Dynamics of refractory carbon in seagrass meadows

University of Technology Sydney. Faculty of Science.The protection and rehabilitation of natural landscapes in order to enhance their role in carbon sequestration is currently a hot topic for scientists and policymakers looking for solutions to reduce atmospheric CO₂ levels. Blue carbon ecosystems (seagrass, mangrove, saltmarsh) have recently been found to match or even exceed the capability of terrestrial ecosystems to sequester carbon. In seagrass habitats, seagrass carbon alone can account for half of the carbon in the top 10 cm of sediment. Litter quality, often measured as refractory carbon content, is one of the main factors that can influence the sequestration and storage of refractory carbon. Yet to-date, there has been little attempt to understand what factors help or hinder refractory carbon preservation in seagrass sediments. The aim of this thesis was to unravel the processes and factors that influence, and even optimise, the preservation of refractory carbon in seagrass meadows beginning with the refractory carbon content in seagrass tissues, its persistence (or remineralisation) during decomposition and finally, its preservation in sediments and the mechanisms that provoke further remineralisation after burial. To accomplish these aims, a multi-variable approach was taken, which involved assessing the main and interaction effects of biological, chemical and environmental/physical variables on refractory carbon remineralisation and storage. The results from this thesis revealed that the processes that affect refractory carbon dynamics in seagrass meadows are complex. It was shown that, while inherent refractory carbon content in the tissues can promote sequestration, decomposition was a strong influence on the persistence of refractory carbon. Anoxic conditions and structural complexity of the tissues promoted refractory carbon preservation and were dependent on the microbial communities present. Sheath and stem tissues were considered to be important carbon contributors due to their high refractory carbon content and chance of in situ burial. Temperature and the availability of labile organic matter and inorganic nutrients enhanced decay in the short-term under oxic conditions, while physical disturbance and habitat loss caused losses of sediment refractory carbon over the course of months to years depending on the type of disturbance. In light of these results, a new conceptual model was developed for seagrass decomposition and have highlighted several important avenues of future blue carbon research, including the functional roles of microbes (bacteria, fungi and protists) in carbon remineralisation via bioinformatics and enzymes kinetics, as well as the conversion, or ‘up-cycling’, of labile carbon to refractory carbon within microbial biomass

Physiology and Biochemistry of the Tropical Seagrass Thalassia testudinum in Response to Hypersalinity Stress and Labyrinthula sp. Infection

Seagrass meadows are essential to coastal ecosystems and have experienced declines in abundance due to a series of environmental stressors including elevated salinity and incidence of disease caused by the pathogen Labyrinthula sp. This thesis evaluated the dynamics between T. testudinum and Labyrinthula concerning the impacts of short term elevated salinity stress on the early stages of infection in Thalassia testudinum (Chapter 2) as well as the presence of anti-labyrinthulid secondary metabolites produced by the tropical seagrass (Chapter 3). The results showed that short term exposure to hypersalinity alters some cellular processes but does not necessarily lead to an immediate increase in wasting disease susceptibility. Specifically, the occurrence of disease was significantly lower in the hypersalinity treatments possibly due to a direct osmotic shock to Labyrinthula or indirectly due to the increase in in vivo H2O2 concentrations that may have inhibited Labyrinthula growth. In addition, it was shown that 4 phenolic acids commonly found in turtlegrass leaf tissue were able to inhibit Labyrinthula growth in culture. Using a bioassay-guided fractionation technique, several purified fractions of T. testudinum leaf tissue showed anti-labyrinthulid activity, however the detailed characterization of the unknown compounds was inconclusive. The results presented in this thesis highlight the halotolerant characteristics of the seagrass T. testudinum as well as suggest that T. testudinum has the capability of defending itself against Labyrinthula infection using secondary metabolites

Comparison of marine macrophytes for their contributions to blue carbon sequestration

