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Importance of bacterial maintenance respiration and baseline respiration for development of coastal hypoxia
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reduced oxygen concentrations and increasing hypoxic zones havebecome more common in the sea due to climate change andeutrophication. The main cause of oxygen loss in oxygenatedenvironments is respiration. Respiration rates can be estimated usingoptode methodologies which utilize dynamic luminescence quenching toestimate the oxygen concentration declines in dark incubations. Apublished optode methodology was improved by using optodes withtitanium housing instead of plastic housing plausibly trapping oxygen.Drift was highly reduced by the titanium casings leading to a higherprecision and lower detection limit of 0.97 mmol O2 m-3 d-1. 28% ofmeasurements were shown to have non-linear oxygen concentrationdeclines. The rate of oxygen change was derived with a 2nd degreepolynomial at 1 hour from the incubation start. The majority of non-lineardeclines were concave and due to carbon substrate limitation. Analyzingnon-linear trends linearly, a common practice, leads to anunderestimation of respiration by up to 64%.

Bacterial maintenance respiration (Rm) was studied using anecophysiological model unverified in natural environments. The modelwas applicable at high productivities but a quadratic model wasdemonstrated to give a better fit. Rm was found to represent a significantpart in the sub-arctic estuary contributing to 58% of the annual specificbacterial respiration. Therefore, Rm may be more important in nature thanpreviously recognized. The ecophysiological model is driven solely by thebacterial specific growth rate (μ) where the relative influence of Rm iselevated as μ decreases. As a consequence, I hypothesize that a reductionin nutrients may not decrease the oxygen consumption but rather shiftbacterial growth based respiration to Rm as μ approaches zero.Baseline respiration (Rbl), defined as ecosystem respiration disconnectedfrom contemporary primary produced carbon, was also studied. Rbl wasshown to be largely supplied by allochthonous carbon in a coastalecosystem and had a contribution of 50% to the annual planktoncommunity respiration in the sub-arctic estuary studied. I claim that Rbland Rm are crucial to include for understanding and managingdevelopment of aquatic hypoxia in an effective and economic manner.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2018. , p. 40
Keywords [en]
Ecology, Respiration, Estuarine, Allochthonous, Maintenance, primary production, bacterial production
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-152587ISBN: 978-91-7601-927-6 (print)OAI: oai:DiVA.org:umu-152587DiVA, id: diva2:1255864
Public defence
2018-11-09, N440, Naturvetarhuset, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2018-10-19 Created: 2018-10-15 Last updated: 2018-11-15Bibliographically approved
List of papers
1. Improved accuracy of optode-based oxygen consumption measurements by removal of system drift and non-linear derivation
Open this publication in new window or tab >>Improved accuracy of optode-based oxygen consumption measurements by removal of system drift and non-linear derivation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This method evaluation aimed to improve the accuracy and precision of the previously published method to measure oxygen consumption using optodes with integrated temperature and salinity correction in dark incubations. Significant short-term system drift currently requires a correction to remove the drift, thus reducing the precision of the oxygen consumption rates. Frequent non-linear declines in oxygen concentration with time also calls for improved data analysis and identification of its origin. Optodes in titanium casings (AanderaaTM model 4330) with low oxygen binding properties showed no significant system drift in autoclaved seawater. A conservative detection limit of 0.97 mmol O2 m-3 d-1 was calculated for the titanium optodes, matching other methods for oxygen consumption reported in the literature.Non-linear oxygen dynamics fitting a quadratic polynomial occurred in 28 % of 230 field samples, independent of season and water depth. Polynomial curve fit resulted in 64 % higher respiration rates when derived within one hour of the quality assured incubation, than obtained when using linear fit. Carbon substrate limitation explained the non-linearity of oxygen decline during dark incubations.Pre-treatment of the optode attached to stoppers with 0.3 mol dm-3 hydrochloric acid resulted in the most stable performance of the sensor and simultaneously provided proper cleaning of the equipment. Thus, we recommend the use of model 4330 optode pretreated with HCl and the derivation of initial respiration rates by a quadratic polynomial function for best accuracy and precision of oxygen consumption in oxygenated surface waters.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-152581 (URN)
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-16
2. Importance of Bacterial Maintenance Respiration in a Subarctic Estuary: a Proof of Concept from the Field
Open this publication in new window or tab >>Importance of Bacterial Maintenance Respiration in a Subarctic Estuary: a Proof of Concept from the Field
2018 (English)In: Microbial Ecology, ISSN 0095-3628, E-ISSN 1432-184XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

