Biology subjects

Farnelid, H.

Publications and source records attributed to Farnelid, H..

3 recordsLinked to original sources

bioRxiv2025

Picophytoplankton Implicated in Productivity and Biogeochemistry in the North Pacific Transition Zone

Marine phytoplankton are central to global seascapes, acting as key conduits in element cycling and oceanic food webs. Phytoplankton cell size spans several orders of magnitude (0.2 to >200 {micro}m) and is an important trait that governs metabolism. Yet, the vast taxonomic diversity within phytoplankton size classes makes it challenging to link specific taxa to bulk community changes in productivity and elemental stoichiometry. To explore phytoplankton biogeography and biogeochemical roles in field populations, we analyzed three years of 16S and 18S rRNA gene amplicon sequencing variant (ASV) data alongside biochemical measurements across the dynamic latitudinal gradient of the North Pacific Transition Zone. We (1) identified picophytoplankton community members associated with patterns in net community production (NCP), particulate organic carbon (POC), and particulate organic nitrogen (PON), and (2) uncovered co-occurring species that may influence their growth and abundance. Multivariate linear mixed modeling revealed that occurrence of chlorophytes explained 22.6% of NCP values, followed by stramenopiles and cyanobacteria. In contrast, POC and PON spatial patterns were best explained by chlorophyte and dinoflagellate spatial patterns. Weighted co-expression network analysis further showed NCP, POC, and PON correlations with a subset of [~]40 ASVs belonging to chlorophytes, cyanobacteria, stramenopiles, haptophytes, and dinoflagellates that range in trophic strategy. Association network inference recapitulated these findings and revealed additional co-occurring phytoplankton, grazers, and heterotrophic bacteria. Together, our integrated computational analyses identified key picophytoplankton and co-occurring mixotrophs as major contributors to shaping regional biogeochemical dynamics in the North Pacific Ocean. ImportancePhytoplankton mediate key biogeochemical processes in dynamic oceanic transition zones. Yet, their vast cell size range and taxonomic diversity makes it challenging to link specific taxa to bulk community changes in productivity and elemental stoichiometry. By integrating molecular and biogeochemical measurements from the North Pacific Transition Zone using combined network and multivariate modeling, we identified specific picophytoplankton strongly linked to community production and organic nutrients levels. These picophytoplankton included specific members of cyanobacteria, pelagophytes, haptophytes, and chlorophytes, and formed tight associations with several nano- and pico-sized protistan mixotrophs highlighting how top-down interactions and microbial consortia shape community structure and elemental fluxes. Our work establishes key microbial players that may control fundamental ecosystem processes like carbon and nitrogen cycling and offers a computational framework to track and identify "microbial neighborhoods" that underpin biogeochemical features of an ecosystem.

microbiology↗

bioRxiv2025

From temperate to polar waters: Transition to non-cyanobacterial diazotrophy upon entering the Atlantic gateway of the Arctic Ocean

Nitrogen fixation, the microbial reduction of dinitrogen to ammonia, is increasingly recognized to occur in the Arctic Ocean. However, knowledge about the composition, biogeography, abundance, and ecology of nitrogen-fixing organisms (diazotrophs) is poor. This ultimately hinders prediction of ecosystem productivity fueled by nitrogen fixation in this rapidly changing and predominantly nitrogen-limited ocean. We assessed the composition and abundance of total and active diazotrophs in sub-surface water (8 m; amplicon sequencing and quantification of the marker gene nifH) over [~]3,400 km from the mouth of the brackish Baltic Sea to the sea ice edge in the Arctic Ocean. Upon entering nutrient-rich waters in the Atlantic gateway to the Arctic, we discovered an abrupt transition from autotrophic to heterotrophic diazotrophic activity. Our findings suggest that diazotrophy is functionally distinct in the Arctic Ocean compared to adjacent temperate-boreal waters - a difference likely driven by inorganic nutrients, salinity, and temperature. We identify three key non-cyanobacterial diazotroph groups in the Arctic Ocean with Arctic-specific (Rhodocyclales and Oceanospirillales) or more widespread (unknown Gammaproteobacterium) distribution patterns and report their nifH gene transcription levels (up to 103 nifH transcripts L-1). In contrast, actively transcribing diazotrophs in the warmer and more nutrient-poor Norwegian Sea with coastal-influenced water were dominated by sublineages of Candidatus Atelocyanobacterium thalassa (UCYN-A1, UCYN-A2, UCYN-A4; up to 104 nifH transcripts L-1). With ongoing atlantification of the Arctic pushing oceanic provinces and biogeographical ranges poleward, we predict a future displacement of the transition from autotrophic to heterotrophic diazotrophic activity with likely significant changes in nitrogen fixation.

ecology↗

bioRxiv2018

A transcriptional cycle suited to daytime N2 fixation in the unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A)

The symbiosis between a marine alga and a N2-fixing cyanobacterium (UCYN-A) is geographically widespread in the oceans and is important in the marine N cycle. UCYN-A is uncultivated, and is an unusual unicellular cyanobacterium because it lacks many metabolic functions, including oxygenic photosynthesis and carbon fixation, which are typical in cyanobacteria. It is now presumed to be an obligate symbiont of haptophytes closely related to Braarudosphaera bigelowii. N2-fixing cyanobacteria use different strategies to avoid inhibition of N2 fixation by the oxygen evolved in photosynthesis. Most unicellular cyanobacteria temporally separate the two incompatible activities by fixing N2 only at night, but surprisingly UCYN-A appears to fix N2 during the day. The goal of this study was to determine how the unicellular UCYN-A coordinates N2 fixation and general metabolism compared to other marine cyanobacteria. We found that UCYN-A has distinct daily cycles of many genes despite the fact that it lacks two of the three circadian clock genes found in most cyanobacteria. We also found that transcription patterns in UCYN-A are most similar to marine cyanobacteria that are capable of aerobic N2 fixation in the light such as Trichodesmium and heterocyst-forming cyanobacteria, rather than Crocosphaera or Cyanothece species, which are more closely related to unicellular marine cyanobacteria evolutionarily. Our findings suggest that the symbiotic interaction has resulted in a shift of transcriptional regulation to coordinate UCYN-A metabolism with the phototrophic eukaryotic host, thus allowing efficient coupling of N2 fixation (by the cyanobacterium) to the energy obtained from photosynthesis (by the eukaryotic unicellular alga) in the light.\n\nImportanceThe symbiotic N2-fixing cyanobacterium UCYN-A and its eukaryotic algal host, which is closely related to Braarudosphaera bigelowii, have been shown to be globally distributed and important in open ocean N2 fixation. These unique cyanobacteria have reduced metabolic capabilities, even lacking genes for oxygenic photosynthesis and carbon fixation. Cyanobacteria generally use energy from photosynthesis for nitrogen fixation, but require mechanisms for avoiding inactivation of the oxygen-sensitive nitrogenase enzyme by ambient oxygen (O2) or the O2 evolved through photosynthesis. This study shows that the symbiosis between the N2-fixing cyanobacterium UCYN-A and its eukaryotic algal host has led to adaptation of its daily gene expression pattern in order to enable daytime aerobic N2 fixation, which is likely more energetically efficient than fixing N2 at night, as in other unicellular marine cyanobacteria.

ecology↗