Many marine ecosystems have the capacity for long-term storage of organic carbon (C) in what are termed "blue carbon" systems. While blue carbon systems (saltmarsh, mangrove, and seagrass) are efficient at long-term sequestration of organic carbon (C), much of their sequestered C may originate from other (allochthonous) habitats. Macroalgae, due to their high rates of production, fragmentation, and ability to be transported, would also appear to be able to make a significant contribution as C donors to blue C habitats. In order to assess the stability of macroalgal tissues and their likely contribution to long-term pools of C, we applied thermogravimetric analysis (TGA) to 14 taxa of marine macroalgae and coastal vascular plants. We assessed the structural complexity of multiple lineages of plant and tissue types with differing cell wall structures and found that decomposition dynamics varied significantly according to differences in cell wall structure and composition among taxonomic groups and tissue function (photosynthetic vs. attachment). Vascular plant tissues generally exhibited greater stability with a greater proportion of mass loss at temperatures > 300 degrees C (peak mass loss -320 degrees C) than macroalgae (peak mass loss between 175-300 degrees C), consistent with the lignocellulose matrix of vascular plants. Greater variation in thermogravimetric signatures within and among macroalgal taxa, relative to vascular plants, was also consistent with the diversity of cell wall structure and composition among groups. Significant degradation above 600 degrees C for some macroalgae, as well as some belowground seagrass tissues, is likely due to the presence of taxon-specific compounds. The results of this study highlight the importance of the lignocellulose matrix to the stability of vascular plant sources and the potentially significant role of refractory, taxon-specific compounds (carbonates, long-chain lipids, alginates, xylans, and sulfated polysaccharides) from macroalgae and seagrasses for their long-term sedimentary C storage. This study shows that marine macroalgae do contain refractory compounds and thus may be more valuable to long-term carbon sequestration than we previously have considered

Estimating the potential blue carbon gains from tidal marsh rehabilitation: A case study from south eastern Australia

© Copyright © 2020 Gulliver, Carnell, Trevathan-Tackett, Duarte de Paula Costa, Masqué and Macreadie. Historically, coastal “blue carbon” ecosystems (tidal marshes, mangrove forests, seagrass meadows) have been impacted and degraded by human intervention, mainly in the form of land acquisition. With increasing recognition of the role of blue carbon ecosystems in climate mitigation, protecting and rehabilitating these ecosystems becomes increasingly more important. This study evaluated the potential carbon gains from rehabilitating a degraded coastal tidal marsh site in south-eastern Australia. Tidal exchange at the study site had been restricted by the construction of earthen barriers for the purpose of reclaiming land for commercial salt production. Analysis of sediment cores (elemental carbon and 210Pb dating) revealed that the site had stopped accumulating carbon since it had been converted to salt ponds 65 years earlier. In contrast, nearby recovered (“control”) tidal marsh areas are still accumulating carbon at relatively high rates (0.54 tons C ha–1year–1). Using elevation and sea level rise (SLR) data, we estimated the potential future distribution of tidal marsh vegetation if the earthen barrier were removed and tidal exchange was restored to the degraded site. We estimated that the sediment-based carbon gains over the next 50 years after restoring this small site (360 ha) would be 9,000 tons C, which could offset the annual emissions of ∼7,000 passenger cars at present time (at 4.6 metric tons pa.) or ∼1,400 Australians. Overall, we recommend that this site is a promising prospect for rehabilitation based on the opportunity for blue carbon additionality, and that the business case for rehabilitation could be bolstered through valuation of other co-benefits, such as nitrogen removal, support to fisheries, sediment stabilization, and enhanced biodiversity

Comment on \u27Geoengineering with seagrasses: Is credit due where credit is given?\u27

Over the past decade scientists around the world have sought to estimate the capacity of seagrass meadows to sequester carbon, and thereby understand their role in climate change mitigation. The number of studies reporting on seagrass carbon accumulation rates is still limited, but growing scientific evidence supports the hypothesis that seagrasses have been efficiently locking away CO2 for decades to millennia (e.g. Macreadie et al 2014, Mateo et al 1997, Serrano et al 2012). Johannessen and Macdonald (2016), however, challenge the role of seagrasses as carbon traps, claiming that gains in carbon storage by seagrasses may be \u27illusionary\u27 and that \u27their contribution to the global burial of carbon has not yet been established\u27. The authors warn that misunderstandings of how sediments receive, process and store carbon have led to an overestimation of carbon burial by seagrasses. Here we would like to clarify some of the questions raised by Johannessen and Macdonald (2016), with the aim to promote discussion within the scientific community about the evidence for carbon sequestration by seagrasses with a view to awarding carbon credits

