Microbial use of low molecular weight DOM in filtered and unfiltered freshwater: Role of ultra-small microorganisms and implications for water quality monitoring

Abstract

Dissolved organic matter (DOM) plays a central role in regulating productivity and nutrient cycling in freshwaters. It is therefore vital that we can representatively sample and preserve DOM in freshwaters for subsequent analysis. Here we investigated the effect of filtration, temperature (5 and 25°C) and acidification (HCl) on the persistence of low molecular weight (MW) dissolved organic carbon (DOC), nitrogen (DON) and orthophosphate in oligotrophic and eutrophic freshwater environments. Our results showed the rapid loss of isotopically-labelled glucose and amino acids from both filtered (0.22 and 0.45μm) and unfiltered waters. We ascribe this substrate depletion in filtered samples to the activity of ultra-small (<0.45μm) microorganisms (bacteria and archaea) present in the water. As expected, the rate of C, N and P loss was much greater at higher temperatures and was repressed by the addition of HCl. Based on our results and an evaluation of the protocols used in recently published studies, we conclude that current techniques used to sample water for low MW DOM characterisation are frequently inadequate and lack proper validation. In contrast to the high degree of analytical precision and rigorous statistical analysis of most studies, we argue that insufficient consideration is still given to the presence of ultra-small microorganisms and potential changes that can occur in the low MW fraction of DOM prior to analysis.

Microbial uptake kinetics of dissolved organic carbon (DOC) compound groups from river water and sediments

“…After sealing with sterile caps, the samples were subsequently incubated on a shaker (200 rev min −1 ) in the dark at 10 °C, to ensure the samples remained well-mixed and aerated for the duration of the experiment. This temperature represents the mean annual temperature within the catchment 23 . The mean water temperature over the duration of the experiment was 8.28 ± 0.34 °C.…”

“…To date, there have been a limited number of studies applying these methods to aquatic environments; such studies have focused on waters only, using simple 14 C-labelled DOC compounds in isolation rather than compound groups, e.g. glucose or phenol 22,23 . However, DOC is a heterogeneous mixture of compounds.…”

Nano-Sized and Filterable Bacteria and Archaea: Biodiversity and Function

“…As discussed earlier, filtration through <0.45 μm pore size filters, is a common method to isolate small cells from aqueous samples. Interestingly, ultrafiltration was considered a method of choice to preserve freshwater samples during their storage and prior the hydrochemistry analysis ( Brailsford et al, 2017 ). 0.22 μm pore size filters were considered as a safe tool for sterilization and for effective removal of microorganisms.…”

“…0.22 μm pore size filters were considered as a safe tool for sterilization and for effective removal of microorganisms. However, a recent study, which monitored the depletion of 14 C-glucose, 14 C-amino acid mixture, and 33 P-orthophosphate in filtered and unfiltered freshwater samples showed significant activity and utilization of substrates by organisms capable of passing this barrier ( Brailsford et al, 2017 ). The previous studies clearly support this claim, as a number of the species were able to pass through ultrafiltration membranes (e.g., Wang et al, 2008 ).…”

Influence of extractants and filter materials in the extraction of dissolved organic matter (DOM) from subtropical agricultural soil

“…Consequently, research based on DOM materials extracted by different methods may lead to different results, because various DOM components play different roles in ecological processes. For example, during the microbial degradation of DOM, components rich in N and P (i.e., amino acids and nucleotides) are transformed into inorganic nutrients (i.e., ammonium, nitrate, phosphate) (Moschonas et al, 2017;Brailsford et al, 2017). Low-molecular weight organic acids can improve the transport of metal ions in soil through the formation of metal-organic acid complexes (Wu et al, 2010 as they are less used by microorganisms (Nebbioso and Piccolo, 2013;Cotrufo et al, 2013).…”