Competition between photochemical and biological degradation of dissolved organic matter from the cyanobacteria Microcystis aeruginosa

Abstract

Extracellular and intracellular dissolved organic matter (E‐DOM and I‐DOM, respectively) produced in axenic cultures by the cosmopolitan cyanobacteria Microcystis aeruginosa were characterized, and photoreactivity, bioreactivity, and the effects of phototransformations on bioavailability of E‐DOM and I‐DOM were assessed in laboratory experiments. Chromophoric (CDOM) and fluorescent (FDOM) properties were analyzed by spectrophotometry, and chemical signatures were obtained by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR‐MS). E‐DOM was more heterogeneous regarding optical characteristics as indicated by the presence of multiple CDOM peaks and the more even distribution of FDOM components, and had less absolute and relative numbers for molecular formulae containing nitrogen (N) than I‐DOM. E‐DOM was more photolabile, with greater photochemical removal of dissolved organic carbon (DOC), CDOM and FDOM, and exhibited more pronounced photochemical alterations in molecular formulae composition than I‐DOM. E‐DOM was less biolabile, with less biological removal of DOC, CDOM, and protein‐like fluorescent material, and supported lower rates of bacterial productivity than I‐DOM. Phototransformations reduced the bioavailability of E‐DOM and I‐DOM, as shown by lower bacterial productivity and longer DOC and CDOM half‐lives in bioassays with photoirradiated DOM in comparison to those with nonirradiated DOM. Analyses of CDOM, FDOM, and FTICR‐MS revealed that the decrease in bioavailability was caused primarily by photolysis of the biolabile DOM, indicating competition between photochemical and biological removal of cyanobacterial DOM. These results provided insights into mechanistic effects of independent and combined photochemical and biological degradation on M. aeruginosa‐derived DOM.

Deciphering ocean carbon in a changing world

“…One-carbon compounds such as methanol (27,29,31), as well as several sulfonates (32), have recently been added to the list. Chemical analyses concur: Photosynthate released directly from phytoplankton is highly complex, consisting of hundreds of different compounds (33,34). This "dissolved primary production"-the material released from living phytoplankton-supports a major fraction of labile carbon flux in the surface ocean (35).…”

Chromatographic methods for the isolation, separation and characterisation of dissolved organic matter

“…This procedure reports the resolution of hundreds of compounds, however, as DOM was extracted through C 18 -functionalised silica SPE disks, the following chromatographic procedure represents a repetition of the extraction procedure, as an analogous stationary phase is used during RP-LC fractionation. 22,75,118,162,182,184 Atmospheric pressure chemical ionisation (APCI) can also be found within DOM MS analysis, especially when attempting to target low polarity compounds. 182 However, in accepting the resolving power of MS detection, one has to also acknowledge potential biases originating from the ionisation source, which can be more efficient for certain classes of compounds over others, and the additional risk of insource fragmentation.…”

Seasonal changes in optical properties of two contrasting tropical freshwater systems

“…The estimated apparent molecular size (M) is obtained by the ratio of two absorption coefficients (a250:a365), based on the fact that declining ratio indicates increasing of molecular size (De Haan, 1987;Peuravuori and Pihlaja, 1997). In recent decades researchers have used these spectral metrics to investigate the optical properties in temperate aquatic systems (Zhang et al, 2007a(Zhang et al, , 2007b(Zhang et al, , 2009Helms et al, 2008;Bracchini et al, 2010), but studies using this approach in tropical environments are still scarce and little is known to Brazilian aquatic systems (Teixeira et al, 2011(Teixeira et al, , 2013Bittar et al, 2015).…”