Björkman
Karin M.
Björkman
Karin M.
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ArticlePolyphosphate dynamics at Station ALOHA, North Pacific subtropical gyre(John Wiley & Sons, 2015-10-27) Diaz, Julia M. ; Björkman, Karin M. ; Haley, Sheean T. ; Ingall, Ellery ; Karl, David M. ; Longo, Amelia ; Dyhrman, Sonya T.Polyphosphate (polyP) was examined within the upper water column (≤ 150 m) of Station ALOHA (22° 45′N, 158° 00′W) during two cruises conducted in May–June 2013 and September 2013. Phosphorus molar ratios of particulate polyP to total particulate phosphorus (TPP) were relatively low, similar to previously reported values from the temperate western North Atlantic, and did not exhibit strong vertical gradients, reflecting a lack of polyP recycling relative to other forms of TPP with depth. Furthermore, relationships among polyP:TPP, soluble reactive phosphorus (SRP), and alkaline phosphatase activity (APA) were also consistent with previous observations from the Atlantic Ocean. To ascertain potential mechanisms of biological polyP production and utilization, surface seawater was incubated following nutrient additions. Results were consistent with polyP:TPP enrichment under opposite extremes of APA, suggesting diverse polyP accumulation/retention mechanisms. Addition of exogenous polyP (45 ± 5 P atoms) to field incubations did not increase chlorophyll content relative to controls, suggesting that polyP was not bioavailable to phytoplankton at Station ALOHA. To clarify this result, phytoplankton cultures were screened for the ability to utilize exogenous polyP. PolyP bioavailability was variable among model diatoms of the genus Thalassiosira, yet chain length did not influence polyP bioavailability. Thus, microbial community composition may influence polyP dynamics in the ocean, and vice versa.
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ArticleIron depletion in the deep chlorophyll maximum: mesoscale eddies as natural iron fertilization experiments(American Geophysical Union, 2021-11-17) Hawco, Nicholas J. ; Barone, Benedetto ; Church, Matthew J. ; Babcock-Adams, Lydia ; Repeta, Daniel J. ; Wear, Emma K. ; Foreman, Rhea K. ; Björkman, Karin M. ; Bent, Shavonna M. ; Van Mooy, Benjamin A. S. ; Sheyn, Uri ; DeLong, Edward F. ; Acker, Marianne ; Kelly, Rachel L. ; Nelson, Alexa ; Ranieri, John ; Clemente, Tara M. ; Karl, David M. ; John, Seth G.In stratified oligotrophic waters, phytoplankton communities forming the deep chlorophyll maximum (DCM) are isolated from atmospheric iron sources above and remineralized iron sources below. Reduced supply leads to a minimum in dissolved iron (dFe) near 100 m, but it is unclear if iron limits growth at the DCM. Here, we propose that natural iron addition events occur regularly with the passage of mesoscale eddies, which alter the supply of dFe and other nutrients relative to the availability of light, and can be used to test for iron limitation at the DCM. This framework is applied to two eddies sampled in the North Pacific Subtropical Gyre. Observations in an anticyclonic eddy center indicated downwelling of iron-rich surface waters, leading to increased dFe at the DCM but no increase in productivity. In contrast, uplift of isopycnals within a cyclonic eddy center increased supply of both nitrate and dFe to the DCM, and led to dominance of picoeukaryotic phytoplankton. Iron addition experiments did not increase productivity in either eddy, but significant enhancement of leucine incorporation in the light was observed in the cyclonic eddy, a potential indicator of iron stress among Prochlorococcus. Rapid cycling of siderophores and low dFe:nitrate uptake ratios also indicate that a portion of the microbial community was stressed by low iron. However, near-complete nitrate drawdown in this eddy, which represents an extreme case in nutrient supply compared to nearby Hawaii Ocean Time-series observations, suggests that recycling of dFe in oligotrophic ecosystems is sufficient to avoid iron limitation in the DCM under typical conditions.
