Experimental assessment of diazotroph responses to elevated seawater pCO₂ in the North Pacific Subtropical Gyre

We examined short-term (24–72 h) responses of naturally occurringmarine N2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater during nine incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) (22°45′N, 158°W) in the North Pacific Subtropical Gyre (NPSG). Rates of N2 fixation, nitrogenase (nifH) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14C-bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated pCO2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (seven out of nine), elevated pCO2 did not significantly influence rates of dinitrogen (N2) fixation or primary productivity (two-way analysis of variance (ANOVA), P>0.05). During two experiments, rates of N2 fixation and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated pCO2 treatments relative to the ambient controls (two-way ANOVA, P<0.05). QPCR amplification of nifH genes and gene transcripts revealed that diazotroph abundances and nifH gene expression were largely unchanged by the perturbation of the seawater pCO2. Our results suggest that naturally occurring N2 fixing plankton assemblages in the NPSG are relatively resilient to large, short-term increases in pCO2.Keywords: Ocean acidification, Anthropogenic carbon, N-2 fixation, Nitrogen-fixation rates, Station Aloha, Marine microbes, Phosphorus limitation, Cyanobacterium Trichodesmium, Inorganic carbo

Short-term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA : a case study of summer 2012

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1145–1164, doi:10.1002/2015GB005141.Time-series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time-series program has maintained near-monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near-monthly time-series observations, an extensive field campaign was conducted during July–September 2012 at Station ALOHA with near-daily sampling of upper water-column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea-surface salinity minimum feature which was prominent in the upper water column (0–50 m) for a period of approximately 30 days. The shipboard observations during July–September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea-surface salinity minimum feature in the NPSG.NOAA Climate Observation Division; National Science Foundation (NSF) Center for Microbial Oceanography: Research and Education (C-MORE) Grant Numbers: EF0424599, OCE-1153656, OCE-1260164; Gordon and Betty Moore Foundation Marine Microbiology Investigator2016-02-1

Short-term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA: A case study of summer 2012

Time-series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time-series program has maintained near-monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near-monthly time-series observations, an extensive field campaign was conducted during July–September 2012 at Station ALOHA with near-daily sampling of upper water-column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea-surface salinity minimum feature which was prominent in the upper water column (0–50 m) for a period of approximately 30 days. The shipboard observations during July–September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea-surface salinity minimum feature in the NPSG.This is the publisher’s final pdf. The article is copyrighted by American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9224

Variability in photosynthetic production of dissolved and particulate organic carbon in the North Pacific Subtropical Gyre

The partitioning of photosynthetically-derived organic carbon between particulate and dissolved phases has important implications for marine carbon cycling. In this study we utilized 14C-bicarbonate assimilation to quantify rates of photosynthetic production of both particulate and dissolved organic carbon (DOC) at Station ALOHA (22˚45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). At near-monthly time scales over ~5 years, we examined retention of 14C-labeled organic matter by both glass fiber filters and 0.2 µm pore size polycarbonate membrane filters that are commonly used for measurements of 14C-based plankton productivity. Use of polycarbonate filters resulted in significantly lower (averaging 60%) estimates of 14C-production compared to glass fiber filters. Coincident measurements of chlorophyll a concentrations from both 0.2 µm polycarbonate and glass fiber filters were not significantly different, suggesting the differences in 14C-productivity between these filter-types did not derive from differences in retention of photosynthetic biomass by these filters. Moreover, consistent with previous studies, results from experiments aimed at quantifying retention of organic matter by these filters suggested the difference between these two types of filters resulted from retention of DOC by glass fiber filters. We also quantified rates of 14C-DOC production to evaluate the partitioning of photosynthetic production between dissolved and particulate phases over daily to monthly time scales in this ecosystem. Unlike the strong depth-dependence observed in measurements of particulate organic carbon production, measured rates of 14C-DOC demonstrated no clear depth-dependence. On average, depth-integrated (0-75 m) rates of 14C-DOC production rates were equivalent to 18 ± 10% of the total (particulate and dissolved) productivity. Our findings indicate that in this oligotrophic ecosystem, rates of dissolved and particulate production can be temporally decoupled over daily to monthly time scales

Percent differences in ¹⁴C-PP between controls and elevated <i>p</i>CO₂ treatments.

<p>Shown are percent differences ([treatments (750 μatm) - controls (387 μatm)] / controls) for rates of ¹⁴C-PP from depth-resolved experiments during cruises in August 2010 and March 2011 for >10 μm size class (panel A), 2–10 μm size class (panel B), and the 0.2–2 μm size class (panel C). Dashed line indicates zero. Bold symbols indicate significant differences between controls and treatments.</p

The influence of abrupt increases in seawater <i>p</i>CO₂ on plankton productivity in the subtropical North Pacific Ocean

<div><p>We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide (<i>p</i>CO₂) in seawater on rates of primary and bacterial production at Station ALOHA (22°45’ N, 158° W) in the North Pacific Subtropical Gyre (NPSG). The majority of experiments (8 of 10 total) displayed no response in rates of primary production (measured by ¹⁴C-bicarbonate assimilation; ¹⁴C-PP) under elevated <i>p</i>CO₂ (~1100 μatm) compared to ambient <i>p</i>CO₂ (~387 μatm). In 2 of 10 experiments, rates of ¹⁴C-PP decreased significantly (~43%) under elevated <i>p</i>CO₂ treatments relative to controls. Similarly, no significant differences between treatments were observed in 6 of 7 experiments where bacterial production was measured via incorporation of ³H-leucine (³H-Leu), while in 1 experiment, rates of ³H-Leu incorporation measured in the dark (³H-Leu_Dark) increased more than 2-fold under high <i>p</i>CO₂ conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of ¹⁴C-PP and ³H-Leu incorporation to abrupt <i>p</i>CO₂ increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of ¹⁴C-PP demonstrated no consistent response to elevated <i>p</i>CO₂. In 2 of 5 depth-resolved experiments, rates of ³H-Leu_Dark incorporation were lower (10% to 15%) under elevated <i>p</i>CO₂ compared to controls. Our results revealed that rates of ¹⁴C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in <i>p</i>CO₂. We postulate that any effects caused by changes in <i>p</i>CO₂ may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.</p></div

Depth-integrated rates of ³H-Leu incorporation.

<p>Depth-integrated rates of ³H-Leu incorporation.</p

Depth-resolved measurements of size fractionated ¹⁴C-PP during cruises in August 2010 and March 2011.

<p>Rates of ¹⁴C-PP in the >10 μm size fraction (panels A and D), 2–10 μm size fraction (panels B and E), and 0.2–2 μm size fraction (panels C and F) under ambient (387 μatm; panels A-C) and elevated <i>p</i>CO₂ (750 μatm) conditions (panels D-F).</p

Results of <i>p</i>CO₂ bubbling experiments.

<p>Measured rates of ¹⁴C-PP (panel A) and ³H-Leu_Dark incorporation (panel C) from all experiments (n = 8 and n = 7, respectively) where no significant differences (NSD) were observed between controls (white circles) and elevated <i>p</i>CO₂ treatments at 1100 μatm (grey squares) and 750 μatm (grey triangles) are depicted. Also shown are rates of ¹⁴C-PP (panel B) and ³H-Leu_Dark incorporation (panel D) from experiments where significant differences (two-way ANOVA; p<0.05) were observed between controls (open symbols) and elevated <i>p</i>CO₂ (black symbols) treatments. Dates of experiments showing significant differences are given in the legend.</p

Depth-integrated rates of ¹⁴C-PP.

<p>Depth-integrated rates of ¹⁴C-PP.</p