Abstract
We monitored the dynamics of extracellular organic matter in 3 mesocosms: one dominated by a heterotrophic (microbial) community with negligible autotrophic activity (net heterotrophic system), a second where a small Phaeocystis bloom developed (production and loss almost balanced), and a third harboring a large diatom bloom (net autotrophic system). In all mesocosms, meso- and macroscopic heterotrophic organisms were excluded to primarily study extracellular organic matter production and turnover by specific algae and microbial loop organisms, respectively. Concentration and composition of dissolved organic matter (DOM), i.e. dissolved organic carbon (DOC), monosaccharides and total carbohydrates (MCHO and TCHO), free and combined neutral carbohydrates (DFCHO and DCCHO), as well as free and combined amino acids (DFAA and DCAA) were measured. In addition, net and gross community production rates were determined to calculate C-budgets. Whereas concentrations and composition of MCHO differed very little among the 3 mesocosms, dynamics of TCHO, DFCHO, and DCCHO differed significantly. Concentrations of DFAA were higher in both algal mesocosms compared to the heterotrophic system, and composition of DFAA was significantly different in the Phaeocystis and Diatom tanks. The composition and concentration of DCAA, however, were similar in all 3 mesocosms. Total dissolved carbohydrates and amino acids comprised a substantial fraction of the DOC pool. Dynamics of these DOC fractions, however, could only partly explain those of DOC, implying either that other dissolved compounds were important for overall C-cycling, or that microbial degradation of DOM affects the detection of carbohydrates and protein components.
Highlights
The majority of organic molecules occurring in the surface ocean are of algal origin (Meon & Kirchmann 2001)
Dynamics of these dissolved organic carbon (DOC) fractions, could only partly explain those of DOC, implying either that other dissolved compounds were important for overall C-cycling, or that microbial degradation of dissolved organic matter (DOM) affects the detection of carbohydrates and protein components
Our carbon budgets show that budgets of dissolved free amino acids (DFAA) and especially of non-neutral carbohydrates (‘other DCHO’) were positive in the 2 phytoplankton mesocosms, indicating that their dynamics were tightly linked to those of the phytoplankton
Summary
The majority of organic molecules occurring in the surface ocean are of algal origin (Meon & Kirchmann 2001). Phytoplankton blooms substantially contribute to the release of dissolved organic carbon (DOC) in seawater (Myklestad 1995). The rate of release of photosynthetically fixed carbon is highly variable, ranging between 1 and 70% (Baines & Pace 1991). This extreme variability may point to methodological problems, such as enzymatic hydrolysis, the uptake of exudates by heterotrophic bacteria, or the disruption of cells during filtration. Large fluctuations in primary production and algal biomass during the development of algal blooms do cause distinct changes in amount and quality of aquatic dissolved organic matter (DOM).
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