Toxins or medicines? Phytoplankton diets mediate host and parasite fitness in a freshwater system.

Diets must satisfy the everyday metabolic requirements of organisms and can also serve as medicines to combat disease. Currently, the medicinal role of diets is much better understood in terrestrial than in aquatic ecosystems. This is surprising because phytoplankton species synthesize secondary metabolites with known antimicrobial properties. Here, we investigated the medicinal properties of phytoplankton (including toxin-producing cyanobacteria) against parasites of the dominant freshwater herbivore, Daphnia. We fed Daphnia dentifera on green algae and toxic cyanobacteria diets known to vary in their nutritional quality and toxin production, and an additional diet of Microcystis with added pure microcystin-LR. We then exposed Daphnia to fungal and bacterial parasites. Anabaena, Microcystis and Chlorella diets prevented infection of Daphnia by the fungal parasite Metschnikowia, while Nodularia toxins increased offspring production by infected hosts. In contrast to their medicinal effects against Metschnikowia, toxic phytoplankton generally decreased the fitness of Daphnia infected with the bacterial parasite, Pasteuria. We also measured the amount of toxin produced by phytoplankton over time. Concentrations of anatoxin-a produced by Anabaena increased in the presence of Metschnikowia, suggesting parasite-induced toxin production. Our research illustrates that phytoplankton can serve as toxins or medicines for their consumers, depending upon the identity of their parasites.

A meta-analysis of the stony coral tissue loss disease microbiome finds key bacteria in unaffected and lesion tissue in diseased colonies.

Stony coral tissue loss disease (SCTLD) has been causing significant whole colony mortality on reefs in Florida and the Caribbean. The cause of SCTLD remains unknown, with the limited concurrence of SCTLD-associated bacteria among studies. We conducted a meta-analysis of 16S ribosomal RNA gene datasets generated by 16 field and laboratory SCTLD studies to find consistent bacteria associated with SCTLD across disease zones (vulnerable, endemic, and epidemic), coral species, coral compartments (mucus, tissue, and skeleton), and colony health states (apparently healthy colony tissue (AH), and unaffected (DU) and lesion (DL) tissue from diseased colonies). We also evaluated bacteria in seawater and sediment, which may be sources of SCTLD transmission. Although AH colonies in endemic and epidemic zones harbor bacteria associated with SCTLD lesions, and aquaria and field samples had distinct microbial compositions, there were still clear differences in the microbial composition among AH, DU, and DL in the combined dataset. Alpha-diversity between AH and DL was not different; however, DU showed increased alpha-diversity compared to AH, indicating that, prior to lesion formation, corals may undergo a disturbance to the microbiome. This disturbance may be driven by Flavobacteriales, which were especially enriched in DU. In DL, Rhodobacterales and Peptostreptococcales-Tissierellales were prominent in structuring microbial interactions. We also predict an enrichment of an alpha-toxin in DL samples which is typically found in Clostridia. We provide a consensus of SCTLD-associated bacteria prior to and during lesion formation and identify how these taxa vary across studies, coral species, coral compartments, seawater, and sediment.

Benthic exometabolites and their ecological significance on threatened Caribbean coral reefs.

Benthic organisms are the architectural framework supporting coral reef ecosystems, but their community composition has recently shifted on many reefs. Little is known about the metabolites released from these benthic organisms and how compositional shifts may influence other reef life, including prolific microorganisms. To investigate the metabolite composition of benthic exudates and their ecological significance for reef microbial communities, we harvested exudates from six species of Caribbean benthic organisms including stony corals, octocorals, and an invasive encrusting alga, and subjected these exudates to untargeted and targeted metabolomics approaches using liquid chromatography-mass spectrometry. Incubations with reef seawater microorganisms were conducted to monitor changes in microbial abundances and community composition using 16 S rRNA gene sequencing in relation to exudate source and three specific metabolites. Exudates were enriched in amino acids, nucleosides, vitamins, and indole-based metabolites, showing that benthic organisms contribute labile organic matter to reefs. Furthermore, exudate compositions were species-specific, and riboflavin and pantothenic acid emerged as significant coral-produced metabolites, while caffeine emerged as a significant invasive algal-produced metabolite. Microbial abundances and individual microbial taxa responded differently to exudates from stony corals and octocorals, demonstrating that exudate mixtures released from different coral species select for specific bacteria. In contrast, microbial communities did not respond to individual additions of riboflavin, pantothenic acid, or caffeine. This work indicates that recent shifts in benthic organisms alter exudate composition and likely impact microbial communities on coral reefs.

Experimental transmission of Stony Coral Tissue Loss Disease results in differential microbial responses within coral mucus and tissue.

Stony coral tissue loss disease (SCTLD) is a widespread and deadly disease that affects nearly half of Caribbean coral species. To understand the microbial community response to this disease, we performed a disease transmission experiment on US Virgin Island (USVI) corals, exposing six species of coral with varying susceptibility to SCTLD. The microbial community of the surface mucus and tissue layers were examined separately using a small subunit ribosomal RNA gene-based sequencing approach, and data were analyzed to identify microbial community shifts following disease acquisition, potential causative pathogens, as well as compare microbiota composition to field-based corals from the USVI and Florida outbreaks. While all species displayed similar microbiome composition with disease acquisition, microbiome similarity patterns differed by both species and mucus or tissue microhabitat. Further, disease exposed but not lesioned corals harbored a mucus microbial community similar to those showing disease signs, suggesting that mucus may serve as an early warning detection for the onset of SCTLD. Like other SCTLD studies in Florida, Rhodobacteraceae, Arcobacteraceae, Desulfovibrionaceae, Peptostreptococcaceae, Fusibacter, Marinifilaceae, and Vibrionaceae dominated diseased corals. This study demonstrates the differential response of the mucus and tissue microorganisms to SCTLD and suggests that mucus microorganisms may be diagnostic for early disease exposure.