Tracing the long-term microbial production of recalcitrant fluorescent dissolved organic matter in seawater

Jørgensen, Louise von Gersdorff; Stedmon, Colin A.; Granskog, Mats A.; Middelboe, Mathias

doi:10.1002/2014gl059428

Cited by 84 publications

(78 citation statements)

References 43 publications

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“…Although it has been recently hypothesized that this relationship could also be caused by further transformations of terrestrial humic-like materials in the open ocean 20 , culture experiments have unequivocally demonstrated that these materials can be produced in situ in the oceans 21 . In fact, we observe positive and significant relationships between the archetypes of both humic-like components (C1 and C2) and the AOU (Fig.…”

Section: Discussionmentioning

confidence: 99%

Turnover time of fluorescent dissolved organic matter in the dark global ocean

et al. 2015

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Marine dissolved organic matter (DOM) is one of the largest reservoirs of reduced carbon on Earth. In the dark ocean (4200 m), most of this carbon is refractory DOM. This refractory DOM, largely produced during microbial mineralization of organic matter, includes humic-like substances generated in situ and detectable by fluorescence spectroscopy. Here we show two ubiquitous humic-like fluorophores with turnover times of 435 ± 41 and 610 ± 55 years, which persist significantly longer than the B350 years that the dark global ocean takes to renew. In parallel, decay of a tyrosine-like fluorophore with a turnover time of 379 ± 103 years is also detected. We propose the use of DOM fluorescence to study the cycling of resistant DOM that is preserved at centennial timescales and could represent a mechanism of carbon sequestration (humic-like fraction) and the decaying DOM injected into the dark global ocean, where it decreases at centennial timescales (tyrosine-like fraction).

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Section: Discussionmentioning

confidence: 99%

Turnover time of fluorescent dissolved organic matter in the dark global ocean

et al. 2015

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“…The overall trends in growth and production of fluorescent CDOM follows global ocean observations in fluorescence patterns and intensity, as well as, net production of DOC. Furthermore, the significant correlations between BEPOM and CDOM fluorescence, especially peaks A and C, challenges the paradigm that humic-like fluorescence in the open ocean is the result of terrestrially-derived material (Andrew et al, 2013;Jørgensen et al, 2014). Finally, hydrolytic enzymatic rates suggest that the bacterial community actively transforms organic matter with significant turnover of carbon and nitrogen, making bacteriallymediated production of RDOM from planktonic biomass a key pathway in global element cycles (e.g., Jiao et al, 2010;Lechtenfeld et al, 2015;Moran et al, 2016).…”

Section: Discussionmentioning

confidence: 99%

Formation of Chromophoric Dissolved Organic Matter by Bacterial Degradation of Phytoplankton-Derived Aggregates

et al. 2018

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Organic matter produced and released by phytoplankton during growth is processed by heterotrophic bacterial communities that transform dissolved organic matter into biomass and recycle inorganic nutrients, fueling microbial food web interactions. Bacterial transformation of phytoplankton-derived organic matter also plays a poorly known role in the formation of chromophoric dissolved organic matter (CDOM) which is ubiquitous in the ocean. Despite the importance of organic matter cycling, growth of phytoplankton and activities of heterotrophic bacterial communities are rarely measured in concert. To investigate CDOM formation mediated by microbial processing of phytoplankton-derived aggregates, we conducted growth experiments with non-axenic monocultures of three diatoms (Skeletonema grethae, Leptocylindrus hargravesii, Coscinodiscus sp.) and one haptophyte (Phaeocystis globosa). Phytoplankton biomass, carbon concentrations, CDOM and base-extracted particulate organic matter (BEPOM) fluorescence, along with bacterial abundance and hydrolytic enzyme activities (α-glucosidase, β-glucosidase, leucine-aminopeptidase) were measured during exponential growth and stationary phase (∼3-6 weeks) and following 6 weeks of degradation. Incubations were performed in rotating glass bottles to keep cells suspended, promoting cell coagulation and, thus, formation of macroscopic aggregates (marine snow), more similar to surface ocean processes. Maximum carbon concentrations, enzyme activities, and BEPOM fluorescence occurred during stationary phase. Net DOC concentrations (0.19-0.46 mg C L −1 ) increased on the same order as open ocean concentrations. CDOM fluorescence was dominated by protein-like signals that increased throughout growth and degradation becoming increasingly humic-like, implying the production of more complex molecules from planktonic-precursors mediated by microbial processing. Our experimental results suggest that at least a portion of open-ocean CDOM is produced by autochthonous processes and aggregation likely facilitates microbial reprocessing of organic matter into refractory DOM.

