Biology subjects

Hawco, N. J.

Publications and source records attributed to Hawco, N. J..

3 recordsLinked to original sources

bioRxiv2022

Seasonal siderophore uptake and biosynthesis associated with carbon flux at Station ALOHA

The North Pacific subtropical gyre is a globally important contributor to carbon uptake and an oligotrophic ecosystem primarily limited by nitrogen. The microbial community is also seasonally exposed to low iron due to biological consumption and seasonally variable iron delivery. In this study, we examined changes in iron uptake rates, dissolved siderophore concentrations, and siderophore biosynthesis at Station ALOHA across time (2013-2016) and depth (surface to 500 m) to observe changes in iron acquisition and internal cycling by the microbial community. The genetic potential for siderophore biosynthesis was widespread throughout the upper water column, and biosynthetic gene clusters peaked in spring and summer along with siderophore concentrations, suggesting changes in nutrient delivery, primary production, and carbon export impact iron acquisition over the seasonal cycle. Dissolved iron turnover times, calculated from iron-amended experiments conducted using surface (15 m) and mesopelagic (300 m) waters, ranged from 9-252 days. The shortest average turnover times at both depths were associated with inorganic iron additions (14{+/-}9 days) and the longest with iron bound to strong siderophores (148{+/-}225 days). Uptake rates of siderophore-bound iron were faster in the mesopelagic waters than in the surface, leading to high Fe:C uptake ratios of heterotrophic bacteria in the upper mesopelagic. The rapid cycling and high demand for Fe at 300 m suggests differences in microbial metabolism and iron acquisition in the mesopelagic compared to surface waters. Together, changes in siderophore production and consumption over the seasonal cycle suggest organic carbon availability impacts iron cycling at Station ALOHA. Scientific Significance StatementMicrobial community production in the subtropical oligotrophic North Pacific is limited by macronutrients such as nitrogen. However, dissolved iron is another important micronutrient that has seasonal inputs from dust and passing eddies, keeping the availability of iron low and episodic. Little attention has been paid to the microbial strategies for dealing with low iron to support primary production in the oligotrophic ocean, or how limited iron availability impacts the processing of sinking particulate organic carbon in this region. In this study, we explore iron cycling including siderophore production and uptake by the microbial community throughout the water column at Station ALOHA to examine how the microbial community adapts and responds to changing iron and carbon availability on seasonal timescales.

bioinformatics↗

bioRxiv2022

Siderophore production and utilization by microbes in the North Pacific Ocean

Siderophores are strong iron-binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron-binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron-replete, low-macronutrient North Pacific Subtropical Gyre (NPSG) through the North Pacific Transition Zone (NPTZ) to the iron-deplete, high-macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0 - 13.9 pM) than the SAFZ (1.2 - 5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust-derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins and iron-free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Additionally, the widespread genetic potential for siderophore biosynthesis and uptake across latitude and the variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin and salmochelin transporter genes at different latitudes suggest that particular microbes actively produce and use siderophores, altering siderophore distributions and the bioavailability of iron across the North Pacific.

ecology↗

bioRxiv2021

Nitrogen fixation in mesoscale eddies of the North Pacific Subtropical Gyre: patterns and mechanisms

1.Mesoscale eddies have been shown to support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), but the mechanisms underlying these observations are not well understood. We explored relationships among NFRs and cyanobacterial diazotroph abundances in eddy pairs of opposite polarity sampled in the North Pacific Subtropical Gyre and compared our observations to seasonal trends from the Hawaii Ocean Time-series (HOT) program. Consistent with previous reports, we found that NFRs were anomalously high for this region (up to 3.7-fold above previous monthly HOT observations) in the centers of the sampled anticyclones, coinciding with elevated abundances of Crocosphaera in the summertime. We then coupled our field-based observations, together with transcriptomic analyses of nutrient stress marker genes and ecological models, to evaluate potential mechanisms controlling diazotroph abundance and activity at the mesoscale. Specifically, we evaluated the role of biological (via estimates of growth and grazing rates) and physical controls on populations of Crocosphaera, Trichodesmium, and diatom symbionts. Our results suggest that increased Crocosphaera abundances associated with summertime anticyclones resulted from the alleviation of phosphate limitation, allowing cells to grow at rates exceeding grazing losses. In contrast, distributions of larger, buoyant taxa (Trichodesmium and diatom symbionts) appeared less affected by eddy-driven biological controls. Instead, they appeared driven by physical dynamics along frontal boundaries that separate cyclonic and anticyclonic eddies. Together, the interplay of eddy-specific changes in bottom-up control, top-down control, and the physical accumulation of cells likely explains the elevated diazotroph abundances and NFRs associated with anticyclones and eddy fronts.

ecology↗