Bacterial respiration contributes to atmospheric carbon dioxide accumulation and development of hypoxia and is a critical, often overlooked, component of ecosystem function. This study investigates the concept that maintenance respiration is a significant proportion of bacterial respiration at natural nutrient levels in the field, advancing our understanding of bacterial living conditions and energy strategies. Two river-sea transects of respiration and specific growth rates were analyzed representing low- and highproductivity conditions (by in situ bacterial biomass production) in a subarctic estuary, using an established ecophysiological linear model (the Pirt model) estimating maintenance respiration. The Pirt model was applicable to field conditions during high, but not low, bacterial biomass production. However, a quadratic model provided a better fit to observed data, accounting for the maintained respiration at low μ. A first estimate of maintenance respiration was 0.58 fmol O2 day−1 cell−1 by the quadratic model. Twenty percent to nearly all of the bacterial respiration was due to maintenance respiration over the observed range of μ (0.21– 0.002 day−1 ). In the less productive condition, bacterial specific respiration was high and without dependence on μ, suggesting enhanced bacterial energy expenditure during starvation. Annual maintenance respiration accounted for 58% of the total bacterioplankton respiration based on μ from monitoring data. Phosphorus availability occasionally, but inconsistently, explained some of the remaining variation in bacterial specific respiration. Bacterial maintenance respiration can constitute a large share of pelagic respiration and merit further study to understand bacterial energetics and oxygen dynamics in the aquatic environment.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Marine, Bacteria, Maintenance, Respiration, Stoichiometry, Model
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-152580 (URN)10.1007/s00248-018-1244-7 (DOI)
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-17
3. Coastal filter effect by microbial mineralization of riverine DOC in a sub-arctic river-estuary gradient
Open this publication in new window or tab >>Coastal filter effect by microbial mineralization of riverine DOC in a sub-arctic river-estuary gradient
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The existence of a coastal filtering effect was studied in a 16 km boreal river-estuary system (RES) during contrasting low and high production conditions. Marked transformations occurred within 5 km (salinity 3) from the river mouth for many of the variables during high productive conditions. During the less productive season changes were small and occurred closer to the river mouth. Active transformation dominated the patterns detected in August (53 %), while mixing of river and coastal marine water was the dominant process in April. Bacterial community respiration was similar during both seasons averaging 2.6 μmol O2 dm-3 d-1, indicating efficient remineralization of riverine dissolved organic carbon (DOC). This was further reflected in a low and variable bacterial growth efficiency (BGE) with a mean of 12 % in the surface water at high productivity, but only 3 % at low productivity conditions. Bacterial community growth (BCG) showed strong relationship to water temperature at both seasons indicating energy limitation. Some indication of removal of nitrogen was observed, while no removal of phosphorus could be demonstrated. Phosphorus concentration showed a strong reciprocal power-function relationship to BCG, suggesting efficient assimilation of the limiting nutrient at carbon sufficiency. Marked spatial changes in diversity of phytoplankton, protozoa and bacterioplankton occurred at low salinities within 1 km from the river mouth. We conclude that this subarctic estuary acts as a coastal filter mainly by remineralizing riverine DOC to CO2.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-152583 (URN)
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-16
4. High influence of baseline respiration in a sub-Arctic coastal ecosystem
Open this publication in new window or tab >>High influence of baseline respiration in a sub-Arctic coastal ecosystem
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Respiration is a key metabolic process in the marine environment that is usually assumed to be driven by phytoplankton production. However, respiration in the absence of contemporary phytoplankton production, termed baseline respiration, can influence the energetics of an ecosystem and its sensitivity to hypoxia. Direct studies of baseline respiration and its importance for coastal oxygen status are currently lacking. This study aims to obtain a first estimate of baseline respiration in a sub-arctic estuary, determine its contribution to annual plankton community respiration and identify the main carbon sources. The four different methods used to define baseline respiration converged on an average rate of 4.2 mmol O2 m-3 d-1 ± 0.1 (SE), corresponding to an annual contribution of 50 % of planktonic respiration on the basin scale. Respiration during the winter season (sporadic ice cover) was significant and comprised 25 % of annual pelagic respiration. No correlation between plankton respiration and phytoplankton production occurred on the intra-annual scale, while there was a weak exponential dependence on riverine total organic carbon inflow combined with phytoplankton production (i.e., major C input). Measured carbon sources could supply most (78 %) of the annual plankton respiration but only 39 % of the winter season respiration, suggesting some missing carbon sources. Nitrification had a negligible (≤ 2.4 %) effect on baseline respiration in the system. The results showed that baseline respiration could constitute a significant share of coastal plankton respiration, when allochthonous carbon sources dominate the carbon supply.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-152584 (URN)
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-12-03

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