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Multiomics approaches need to be applied in the central Arctic Ocean to benchmark biodiversity change and to identify novel species and their genes. As part of MOSAiC, EcoOmics will therefore be essential for conservation and sustainable bioprospecting in one of the least explored ecosystems on Earth.

Terrestrial fungi play critical roles in nutrient cycling and food webs and can shape macroorganism communities as parasites and mutualists. Although estimates for the number of fungal species on the planet range from 1.5 to over 5 million, likely fewer than 10% of fungi have been identified so far. To date, a relatively small percentage of described species are associated with marine environments, with ∼1,100 species retrieved exclusively from the marine environment. Nevertheless, fungi have been found in nearly every marine habitat explored, from the surface of the ocean to kilometers below ocean sediments. Fungi are hypothesized to contribute to phytoplankton population cycles and the biological carbon pump and are active in the chemistry of marine sediments. Many fungi have been identified as commensals or pathogens of marine animals (e.g., corals and sponges), plants, and algae. Despite their varied roles, remarkably little is known about the diversity of this major branch of eukaryotic life in marine ecosystems or their ecological functions. This perspective emerges from a Marine Fungi Workshop held in May 2018 at the Marine Biological Laboratory in Woods Hole, MA. We present the state of knowledge as well as the multitude of open questions regarding the diversity and function of fungi in the marine biosphere and geochemical cycles.

Polymorphic yeasts can switch between unicellular division and multicellular filamentous growth. Although prevalent in aquatic ecosystems, such as the open ocean, we have a limited understanding of the controlling factors on their morphological variation in an aquatic ecology context. Here we show that substrate concentration regulates cell morphogenesis in a cosmopolitan polymorphic yeast, Aureobasidium pullulans, isolated from the pelagic open ocean and analyzed in liquid batch culture. Filamentous cell development was triggered only under high initial substrate conditions, suggesting that hyphal growth could be more advantageous under eutrophic conditions and may influence pelagic fungal interactions with particulate organic matter. Filamentous growth proportionally declined before the exhaustion of substrate and before budding yeast-type cell division entered stationary phase, possibly modulated by quorum sensing as previously evidenced in other polymorphic yeasts. We also found that budding yeast-type unicells decreased in size and became more elongated in shape in response to substrate depletion, resulting in higher cell surface area to volume ratios, which could affect yeast dispersal and/or provide a nutrient uptake advantage under oligotrophic conditions. Our results demonstrate resource-responsive morphological plasticity in a marine-derived polymorphic yeast, providing mechanistic insight into the ability of fungi to survive fluctuating environmental conditions such as in the open ocean. [ABSTRACT FROM AUTHOR]

Sinking particles transport carbon from the surface to the deep ocean. Microbial colonization and remineralization are important ecosystem services constraining ocean biogeochemistry by recycling and redistributing nutrients from the surface to the deep ocean. Fragmentation of particles by zooplankton and the resulting colonization by microorganisms before ingestion, known as 'microbial gardening', allows for trophic upgrading and increased microbial biomass for detritivorous zooplankton. Using model chitin particles incubated with seawater collected from the surface, mesopelagic and bathypelagic depths in the Northeast Atlantic Ocean, we determined particle-attaching bacterial communities to identify general and depth-specific candidates of particle colonization. Comparison of particle-attached communities at the amplicon sequence variant level showed that bacteria found on surface particles were also colonizers in the bathypelagic, in line with sinking particles promoting vertical connectivity. Bathypelagic particle-attached communities were most diverse. We propose that some particle colonizers attach to the surface and sink out with the particle, whilst other colonizers are depth-specific. This suggests that candidates for particle colonization differ with depth, which may be important when considering the implications for the delivery of ecosystem services, including carbon cycling and the role they play for zooplankton grazers. [ABSTRACT FROM AUTHOR]

Summary: Lichens are exemplar symbioses based upon carbon exchange between photobionts and their mycobiont hosts. Historically considered a two‐way relationship, some lichen symbioses have been shown to contain multiple photobiont partners; however, the way in which these photobiont communities react to environmental change is poorly understood.Lichina pygmaea is a marine cyanolichen that inhabits rocky seashores where it is submerged in seawater during every tidal cycle. Recent work has indicated that L. pygmaea has a complex photobiont community including the cyanobionts Rivularia and Pleurocapsa. We performed rRNA‐based metabarcoding and mRNA metatranscriptomics of the L. pygmaea holobiont at high and low tide to investigate community response to immersion in seawater.Carbon exchange in L. pygmaea is a dynamic process, influenced by both tidal cycle and the biology of the individual symbiotic components. The mycobiont and two cyanobiont partners exhibit distinct transcriptional responses to seawater hydration.Sugar‐based compatible solutes produced by Rivularia and Pleurocapsa in response to seawater are a potential source of carbon to the mycobiont. We propose that extracellular processing of photobiont‐derived polysaccharides is a fundamental step in carbon acquisition by L. pygmaea and is analogous to uptake of plant‐derived carbon in ectomycorrhizal symbioses. See also the Commentary on this article by Grube & Balestrini, 241: 1888–1890. [ABSTRACT FROM AUTHOR]

Summary: Diatoms are globally abundant microalgae that form extensive blooms in aquatic ecosystems. Certain bacteria behave antagonistically towards diatoms, killing or inhibiting their growth. Despite their crucial implications to diatom blooms and population health, knowledge of diatom antagonists in the environment is fundamentally lacking.We report systematic characterisation of the diversity and seasonal dynamics of bacterial antagonists of diatoms via plaque assay sampling in the Western English Channel (WEC), where diatoms frequently bloom. Unexpectedly, peaks in detection did not occur during characteristic spring diatom blooms, but coincided with a winter bloom of Coscinodiscus, suggesting that these bacteria likely influence distinct diatom host populations.We isolated multiple bacterial antagonists, spanning 4 classes and 10 bacterial orders. Notably, a diatom attaching Roseobacter Ponticoccus alexandrii was isolated multiple times, indicative of a persistent environmental presence. Moreover, many isolates had no prior reports of antagonistic activity towards diatoms. We verified diatom growth inhibitory effects of eight isolates. In all cases tested, these effects were activated by pre‐exposure to diatom organic matter. Discovery of widespread 'cryptic' antagonistic activity indicates that bacterial pathogenicity towards diatoms is more prevalent than previously recognised.Finally, examination of the global biogeography of WEC antagonists revealed co‐occurrence patterns with diatom host populations in marine waters globally. [ABSTRACT FROM AUTHOR]

Earlier studies had already shown that some fungi collected from the marine environment are the same or at least similar to non-marine taxa (Sparrow, [24]), and an updated definition of marine fungi was made to separate the obligate marine fungi (i.e., fungi that only grow and sporulate in the marine environment) from facultative marine fungi (i.e., fungi that are from the non-marine ecosystems that are still able to grow and sporulate in the marine environment) (Kohlmeyer & Kohlmeyer, [17]). This could account for why many marine fungi in short read metabarcode studies appear the same or similar to terrestrial/freshwater fungi. Many of the marine fungi identified with culture-independent DNA-based tools are not on lists of "obligate marine fungi" but are instead currently taxa more familiar to non-marine habitats and potentially fall into Kohlmeyer's category of "facultative marine fungi". Fungi are widespread throughout marine habitats. [Extracted from the article]