Changes In Dissolved Organic Matter Quality Research Articles

Fluorescent properties of marine phytoplankton exudates and lability to marine heterotrophic prokaryotes degradation

AbstractExudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low‐ and high‐molecular‐weight fractions. To detect changes in the polarity of proteins, reverse‐phase high‐performance liquid chromatography (HPLC) was applied to the high‐molecular‐weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high‐molecular‐weight DOM and 40% of fluorescence attributed to protein‐like DOM were degraded over the first days of the incubation indicating that protein‐like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic‐like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short‐term rates of high‐molecular‐weight and low‐molecular‐weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.

Differential recruitment of opportunistic taxa leads to contrasting abilities in carbon processing by bathypelagic and surface microbial communities.

Different factors affect the way dissolved organic matter (DOM) is processed in the ocean water column, including environmental conditions and the functional capabilities of the communities. Recent studies have shown that bathypelagic prokaryotes are metabolically flexible, but whether this versatility translates into a higher ability to process DOM has been barely explored. Here we performed a multifactorial transplant experiment to compare the growth, activity and changes in DOM quality in surface and bathypelagic waters inoculated with either surface or bathypelagic prokaryotic communities. The effect of nutrient additions to surface waters was also explored. Despite no differences in the cell abundance of surface and deep ocean prokaryotes were observed in any of the treatments, in surface waters with nutrients the heterotrophic production of surface prokaryotes rapidly decreased. Conversely, bathypelagic communities displayed a sustained production throughout the experiment. Incubations with surface prokaryotes always led to a significant accumulation of recalcitrant compounds, which did not occur with bathypelagic prokaryotes, suggesting they have a higher ability to process DOM. These contrasting abilities could be explained by the recruitment of a comparatively larger number of opportunistic taxa within the bathypelagic assemblages, which likely resulted in a broader community capability of substrate utilization.

Mismatched dynamics of dissolved organic carbon and chromophoric dissolved organic matter in the coastal NW Mediterranean Sea

Coastal marine ecosystems are strongly influenced by different occasional events, such as intense winds, mixing, rain and river discharges. These events can directly or indirectly cause changes in dissolved organic matter (DOM) quality through a cascade of different biotic and abiotic processes. Changes in DOM quality are often associated with changes in DOM optical properties. Thus, examining the dynamics of chromophoric DOM (CDOM) can provide valuable information about biological and physical processes that have occurred in the ecosystem. Episodic meteorological events, particularly in temperate areas, appear very abruptly and induce very rapid responses; therefore, high time-resolved measurements are needed to capture them. We used a weekly sampling scheme to characterize DOM and nutrient dynamics in the NW Mediterranean coastal station ‘SOLA’. From February 2013 to April 2014, we measured several physical and chemical variables including temperature, salinity, inorganic nutrients, chlorophyll a dissolved organic carbon (DOC), CDOM and fluorescent DOM (FDOM). During this period, two extremely high fresh water intrusions greatly influenced the dynamics of some DOM fractions, in particular the FDOM. Inorganic nutrients and chlorophyll a showed seasonal patterns: A winter period characterized by a high nutrient concentration in surface waters favored the phytoplankton spring bloom; then, summer stratification extended until autumn. This stratification led to nutrient depletion and, consequently, lower chlorophyll a values in the photic zone. The CDOM and FDOM optical active fractions did not follow temporal trends similar to total DOC. This was likely because the potential sources and sinks of these DOM pools are microbial activity and light exposure, and these were acting simultaneously but in opposite directions. Interestingly, DOC exhibited the highest concentrations in summer, coinciding with nutrient and chlorophyll a minima. To explain this mismatch, we propose a sequence of abiotic and biotic phenomena that drive DOC temporal dynamics.

High field FT-ICR mass spectrometry data sets enlighten qualitative DOM alteration in lake sediment porewater profiles

This study investigates how changes in dissolved organic matter (DOM) quality can be evaluated considering all existing principal data pools from high field Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS). We recommend five steps, including suggestions on underlying processes. Two different sediment profiles from a highly acidic mining lake in Germany with different mixis of the overlying lake water (dimictic vs. meromictic) were selected as a model for the analysis of sediment porewater sample to a depth of 30cm. FT-ICR-MS datasets of elemental composition were evaluated for the presence of specific CHO molecular series. In six (seven) porewater depth groups of 63 (127) different data pools were identified. In the first step, the data pools were evaluated with components present in all six (seven) samples (total common presence). This group of components was analysed via a statistical rank analysis of the mass peak intensities (inter sample rankings analysis). The second step comprised the allocation of components which were not present in all samples to specific data pools (partial common presence and different presence pools). Of these pools the largest (containing most components) were selected in the third step for visualization of DOM quality change using van Krevelen diagrams. Underlying DOM transformations were discussed in the fourth step. The fifth step comprised the parallel allocation of changes in DOM quality and in concentrations of inorganic compounds. We identified the principal changes in both depth profiles and thereby demonstrated the potential of DOM data pool evaluation for elucidating biogeochemical processes.

Carbon dynamics and their link to dissolved organic matter quality across contrasting stream ecosystems

Streams represent active components of the carbon cycle as emitters of carbon dioxide (CO2) and methane to the atmosphere at a global scale. However, the mechanisms and governing factors of these emissions are still largely unknown, especially concerning the effect of land use. We compared dissolved and gaseous carbon dynamics in streams bordered by contrasting types of land use, specifically agriculture and forest. Carbon dioxide and methane partial pressures (pCO2 and pCH4, respectively) in the water body and carbon emissions via both gases were studied for 24h during four field expeditions. pCH4 did not differ between the two system types. pCO2 was constantly oversaturated in all streams and significantly higher in agricultural streams (annual mean 4282ppm) compared to forest streams (annual mean 2189ppm) during all seasons. However, emissions of CO2 were not significantly different between the stream types due to significantly higher gas transfer velocity in forest compared to agricultural streams. pCO2 was significantly positively correlated to the concentrations of dissolved organic carbon, dissolved nitrogen and soluble reactive phosphorus in the water. Furthermore, pCO2 was correlated to optical parameters of dissolved organic matter (DOM) quality, e.g., it increased with indicators of molecular size and an allochthonous fluorescent component identified by Parallel Factor Analysis (PARAFAC). This study demonstrates that different forms of land use may trigger a cascade of effects on the carbon production and emission of streams linked to changes in DOM quality.

Dissolved organic matter concentration, optical parameters and attenuation of solar radiation in high-latitude lakes across three vegetation zones

High-latitude lakes are usually transparent, due to their low productivity and low concentration of dissolved organic matter (DOM), but large variations in lake optical properties can be found within and between regions. We investigated the light regimes in relation to DOM in 18 oligotrophic, high-latitude lakes across mountain birch woodland, shrub tundra and barren tundra in north-west Finnish Lapland. In 12 lakes >1% of photosynthetically active radiation (PAR) reached the lake bottom, while 1% UV-B depth ranged from 0.1 to >12 m. Lakes located in barren tundra had highest transparency, lowest dissolved organic carbon (DOC) concentration and lowest DOM absorption (a440) (mean values: Kd PAR 0.3m–1, DOC 2.1mg l–1, a440 0.4m–1), while lakes in shrub tundra and mountain birch forest were less transparent (DOC 4.7 mg l–1, a440 1.4 m–1). Solar attenuation and lake transparency was best explained by a440. Our survey emphasizes the importance of catchment type on DOM characteristics and lake optics. We predict that even small changes in DOM quality may largely change the UV radiation exposure of lakes while changes in PAR may have smaller biological effects in these shallow lakes that are already illuminated to the bottom.