Microbial plankton communities play key roles in biogeochemical cycling and primary production in marine coastal ecosystems. Given the impacts of global change on Norwegian coastal waters, there is a need to understand the drivers of microbial plankton community structure and diversity. Environmental drivers such as temperature, salinity, and light influence the dynamics of microbial community structure and abundance in temperate oceans. Here we characterize the summer diversity of protist and prokaryotic plankton communities, using DNA metabarcoding and light microscopy, in the surface, upper mixed layer, and deep waters of Spindsfjorden, a South Norwegian fjord facing the North Sea. The sampling site was vertically stratified throughout the summer, with compositionally variable communities dominated by phototrophs and mixotrophs in the surface and upper mixed layers, and stable communities dominated by heterotrophs and parasites in the deep layer. Late summer blooms were dominated by the diatom Cerataulina pelagica in the surface and dinoflagellates of Tripos spp. in the upper mixed layer. Positive co-occurrences between certain diatom taxa and flavobacteria, and between diatoms and Alphaproteobacteria suggest potential symbiotic relationships or overlapping environmental preferences. Negative associations were observed between certain dinoflagellate groups and Syndiniales, possibly due to parasitic interactions. Temperature emerged as a key environmental driver of community change, underscoring its role in shaping the dynamics of microbial communities in temperate, coastal ecosystems.

Phytoplankton are essential components of marine food webs and biogeochemical cycles; thus, elucidating the factors that mediate their growth and composition are essential for understanding the marine environment. Vitamins have been found to be a fundamental requirement for the growth of many marine phytoplankton. Here, 20 incubation experiments were conducted over the course of 1 yr in a coastal ecosystem to examine the addition of thiamin (B1) and its components 4-methyl-5-thiazoleethanol (HET) and 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) on net phytoplankton growth rates. While vitamin addition periodically resulted in significant changes in growth rates, no uniform compound-specific or temporal patterns in rate changes were observed. Additionally, no significant changes in rates were found to correlate with starting phytoplankton abundance. Together, these findings suggest that the composition of the starting phytoplankton community likely influenced the population level response to vitamin addition and highlights the complexity of understanding the ramifications of vitamin fluxes in the marine environment.

Ammonia-oxidizing bacteria (AOB) are responsible for the first step of nitrification and are thereby important players in the global nitrogen cycle. Microorganisms in aquatic environments are frequently exposed to oxidative stress from metabolic byproducts and photochemical processes. Heterotrophic bacteria aid AOB under oxidative stress; however, the impacts of oxidative stress on the heterotrophic communities in co-culture with the autotrophic microorganisms have not been investigated in detail. In this study, we exposed 2 AOB enrichment cultures to oxidative stress via hydrogen peroxide (H2O2) and evaluated the effects of H2O2 on the activity of the AOB and the heterotrophic communities. Cultures received different frequencies of exposure to 10 or 100 µM H2O2 as well as time to recover after exposure to H2O2. Ammonia-oxidizing activity was unaffected by 10 µM H2O2; however, an increase in lag phase was observed in the presence of 100 µM H2O2. The heterotrophic community structure changed with repeated exposure to 100 µM H2O2. Microorganisms such as Xanthobacter sp. and Variovorax sp. increased in abundance in the presence of 100 µM H2O2, and Methyloversatilis sp. stayed stable under all conditions. It is likely that these microorganisms were resistant to the H2O2 stress and supported the AOB. Ammonia-oxidizing activity recovered from repeated exposure to H2O2 stress immediately after removal of the H2O2 stress. In contrast, the heterotrophic community needed repeated exposure to non-stress conditions to change back towards pre-stress conditions, indicating that different heterotrophic communities can provide the same support to the ammonia oxidizers.

