Character and environmental lability of cyanobacteria-derived dissolved organic matter

Autotrophic dissolved organic matter (DOM) is central to the carbon biogeochemistry of aquatic systems, and the full complexity of autotrophic DOM has not been extensively studied, particularly by high-resolution mass spectrometry (HRMS). Terrestrial DOM tends to dominate HRMS studies in freshwaters due to the propensity of such compounds to ionize by negative mode electrospray, and possibly also because ionizable DOM produced by autotrophy is decreased to low steady-state concentrations by heterotrophic bacteria. In this study, we investigated the character of DOM produced by the widespread cyanobacteriaMicrocystis aeruginosausing high-pressure liquid chromatography-electrospray ionization-high-resolution mass spectrometry.M. aeruginosaproduced thousands of detectable compounds in axenic culture. These compounds were chromatographically resolved and the majority were assigned to aliphatic formulas with a broad polarity range. We found that the DOM produced byM. aeruginosawas highly susceptible to removal by heterotrophic freshwater bacteria, supporting the hypothesis that this autotroph-derived organic material is highly labile and accordingly only seen at low concentrations in natural settings

Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe

KOSMOS mesocosm unit and conceptual figure of element pools and fluxes.

<p>(A) Schematic illustration of a KOSMOS unit, including the floatation frame at the sea surface and the enclosure bag reaching down to the sediment trap at the bottom. (B) Element pools (inorganic nutrients [IN], dissolved organic matter [DOM], particulate matter [PM], particulate matter of copepods [PM_COP]) and fluxes (air-sea gas exchange of CO₂, sedimentation of particulate matter [PM_SED]) included in the mass balance calculations of carbon, nitrogen, phosphorus, and silica (C, N, P, and Si). Grey arrows indicate exchange between the individual element pools in the water column. Illustration of the KOSMOS unit modified from Rita Erven (GEOMAR).</p

Colour code, line types, and abbreviations of the different element pools and their calculated net community production.

<p>Colour code, line types, and abbreviations of the different element pools and their calculated net community production.</p

Conditions of the element pools during the reference period of the experiment.

<p>Conditions of the element pools during the reference period of the experiment.</p

Time course the particulate carbon to nitrogen ratio at ambient and high CO₂.

<p>Solid lines show mean values of the particulate carbon (TPC) to nitrogen (TPN) ratio in (A) the water column, (B) collected sediment trap samples, and of the suspended particle size fractions (C) larger and (D) smaller than 200 μm in the ambient (blue) and high (red) CO₂ treatment. Coloured areas indicate standard deviation of replicated (n = 5) treatments. Roman numbers denote the four different phases of the experiment. Vertical dashed lines represent the Redfield ratio of carbon to nitrogen (6.6).</p

Time course of net changes of the element pools at ambient and high CO₂.

<p>Solid lines indicate temporal net changes (Δ values) of the element pools and net community production of (A–E) carbon, (F–J) nitrogen, and (K–O) phosphorus as average values of the ambient (blue) and high (red) CO₂ mesocosms. Coloured areas indicate the standard deviation of replicated (n = 5) treatments. Roman numbers denote the four different phases of the experiment. Black asterisks identify significant CO₂ effects (PERMANOVA, p <0.05).</p