Dumpster diving for diatom plastid 16S rRNA genes

High throughput sequencing is improving the efficiency of monitoring diatoms, which inhabit and support aquatic ecosystems across the globe. In this study, we explored the potential of a standard V4 515F-806RB primer pair in recovering diatom plastid 16S rRNA sequences. We used PhytoREF to classify the 16S reads from our freshwater biofilm field sampling from three stream segments across two streams in south-eastern Australia and retrieved diatom community data from other, publicly deposited, Australian 16S amplicon datasets. When these diatom operational taxonomic units (OTUs) were traced using the default RDPII and NCBI databases, 68% were characterized as uncultured cyanobacteria. We analysed the 16S rRNA sequences from 72 stream biofilm samples, separated the chloroplast OTUs, and classified them using the PhytoREF database. After filtering the reads attributed to Bacillariophyta (relative abundance >1%), 71 diatom OTUs comprising more than 90% of the diatom reads in each stream biofilm sample were identified. Beta-diversity analyses demonstrated significantly different diatom assemblages and discrimination among river segments. To further test the approach, the diatom OTUs from our biofilm sampling were used as reference sequences to identify diatom reads from other Australian 16S rRNA datasets in the NCBI-SRA database. Across the three selected public datasets, 67 of our 71 diatom OTUs were detected in other Australian ecosystems. Our results show that diatom plastid 16S rRNA genes are readily amplified with existing 515F-806RB primer sets. Therefore, the volume of existing 16S rRNA amplicon datasets initially generated for microbial community profiling can also be used to detect, characterize, and map diatom distribution to inform phylogeny and ecological health assessments, and can be extended into a range of ecological and industrial applications. To our knowledge, this study represents the first attempt to classify freshwater samples using this approach and the first application of PhytoREF in Australia

Oxygen Consumption and Sulfate Reduction in Vegetated Coastal Habitats: Effects of Physical Disturbance

Vegetated coastal habitats (VCHs), such as mangrove forests, salt marshes and seagrass meadows, have the ability to capture and store carbon in the sediment for millennia, and thus have high potential for mitigating global carbon emissions. Carbon sequestration and storage is inherently linked to the geochemical conditions created by a variety of microbial metabolisms, where physical disturbance of sediments may expose previously anoxic sediment layers to oxygen (O2), which could turn them into carbon sources instead of carbon sinks. Here, we used O2, hydrogen sulfide (H2S) and pH microsensors to determine how biogeochemical conditions, and thus aerobic and anaerobic metabolic pathways, vary across mangrove, salt marsh and seagrass sediments (case study from the Sydney area, Australia). We measured the biogeochemical conditions in the top 2.5 cm of surface (0–10 cm depth) and experimentally exposed deep sediments (>50 cm depth) to simulate undisturbed and physically exposed sediments, respectively, and how these conditions may affect carbon cycling processes. Mangrove surface sediment exhibited the highest rates of O2 consumption and sulfate (SO42-) reduction based on detailed microsensor measurements, with a diffusive O2 uptake rate of 102 mmol O2 m-2 d-1 and estimated sulfate reduction rate of 57 mmol Stot2- m-2 d-1. Surface sediments (0–10 cm) across all the VCHs generally had higher O2 consumption and estimated sulfate reduction rates than deeper layers (>50 cm depth). O2 penetration was <4 mm for most sediments and only down to ∼1 mm depth in mangrove surface sediments, which correlated with a significantly higher percent organic carbon content (%Corg) within sediments originating from mangrove forests as compared to those from seagrass and salt marsh ecosystems. Additionally, pH dropped from 8.2 at the sediment/water interface to <7–7.5 within the first 20 mm of sediment within all ecosystems. Prevailing anoxic conditions, especially in mangrove and seagrass sediments, as well as sediment acidification with depth, likely decreased microbial remineralisation rates of sedimentary carbon. However, physical disturbance of sediments and thereby exposure of deeper sediments to O2 seemed to stimulate aerobic metabolism in the exposed surface layers, likely reducing carbon stocks in VCHs

A global assessment of the chemical recalcitrance of seagrass tissues: Implications for long-term carbon sequestration

Seagrass ecosystems have recently been identified for their role in climate change mitigation due to their globally-significant carbon sinks; yet, the capacity of seagrasses to sequester carbon has been shown to vary greatly among seagrass ecosystems. The recalcitrant nature of seagrass tissues, or the resistance to degradation back into carbon dioxide, is one aspect thought to influence sediment carbon stocks. In this study, a global survey investigated how the macromolecular chemistry of seagrass leaves, sheaths/stems, rhizomes and roots varied across 23 species from 16 countries. The goal was to understand how this seagrass chemistry might influence the capacity of seagrasses to contribute to sediment carbon stocks. Three non-destructive analytical chemical analyses were used to investigate seagrass chemistry: thermogravimetric analysis (TGA) and solid state 13 C-NMR and infrared spectroscopy. A strong latitudinal influence on carbon quality was found, whereby temperate seagrasses contained 5% relatively more labile carbon, and tropical seagrasses contained 3% relatively more refractory carbon. Sheath/stem tissues significantly varied across taxa, with larger morphologies typically containing more refractory carbon than smaller morphologies. Rhizomes were characterized by a higher proportion of labile carbon (16%of total organic matter compared to 8–10%in other tissues); however, high rhizome biomass production and slower remineralization in anoxic sediments will likely enhance these below-ground tissues’ contributions to long-termcarbon stocks. Our study provides a standardized and global dataset on seagrass carbon quality across tissue types, taxa and geography that can be incorporated in carbon sequestration and storage models as well as ecosystem valuation and management strategies

