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Phytoplankton bloom events result in distinct changes in the composition and availability of nutrients as well as physical conditions within aquatic ecosystems, resulting in significant effects on bacterial communities. Using a metabarcoding approach, this study investigated the effect of a harmful algal bloom (HAB) of Heterosigma akashiwo in the Sundays Estuary, South Africa, on bacterial community structures in this estuarine ecosystem over an entire bloom event. The occurrence of bacterial lineages occurring in both the oxygen-rich surface water and hypoxic bottom waters in this study reflects the respiratory flexibility and potential for both aerobic and anaerobic metabolism across a wide range of bacterial phylogenetic lineages. A close correlation between the bacterial community profiles and the physiological state of the bloom was observed, with Flavobacteria found in increased relative abundances during the H. akashiwo HAB. Flavobacteria are commonly reported in the literature associated with algal blooms, which is indicative of their central role in the degradation of algal-derived organic matter. Halieaceae were prevalent during the bloom, whilst Synechococcales, Cryomorphaceae and Sporichthyaceae were found to be positively correlated with the decay of the H. akashiwo bloom. Rhodobacteraceae correlated significantly with the H. akashiwo bloom; however, unlike the Rhodobacteraceae specific bloom-associated genera reported in literature (predominantly Roseobacter), up to 74% of the Rhodobacteraceae sequence reads in this study were assigned to the genus Litorimicrobium. The distinct bacterial community profiles recorded throughout the H. akashiwo HAB can be attributed to the influence of the bloom-forming species as well as under-representation of estuarine-occurring HABs in the literature.

Coastal areas host some of the planet’s most productive ecosystems, providing life-sustaining ecological services and several benefits to humankind, while also being some of the most threatened areas (e.g., by globalization, climate change, and biological invasion). Salt marshes are coastal habitats with a key role in food and shelter provisioning, sediment deposition, nutrient cycling and carbon storage. Spartina spp. is a genus of grass halophytes which occurs in salt marshes worldwide, and includes species with different invasive potential. We evaluated the effect of climate change in the distribution and invasion potential of five Spartina species (S. anglica, S. alterniflora, S. densiflora, S. patens, and S. maritima) at a global scale. Species distribution models (SDMs) were applied on species occurrence data and atmospheric environmental predictors (WorldClim 2.1) to project potential changes in habitat suitability and associated changes in distribution and species co-occurrence until the end of the century, across four Shared Socioeconomic Pathway scenarios (i.e., SSP1-2.6 to SSP5-8.5). Projections showed a global trend for increasing species co-occurrence, with a general range expansion potentiated by increasing pathway severity. This study suggests that Spartina species can potentially benefit from climate change, predicting poleward expansions in the Northern Hemisphere for most species, with results pointing at increased conflict and invasion potential in Northern Europe and East Asian shorelines, already under strong invasive pressure. S. anglica is projected to remain a successful invader, with more severe scenarios likely favoring greater expansions. S. alterniflora exhibits very low expansion comparatively, despite exhibiting the same northward distribution shift. SSP1-2.6 produced the smallest change to species co-occurrence, suggesting a smaller potential for invasion-related conflicts, although still registering a potential net expansion for the Genus. Despite their limitations, SDMs can help establish general trends in climate change ecology and inform policymakers and environmental agents to ensure the correct management of these habitats and, ultimately, ecosystems.