Nadia S. Santini

Contemporary Rates of Carbon Sequestration Through Vertical Accretion of Sediments in Mangrove Forests and Saltmarshes of South East Queensland, Australia

Mangrove forests and saltmarshes are important habitats for carbon (C) sequestration in the coastal zone but variation in rates of C sequestration and the factors controlling sequestration are poorly understood. We assessed C sequestration in Moreton Bay, South East Queensland in mangrove forests and tidal marshes that span a range of environmental settings and plant communities, including mangrove forests and tidal marshes on the oligotrophic sand islands of the eastern side of Moreton Bay and on the nutrient enriched, western side of the bay adjacent to the city of Brisbane. We found that rates of C sequestration in sediments were similar among mangrove forests over the bay, despite large differences in the C density of sediments, because of different rates of vertical accretion of sediments. The C sequestration on the oligotrophic sand island tidal marshes, dominated by Juncus kraussii, had the highest rate of C sequestration in the bay while the western saltmarshes, which were dominated by Sarcocornia quinqueflora, had the lowest rate of C sequestration. Our data indicate C sequestration varies among different tidal wetland plant community types, due to variation in sediment characteristics and rates of sediment accretion over time.

Variation in wood density and anatomy in a widespread mangrove species

Wood density is an important plant trait that influences a range of ecological processes, including resistance to damage and growth rates. Wood density is highly dependent on anatomical characteristics associated with the conductive tissue of trees (xylem and phloem) and the fibre matrix in which they occur. Here, we investigated variation in the wood density of the widespread mangrove species Avicennia marina in the Exmouth Gulf in Western Australia and in the Firth of Thames in New Zealand. We assessed how variation in xylem vessel size, fibre wall thickness and proportion of phloem within the wood contributed to variation in wood density and how these characteristics were linked to growth rates. We found the wood density of A. marina to be higher in Western Australia than in New Zealand and to be higher in taller seaward fringing trees than in scrub trees growing high in the intertidal. At the cellular level, high wood density was associated with large xylem vessels and thick fibre walls. Additionally, wood density increased with decreasing proportions of phloem per growth layer of wood. Tree growth rates were positively correlated with xylem vessel size and wood density. We conclude that A. marina can have large xylem vessel sizes and high growth rates while still maintaining high wood density because of the abundance and thickness of fibres in which vessels are found.

The Pleistocene glacial cycles shaped the historical demography and phylogeography of a pine fungal endophyte

The fungal endophyte Lophodermium nitens is an obligate symbiont of soft pines inhabiting only two pine species in Mexico with a broad distribution of geographically isolated populations. A previous study for the hosts indicated a main east–west subdivision with recurrent gene flow within these regions and demographic expansion of populations. We took these patterns as null hypotheses to test for the demography and phylogeographical patterns of the fungus, given the obligatory relationship of the endophyte to the host and its reduced capacity for long-distance dispersal. For this purpose, we employed two nuclear DNA loci, fragments of the actin and chitin synthase I genes. Both loci showed high genetic variation, consisting of private single-copy alleles, as well as few ones at high frequency that were shared among almost all populations. In order to distinguish between shared polymorphism due to incomplete lineage sorting and gene flow posterior to population divergence, we applied the coalescent-based Isolation–Migration (IM) model. We found patterns of gene flow and isolation similar to those of the hosts as well as signs of population expansion. Mean migration time and divergence time estimates fell within the Pleistocene, previous to Last Glacial Maximum. The results presented here for L. nitens emphasize the potential use of endophytic fungi to deepen the knowledge of historical patterns and processes of their host plants.

Differences in osmotic adjustment, foliar abscisic acid dynamics, and stomatal regulation between an isohydric and anisohydric woody angiosperm during drought

Species are often classified along a continuum from isohydric to anisohydric, with isohydric species exhibiting tighter regulation of leaf water potential through stomatal closure in response to drought. We investigated plasticity in stomatal regulation in an isohydric (Eucalyptus camaldulensis) and an anisohydric (Acacia aptaneura) angiosperm species subject to repeated drying cycles. We also assessed foliar abscisic acid (ABA) content dynamics, aboveground/belowground biomass allocation and nonstructural carbohydrates. The anisohydric species exhibited large plasticity in the turgor loss point (ΨTLP ), with plants subject to repeated drying exhibiting lower ΨTLP and correspondingly larger stomatal conductance at low water potential, compared to plants not previously exposed to drought. The anisohydric species exhibited a switch from ABA to water potential-driven stomatal closure during drought, a response previously only reported for anisohydric gymnosperms. The isohydric species showed little osmotic adjustment, with no evidence of switching to water potential-driven stomatal closure, but did exhibit increased root:shoot ratios. There were no differences in carbohydrate depletion between species. We conclude that a large range in ΨTLP and biphasic ABA dynamics are indicative of anisohydric species, and these traits are associated with exposure to low minimum foliar water potential, dense sapwood and large resistance to xylem embolism.