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ArticleMicrobial group specific uptake kinetics of inorganic phosphate and adenosine-5′-triphosphate (ATP) in the North Pacific Subtropical Gyre(Frontiers Media, 2012-06-11) Björkman, Karin M. ; Duhamel, Solange ; Karl, David M.We investigated the concentration dependent uptake of inorganic phosphate (Pi) and adenosine-5′-triphosphate (ATP) in microbial populations in the North Pacific Subtropical Gyre (NPSG). We used radiotracers to measure substrate uptake into whole water communities, differentiated microbial size classes, and two flow sorted groups; Prochlorococcus (PRO) and non-pigmented bacteria (NPB). The Pi concentrations, uptake rates, and Pi pool turnover times (Tt) were (mean, ±SD); 54.9 ± 35.0 nmol L−1 (n = 22), 4.8 ± 1.9 nmol L−1 day−1 (n = 19), and 14.7 ± 10.2 days (n = 19), respectively. Pi uptake into >2 μm cells was on average 12 ± 7% (n = 15) of the total uptake. The kinetic response to Pi (10–500 nmol L−1) was small, indicating that the microorganisms were close to their maximum uptake velocity (Vmax). Vmax averaged 8.0 ± 3.6 nmol L−1 day−1 (n = 19) in the >0.2 μm group, with half saturation constants (Km) of 40 ± 28 nmol L−1 (n = 19). PRO had three times the cell specific Pi uptake rate of NPB, at ambient concentrations, but when adjusted to cells L−1 the rates were similar, and these two groups were equally competitive for Pi. The Tt of γ-P-ATP in the >0.2 μm group were shorter than for the Pi pool (4.4 ± 1.0 days; n = 6), but this difference diminished in the larger size classes. The kinetic response to ATP was large in the >0.2 μm class with Vmax exceeding the rates at ambient concentrations (mean 62 ± 27 times; n = 6) with a mean Vmax for γ-P-ATP of 2.8 ± 1.0 nmol L−1 day−1, and Km at 11.5 ± 5.4 nmol L−1 (n = 6). The NPB contribution to γ-P-ATP uptake was high (95 ± 3%, n = 4) at ambient concentrations but decreased to ∼50% at the highest ATP amendment. PRO had Km values 5–10 times greater than NPB. The above indicates that PRO and NPB were in close competition in terms of Pi acquisition, whereas P uptake from ATP could be attributed to NPB. This apparent resource partitioning may be a niche separating strategy and an important factor in the successful co-existence within the oligotrophic upper ocean of the NPSG.
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ArticlePhosphorus dynamics in biogeochemically distinct regions of the southeast subtropical Pacific Ocean(Elsevier, 2016-12-29) Duhamel, Solange ; Björkman, Karin M. ; Repeta, Daniel J. ; Karl, David M.The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of Chile and Easter Island. This remote area of the world’s ocean presents strong gradients in physical (e.g., temperature, density and light), chemical (e.g., salinity and nutrient concentrations) and microbiological (e.g., cell abundances, biomass and specific growth rates) properties. The goal of this study was to describe the phosphorus (P) dynamics in three main ecosystems along this transect: the upwelling regime off the northern Chilean coast, the oligotrophic area associated with the southeast subtropical Pacific gyre and the transitional area in between these two biomes. We found that inorganic phosphate (Pi) concentrations were high and turnover times were long (>210 nmol l−1 and >31 d, respectively) in the upper water column, along the entire transect. Pi uptake rates in the gyre were low (euphotic layer integrated rates were 0.26 mmol m−2 d−1 in the gyre and 1.28 mmol m−2 d−1 in the upwelling region), yet not only driven by decreases in particle mass or cell abundance (particulate P- and cell- normalized Pi uptake rates in the euphotic layer were ∼1–4 times and ∼3–15 times lower in the gyre than in the upwelling, respectively). However these Pi uptake rates were at or near the maximum Pi uptake velocity (i.e., uptake rates in Pi amended samples were not significantly different from those at ambient concentration: 1.5 and 23.7 nmol l−1 d−1 at 50% PAR in the gyre and upwelling, respectively). Despite the apparent Pi replete conditions, selected dissolved organic P (DOP) compounds were readily hydrolyzed. Nucleotides were the most bioavailable of the DOP substrates tested. Microbes actively assimilated adenosine-5′-triphosphate (ATP) leading to Pi and adenosine incorporation as well as Pi release to the environment. The southeast subtropical Pacific Ocean is a Pi-sufficient environment, yet DOP hydrolytic processes are maintained and contribute to P-cycling across the wide range of environmental conditions present in this ecosystem.