Similar but different: Characterization of dddD gene-mediated DMSP metabolism among coral-associated Endozoicomonas

Horizontal transmission of symbiotic bacteria and host selective sweep in the giant clam Tridacna crocea

Giant clams, with their significant ecological importance, depend on associated bacteria for their health and development, yet the transmission modes and succession of community dynamics of these bacteria remain poorly understood. This study employed 16S rRNA gene sequencing and microscopy to investigate the transmission and community dynamics of symbiotic bacteria in the giant clam Tridacna crocea during early developmental stages (fertilized eggs, blastocyst, D-larvae, and pediveliger larvae). Fluorescence in situ hybridization and transmission electron microscopy did not detect internal symbiotic bacteria in fertilized eggs and adult gonad gametes, but scanning electron microscopy revealed microbial structures on egg surface microvilli, suggesting their role as microbial carriers. 16S rRNA sequencing confirmed microbial presence in fertilized eggs, indicating bacterial acquisition via external vertical transmission (adherence to microvilli) or horizontal transmission. Given the lack of internalized bacteria in reproductive organs, we prefer to classify the symbiotic bacteria acquisition as horizontal transmission. Microbial community analysis showed that T. crocea acquired a significant portion of its microbiome from seawater throughout its development. Before reaching the pediveliger stage, the bacterial community composition closely resembled that of the surrounding seawater, primarily featuring the family Rhodobacteraceae. As T. crocea matured, the host's selective pressure increased (e.g. deterministic assembly), which simplified the microbial community and reduced diversity. During the pediveliger stage, the genus Endozoicomonas became dominant, forming a large proportion of the bacterial community within the gonads. This highlights the ecological significance of host-microbe interactions in maintaining biodiversity and driving ecosystem stability through dynamic community assembly processes.

Seasonal dynamics and environmental drivers of tissue and mucus microbiomes in the staghorn coral Acropora pulchra

Background

Rainfall-induced coastal runoff represents an important environmental impact in near-shore coral reefs that may affect coral-associated bacterial microbiomes. Shifts in microbiome community composition and function can stress corals and ultimately cause mortality and reef declines. Impacts of environmental stress may be site specific and differ between coral microbiome compartments (e.g., tissue versus mucus). Coastal runoff and associated water pollution represent a major stressor for near-shore reef-ecosystems in Guam, Micronesia.

Methods

Acropora pulchra colonies growing on the West Hagåtña reef flat in Guam were sampled over a period of 8 months spanning the 2021 wet and dry seasons. To examine bacterial microbiome diversity and composition, samples of A. pulchra tissue and mucus were collected during late April, early July, late September, and at the end of December. Samples were collected from populations in two different habitat zones, near the reef crest (farshore) and close to shore (nearshore). Seawater samples were collected during the same time period to evaluate microbiome dynamics of the waters surrounding coral colonies. Tissue, mucus, and seawater microbiomes were characterized using 16S DNA metabarcoding in conjunction with Illumina sequencing. In addition, water samples were collected to determine fecal indicator bacteria (FIB) concentrations as an indicator of water pollution. Water temperatures were recorded using data loggers and precipitation data obtained from a nearby rain gauge. The correlation structure of environmental parameters (temperature and rainfall), FIB concentrations, and A. pulchra microbiome diversity was evaluated using a structural equation model. Beta diversity analyses were used to investigate spatio-temporal trends of microbiome composition.

Results

Acropora pulchra microbiome diversity differed between tissues and mucus, with mucus microbiome diversity being similar to the surrounding seawater. Rainfall and associated fluctuations of FIB concentrations were correlated with changes in tissue and mucus microbiomes, indicating their role as drivers of A. pulchra microbiome diversity. A. pulchra tissue microbiome composition remained relatively stable throughout dry and wet seasons; tissues were dominated by Endozoicomonadaceae, coral endosymbionts and putative indicators of coral health. In nearshore A. pulchra tissue microbiomes, Simkaniaceae, putative obligate coral endosymbionts, were more abundant than in A. pulchra colonies growing near the reef crest (farshore). A. pulchra mucus microbiomes were more diverse during the wet season than the dry season, a distinction that was also associated with drastic shifts in microbiome composition. This study highlights the seasonal dynamics of coral microbiomes and demonstrates that microbiome diversity and composition may differ between coral tissues and the surface mucus layer.

Cytology in cnidaria using Exaiptasia as a model

A need exists for additional methods to examine cnidaria at the cellular level to aid our understanding of health, anatomy, and physiology of this important group of organisms. This need is particularly acute given that disease is emerging as a major factor in declines of ecologically important functional groups such as corals. Here we describe a simple method to process cnidarian cells for microscopic examination using the model organism Exaiptasia. We show that this organism has at least 18 cell types or structures that can be readily distinguished based on defined morphological features. Some of these cells can be related back to anatomic features of the animal both at the light microscope and ultrastructural level. The cnidome of Exaiptasia may be more complex than what is currently understood. Moreover, cnidarian cells, including some types of cnidocytes, phagocytize cells other than endosymbionts. Finally, our findings shed light on morphologic complexity of cell-associated microbial aggregates and their intimate intracellular associations. The tools described here could be useful for other cnidaria.