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“…Bioavailable dissolved organic carbon (BDOC) has been estimated to constitute on average <1% of the oceanic DOC pool, however elevated in the surface waters (Hansell, 2013). Studies in the Greenland Sea, Fram Strait, and Kobbefjord have shown that BDOC constitutes 13-36% of total DOC in surface water Middelboe et al, 2012;Jørgensen et al, 2014). Glacial meltwater from both Alaskan (Hood et al, 2009) and Alpine glaciers (Singer et al, 2012) contained highly bioavailable (>60%) DOC.…”

Section: Introductionmentioning

confidence: 99%

Heterotrophic nanoflagellate grazing facilitates subarctic Atlantic spring bloom development

Paulsen¹,

Riisgaard²,

John³

et al. 2017

Aquat. Microb. Ecol.

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My study underpins that picophytoplankton are important contributors to primary production, especially during the winter-spring transition (Paper I and III) and autumn (Paper V). They boosted the growth of heterotrophic microorganisms before the onset of the diatom spring bloom in the Subarctic Atlantic (Paper I) and dominated the phytoplankton biomass in the high turbid parts of a NE Greenland fjord influenced by glacial meltwater (Paper V). Picophytoplankton were better adapted to low light conditions and demonstrated higher growth rates, than larger phytoplankton (Paper I, II, III, V). In the Polar-influenced water near Greenland, Synechococcus were negligible, while in the Atlantic influenced waters picoeukaryotes and Synechococcus were often equally abundant and the latter dominated on several occasions during autumn and winter (Paper I and III). Unexpectedly, abundances of Synechococcus were as high at 65°N as at 79°N, and molecular analysis suggests the presence of new clades specially adapted to Arctic conditions. Bacteria were generally rather carbon-than nutrient limited, and their abundance increased rapidly in response to the pre-bloom picophytoplankton production of labile carbon in both the Arctic and Subarctic (Paper I and V). In NE Greenland the terrestrial DOM supplied from the Greenland ice sheet proved to be highly bioavailable compared to the 7 autochthonous fjord DOM (Paper IV). The in situ changes in DOM, which were examined via fluorescence signal of different DOM components (FDOM), surprisingly demonstrated that the highest net-growth of bacteria was not coupled to the labile glacial runoff in the surface, but rather to sub-surface the humic-DOM, commonly considered to be refractory. This may be explained by the presence of specific dominating taxa of bacteria that had the ability to degrade humic-DOM (Paper V).Across regions, HNF exerted strong control of picophytoplankton and bacteria (Paper I, II, III, V). HNF grew significantly faster than microzooplankton and were therefore less affected by mixing and relatively more important grazers than their micro-sized counterparts in well-mixed water columns (Paper I and II). HNF larger than 5µm controlled picophytoplankton particularly in the early productive season, while small HNF (3-5µm) mainly kept bacteria in check in autumn (Paper III, V). In conclusion, the studies underline that pico-sized plankton play a fundamental part in the carbon transfer in high latitude ecosystems both as primary producers and via the microbial loop. Picophytoplankton appeared better adapted than larger phytoplankton to low light conditions, and bacteria were capable of degrading terrestrial derived DOM, however, these abilities are highly community specific. The data suggest that a change in mixing patters will affect the microbial food structure and that shifts in coastal microbial community composition should be anticipated with increased runoff. 8 List of publicationsThe thesis is based on preliminary data and the following fived papers referred to in the t...

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Tracing the long-term microbial production of recalcitrant fluorescent dissolved organic matter in seawater

Cited by 84 publications

References 43 publications

Turnover time of fluorescent dissolved organic matter in the dark global ocean

Turnover time of fluorescent dissolved organic matter in the dark global ocean

Formation of Chromophoric Dissolved Organic Matter by Bacterial Degradation of Phytoplankton-Derived Aggregates

Heterotrophic nanoflagellate grazing facilitates subarctic Atlantic spring bloom development

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