The anatomical basis of the link between density and mechanical strength in mangrove branches

Tree branches are important as they support the canopy, which controls photosynthetic carbon gain and determines ecological interactions such as competition with neighbours. Mangrove trees are subject to high wind speeds, strong tidal flows and waves that can damage their branches. The survival and establishment of mangroves partly depend on the structural and mechanical characteristics of their branches. In addition, mangroves are exposed to soils that vary in salinity. Highly saline conditions can increase the tension in the water column, imposing mechanical stresses on the xylem vessels. Here, we investigated how mechanical strength, assessed as the modulus of elasticity (MOE) and the modulus of rupture (MOR), and density relate to the anatomical characteristics of intact mangrove branches from southeast Queensland and whether the mechanical strength of branches varies among mangrove species. Mechanical strength was positively correlated with density of mangrove intact branches. Mechanical strength (MOE) varied among species, with Avicennia marina (Forssk.) Vierh. branches having the highest mechanical strength (2079±176MPa), and Rhizophora stylosa Griff. and Bruguiera gymnorrhiza (L.) Savigny ex Lam. and Poiret having the lowest mechanical strength (536.8±39.2MPa in R. stylosa and 554±58.2MPa in B. gymnorrhiza). High levels of mechanical strength were associated with reductions in xylem vessel lumen area, pith content and bark content, and positively associated with increases in fibre wall thickness. The associations between mechanical strength and anatomical characteristics in mangrove branches suggest trade-offs between mechanical strength and water supply, which are linked to tree growth and survival.

Radiocarbon Dating and Wood Density Chronologies of Mangrove Trees in Arid Western Australia

Mangrove trees tend to be larger and mangrove communities more diverse in tropical latitudes, particularly where there is high rainfall. Variation in the structure, growth and productivity of mangrove forests over climatic gradients suggests they are sensitive to variations in climate, but evidence of changes in the structure and growth of mangrove trees in response to climatic variation is scarce. Bomb-pulse radiocarbon dating provides accurate dates of recent wood formation and tree age of tropical and subtropical tree species. Here, we used radiocarbon techniques combined with X-ray densitometry to develop a wood density chronology for the mangrove Avicennia marina in the Exmouth Gulf, Western Australia (WA). We tested whether wood density chronologies of A. marina were sensitive to variation in the Pacific Decadal Oscillation Index, which reflects temperature fluctuations in the Pacific Ocean and is linked to the instrumental rainfall record in north WA. We also determined growth rates in mangrove trees from the Exmouth Gulf, WA. We found that seaward fringing A. marina trees (∼10 cm diameter) were 48±1 to 89±23 years old (mean ± 1σ) and that their growth rates ranged from 4.08±2.36 to 5.30±3.33 mm/yr (mean ±1σ). The wood density of our studied mangrove trees decreased with increases in the Pacific Decadal Oscillation Index. Future predicted drying of the region will likely lead to further reductions in wood density and their associated growth rates in mangrove forests in the region.

Synthetic microbe communities provide internal reference standards for metagenome sequencing and analysis

The complexity of microbial communities, combined with technical biases in next-generation sequencing, pose a challenge to metagenomic analysis. Here, we develop a set of internal DNA standards, termed “sequins” (sequencing spike-ins), that together constitute a synthetic community of artificial microbial genomes. Sequins are added to environmental DNA samples prior to library preparation, and undergo concurrent sequencing with the accompanying sample. We validate the performance of sequins by comparison to mock microbial communities, and demonstrate their use in the analysis of real metagenome samples. We show how sequins can be used to measure fold change differences in the size and structure of accompanying microbial communities, and perform quantitative normalization between samples. We further illustrate how sequins can be used to benchmark and optimize new methods, including nanopore long-read sequencing technology. We provide metagenome sequins, along with associated data sets, protocols, and an accompanying software toolkit, as reference standards to aid in metagenomic studies.Complex microbial communities pose a challenge to metagenomic analysis. Here the authors develop ‘sequins’, internal DNA standards that represent a synthetic community of artificial genomes.