Seagrass meadow ecosystems offer several valuable ecosystem services in coastal regions around the world. Recent studies have suggested that one such important service is reduction of pathogenic bacteria, specifically Vibrio spp., in adjacent waters. The specific mechanisms of pathogen reduction remain unclear, although increased sedimentation has been suggested as one likely process for pathogens to be quenched from the water column. Whether Vibrio spp. persist in the sediment or in other compartments of the seagrass meadow is currently unknown, but it has been shown that marine surface biofilms can function as reservoirs of pathogenic vibrios. This general feature may also apply to seagrass and sediment surfaces. In this study, we investigated the relative abundance and community ecology of Vibrio spp. bacteria in Baltic Sea seagrass meadows using both culturing and culture-independent methods. While we did not detect a significant reduction of Vibrio spp. in the water column above unvegetated sites as compared to seagrass meadows, we observed high relative abundances of Vibrio spp. on seagrass roots. This supports previous observations that marine surfaces are selectively colonized by Vibrio spp., implying that these habitats are important for the persistence and possibly release of Vibrio spp. into the water column. Our results emphasize the need to understand the interactions of pathogenic bacteria with coastal habitats, including interactions with host organisms such as seagrasses that provide biofilm microenvironments, in order to understand how diseases associated with these organisms develop.

Phytoplankton play crucial roles in aquatic ecosystems, serving as the foundation of marine food webs and being responsible for ~50% of the world’s oxygen production. Predation by microzooplankton and viral lysis are the major sources of phytoplankton mortality, with the balance between these 2 processes affecting microbial food webs and biogeochemical cycles. However, determining the dominant mortality process in time and space remains an open question. This study investigated microzooplankton grazing and viral lysis rates during a mesocosm experiment in western Norway. High-resolution measurements were determined on phytoplankton groups using flow cytometry to observe changes in mortality rates and carbon flow during phytoplankton blooms. Digital droplet PCR was employed to detect Emiliania huxleyi and Micromonas spp. and associated viruses within environmental samples, to explore its use for determining mortality processes and understanding the impact on the phytoplankton community. The results showed that grazing and viral lysis dominated mortality at different times, with only one significant instance of both processes being observed. Microzooplankton grazing primarily affected picoplankton, while nanoeukaryotes and E. huxleyi were more susceptible to viral lysis. Molecular detection did not always match with abundances or rates determined by flow cytometry; however, it did provide insights into their dynamics throughout the mesocosm. These findings provide insights into the complex interactions between microzooplankton, viruses and phytoplankton communities. Understanding the balance between microzooplankton grazing and viral lysis can contribute to a more comprehensive understanding of carbon flow in aquatic ecosystems, which has significant implications for food webs and biogeochemical cycles.

Seawater microorganisms impact ecological and biogeochemical cycling on coral reefs and are sensitive indicators of ecosystem status. Microbialization, a shift towards trophic collapse and resultant high microbial biomass, is a global concern on coral reefs. Indeed, macroorganisms can influence microbial processes and community composition on reefs, which is best understood as increased macroalgae resulting in copiotrophic microbial growth and oxygen reduction. Whether or not smaller-scale changes in macroorganisms influence the overlying seawater microbial communities is largely unknown. Here, we assessed seawater microorganisms across 3 coral reefs to understand their connection to reef site and within-reef benthic characteristics. At 3 coral reefs in St. John, US Virgin Islands, we collected 60 ml seawater samples 2 cm above the seafloor, spaced 2 m apart in a grid pattern, and assessed bacterial and archaeal communities via sequencing of small subunit ribosomal RNA genes. Benthic cover within 1 m of each sample was determined at 10 cm resolution through photogrammetry. Our results reveal that overall reef site overwhelmingly shapes microbial community structure, while within-reef benthic cover surrounding sample locations has minimal influence. However, ecospheres as areas that reflect the small-scale effects of benthic cover directly under each sample, significantly explain as much as 12.1% of within-reef microbial variation and may even outweigh variation attributable to reef site alone. These findings provide new insights into fine-scale spatial variability in reef seawater microbiomes that are crucial for the use of microorganisms as indicators of microbialization and coral reef health.