Oxygen consumption and sulfate reduction in vegetated coastal habitats: Effects of physical disturbance

© 2019 Brodersen, Trevathan-Tackett, Nielsen, Connolly, Lovelock, Atwood and Macreadie. Vegetated coastal habitats (VCHs), such as mangrove forests, salt marshes and seagrass meadows, have the ability to capture and store carbon in the sediment for millennia, and thus have high potential for mitigating global carbon emissions. Carbon sequestration and storage is inherently linked to the geochemical conditions created by a variety of microbial metabolisms, where physical disturbance of sediments may expose previously anoxic sediment layers to oxygen (O 2 ), which could turn them into carbon sources instead of carbon sinks. Here, we used O 2 , hydrogen sulfide (H 2 S) and pH microsensors to determine how biogeochemical conditions, and thus aerobic and anaerobic metabolic pathways, vary across mangrove, salt marsh and seagrass sediments (case study from the Sydney area, Australia). We measured the biogeochemical conditions in the top 2.5 cm of surface (0-10 cm depth) and experimentally exposed deep sediments (> 50 cm depth) to simulate undisturbed and physically exposed sediments, respectively, and how these conditions may affect carbon cycling processes. Mangrove surface sediment exhibited the highest rates of O 2 consumption and sulfate (SO 42- ) reduction based on detailed microsensor measurements, with a diffusive O 2 uptake rate of 102 mmol O 2 m -2 d -1 and estimated sulfate reduction rate of 57 mmol S tot2- m -2 d -1 . Surface sediments (0-10 cm) across all the VCHs generally had higher O 2 consumption and estimated sulfate reduction rates than deeper layers (> 50 cm depth). O 2 penetration was < 4 mm for most sediments and only down to 1 mm depth in mangrove surface sediments, which correlated with a significantly higher percent organic carbon content (%C org ) within sediments originating from mangrove forests as compared to those from seagrass and salt marsh ecosystems. Additionally, pH dropped from 8.2 at the sediment/water interface to < 7-7.5 within the first 20 mm of sediment within all ecosystems. Prevailing anoxic conditions, especially in mangrove and seagrass sediments, as well as sediment acidification with depth, likely decreased microbial remineralisation rates of sedimentary carbon. However, physical disturbance of sediments and thereby exposure of deeper sediments to O 2 seemed to stimulate aerobic metabolism in the exposed surface layers, likely reducing carbon stocks in VCHs

Effects of nutrient loading on sediment bacterial and pathogen communities within seagrass meadows

Eutrophication can play a significant role in seagrass decline and habitat loss. Microorganisms in seagrass sediments are essential to many important ecosystem processes, including nutrient cycling and seagrass ecosystem health. However, current knowledge of the bacterial communities, both beneficial and detrimental, within seagrass meadows in response to nutrient loading is limited. We studied the response of sediment bacterial and pathogen communities to nutrient enrichment on a tropical seagrass meadow in Xincun Bay, South China Sea. The bacterial taxonomic groups across all sites were dominated by the Gammaproteobacteria and Firmicutes. Sites nearest to the nutrient source and with the highest NH4+ and PO43&minus; content had approximately double the relative abundance of putative denitrifiers Vibrionales, Alteromonadales, and Pseudomonadales. Additionally, the relative abundance of potential pathogen groups, especially Vibrio spp. and Pseudoalteromonas spp., was approximately 2‐fold greater at the sites with the highest nutrient loads compared to sites further from the source. These results suggest that proximity to sources of nutrient pollution increases the occurrence of potential bacterial pathogens that could affect fishes, invertebrates and humans. This study shows that nutrient enrichment does elicit shifts in bacterial community diversity and likely their function in local biogeochemical cycling and as a potential source of infectious diseases within seagrass meadows