suggest that oxygen availability constrains the stimulatory effects of temperature increases on bacterial carbon remineralization, possibly through differential temperature effects on bacterial functional groups, including putative aerobic heterotrophs (e.g. Erythrobacteraceae, Hyphomicrobiaceae) and sulfate reducers (e.g. Desulfobacteraceae). Consequently, under elevated seawater temperatures, carbon accumulation rates may diminish due to higher remineralization rates at the sediment surface. Nonetheless, the anoxic conditions ubiquitous to seagrass sediments can provide a degree of carbon protection under warming seawater temperatures. &NA; Graphical Abstract Figure. While elevated seawater temperatures may diminish carbon accumulation at the sediment surface, the anoxic conditions in coastal sediments can provide carbon protection under warming temperatures, thus promoting carbon storage.

The First Isolation and Characterisation of the Protist Labyrinthula sp. in Southeastern Australia

As a result of anthropogenic influences and global climate change, emerging infectious marine diseases are thought to be increasingly more common and more severe than in the past. The aim of our investigation was to confirm the presence of Labyrinthula, the aetiological agent of the seagrass wasting disease, in Southeastern Australia and provide the first isolation and characterisation of this protist, in Australia. Colonies and individual cells were positively identified as Labyrinthula using published descriptions, diagrams, and photographs. Their identity was then confirmed using DNA barcoding of a region of the 18S rRNA gene. Species level identification of isolates was not possible as the taxonomy of the Labyrinthula is still poorly resolved. Still, a diversity of Labyrinthula was isolated from small sections of the southeast coast of Australia. The isolates were grouped into three haplotypes that are biogeographically restricted. These haplotypes are closely related to previously identified saprotrophic clades. The study highlights the need for further investigation into the global distribution of Labyrinthula, including phylogenetic pathogenicity and analysis of host‐parasite interactions in response to stressors. Given the results of our analyses, it is prudent to continue research into disease and epidemic agents to better prepare researchers for potential future outbreaks.

Sediment Sampling in Estuaries: Site Selection and Sampling Techniques

In this chapter a range of sediment sampling techniques specifically suited to estuarine conditions are briefly described and discussed. Advice is provided about the selection of appropriate coring sites and techniques for a variety of conditions, including water depth, varying sediment composition, and sample analytical requirements. In the section on experimental design we briefly consider issues to do with sample replication from both a biological and geological perspective. During coring, alterations are inevitably made to the texture of the sediment, including compaction and water loss, resulting in changes to bulk density and the structure of the pore spaces, and physical disruption to layering. We comment on the nature of some of these disturbances, their dependency on sediment composition, which techniques to choose to minimise occurrence and, if necessary, how and when to make measurements to determine the amount of change caused by coring. Several factors need to be considered during the core recovery phase to ensure optimal retrieval of the core. These include use of core catchers and plugs to minimise or prevent loss of sediment during recovery. Freeze coring is recommended where the sediment-water interface is poorly defined or the sediments are particularly watery. Finally, we discuss transport and initial storage of cores.

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.

Detachment and flow cytometric quantification of seagrass-associated bacteria.

A new protocol was developed to detach bacteria from seagrass tissue and subsequently enumerate cells using flow cytometry (FCM). A method involving addition of the surfactant Tween 80 and vortexing resulted in maximum detachment efficiency of seagrass attached bacteria, providing a robust protocol for precisely enumerating seagrass-associated bacteria with FCM. Using this approach we detected cell concentrations between 2.0×10(5) and 8.0×10(6)cells mg(-1) DW tissue.

Effects of small‐scale, shading‐induced seagrass loss on blue carbon storage: Implications for management of degraded seagrass ecosystems

1Climate Change Cluster, University of Technology Sydney, Ultimo, NSW, Australia; 2Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Burwood, VIC, Australia; 3Dauphin Island Sea Laboratory, Dauphin Island, AL, USA; 4Department of Marine Sciences, University of South Alabama, Mobile, AL, USA; 5School of Science, Edith Cowan University, Joondalup, WA, Australia; 6Departament de Física & Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, Bellaterra, Spain and 7Oceans Institute & School of Physics, University of Western Australia, Crawley, WA, Australia

Leaching of dissolved organic matter from seagrass leaf litter and its biogeochemical implications

Dissolved organic matter (DOM) represents a significant source of nutrients that supports the microbial-based food web in seagrass ecosystems. However, there is little information on how the various fractions of DOM from seagrass leaves contributed to the coastal biogeochemical cycles. To address this gap, we carried out a 30-day laboratory chamber experiment on tropical seagrasses Thalassia hemprichii and Enhalus acoroides. After 30 days of incubation, on average 22% carbon (C), 70% nitrogen (N) and 38% phosphorus (P) of these two species of seagrass leaf litter was released. The average leached dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) of these two species of seagrass leaf litter accounted for 55%, 95% and 65% of the total C, N and P lost, respectively. In the absence of microbes, about 75% of the total amount of DOC, monosaccharides (MCHO), DON and DOP were quickly released via leaching from both seagrass species in the first 9 days. Subsequently, little DOM was released during the remainder of the experiment. The leaching rates of DOC, DON and DOP were approximately 110, 40 and 0.70 µmol/(g·d). Leaching rates of DOM were attributed to the nonstructural carbohydrates and other labile organic matter within the seagrass leaf. Thalassia hemprichii leached more DOC, DOP and MCHO than E. acoroides. In contrast, E. acoroides leached higher concentrations of DON than T. hemprichii, with the overall leachate also having a higher DON: DOP ratio. These results indicate that there is an overall higher amount of DOM leachate from T. hemprichii than that of E. acoroides that is available to the seagrass ecosystem. According to the logarithmic model for DOM release and the in situ leaf litter production (the Xincun Bay, South China Sea), the seagrass leaf litter of these two seagrass species could release approximately 4×103 mol/d DOC, 1.4×103 mol/d DON and 25 mol/d DOP into the seawater. In addition to providing readily available nutrients for the microbial food web, the remaining particulate organic matter (POM) from the litter would also enter microbial remineralization processes. What is not remineralized from either DOM or POM fractions has potential to contribute to the permanent carbon stocks.

The Microbiology of Seagrasses

Like both terrestrial plants and other benthic marine organisms, seagrasses host abundant and diverse communities of microorganisms. These microbes fundamentally influence seagrass physiology and health, while also regulating the biogeochemical dynamics of entire seagrass meadows. Discrete populations of bacteria, fungi, microalgae, archaea and viruses inhabit seagrass leaves, roots and rhizomes and the surrounding sediments. The plethora of ecological interactions taking place between seagrasses and this microbiome span the continuum of symbiotic relationships from mutualism to parasitism. Indeed, the metabolic activities of some seagrass associated microbes, such as diazotrophic and sulphur oxidizing bacteria, govern the local chemical environment in ways that facilitate seagrass survival. On the other hand, pathogens, such as the protozoan parasite Labyrinthula cause disease outbreaks that can lead to mass seagrass die offs. While the role of the seagrass microbiome in defining the success of seagrass habitats is becoming increasingly apparent, there is still much to be learnt. For instance, the development of an understanding of how seagrass associated microbes may buffer or augment the negative impacts of growing environmental pressures will be valuable for informing decisions regarding the management and conservation of threatened seagrass habitats. In this chapter we will synthesise the current state of knowledge on the microbiology of seagrasses, with a goal of conveying the often overlooked importance of the seagrass microbiome in governing seagrass health and the biogeochemical stability of seagrass ecosystems.