Fate of organo-mineral particles in streams: Microbial degradation by streamwater & biofilm assemblages

Inland waters are of global biogeochemical importance. They receive carbon inputs of ~ 4.8 Pg C/ y of which, 12 % is buried, 18 % transported to the oceans, and 70 % supports aquatic secondary production. However, the mechanisms that determine the fate of organic matter (OM) in these systems are poorly defined. One aspect of this is the formation of organo-mineral complexes in aquatic systems and their potential as a route for OM transport and burial vs. their use as carbon (C) and nitrogen (N) sources within aquatic systems. Organo-mineral particles form by sorption of dissolved OM to freshly eroded mineral surfaces and may contribute to ecosystem-scale particulate OM fluxes. We experimentally tested the availability of mineral-sorbed OM as a C & N source for streamwater microbial assemblages and streambed biofilms. Organo-mineral particles were constructed in vitro by sorption of ¹³C:¹⁵N-labelled amino acids to hydrated kaolin particles, and microbial degradation of these particles compared with equivalent doses of ¹³C:¹⁵N-labelled free amino acids. Experiments were conducted in 120 ml mesocosms over 7 days using biofilms and water sampled from the Oberer Seebach stream (Austria). Each incubation experienced a 16:8 light:dark regime, with metabolism monitored via changes in oxygen concentrations between photoperiods. The relative fate of the organo-mineral particles was quantified by tracing the mineralization of the ¹³C and ¹⁵N labels and their incorporation into microbial biomass. Here we present the initial results of ¹³C-label mineralization, incorporation and retention within dissolved organic carbon pool. The results indicate that 514 (± 219) μmol/ mmol of the ¹³:¹⁵N labeled free amino acids were mineralized over the 7-day incubations. By contrast, 186 (± 97) μmol/ mmol of the mineral-sorbed amino acids were mineralized over a similar period. Thus, organo-mineral complexation reduced amino acid mineralization by ~ 60 %, with no differences observed between the streamwater and biofilm assemblages. Throughout the incubations, biofilms were observed to leach dissolved organic carbon (DOC). However, within the streamwater assemblage the presence of both organo-mineral particles and kaolin particles was associated with significant DOC removal (-1.7 % and -7.5 % respectively). Consequently, the study demonstrates that mineral and organo-mineral particles can limit the availability of DOC in aquatic systems, providing nucleation sites for flocculation and fresh mineral surfaces, which facilitate OM-sorption. The formation of these organo-mineral particles subsequently restricts microbial OM degradation, potentially altering the transport and facilitating the burial of OM within streams.

Increase in microbial contamination risk with compression of the lid margin in eyes having cataract surgery

Purpose: To assess the bacterial contamination risk in cataract surgery associated with mechanical compression of the lid margin immediately after sterilization of the ocular surface.

Setting: Department of Cataract, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China.

Design: Prospective randomized controlled double-masked trial.

Methods: Patients with age-related cataract were randomly assigned to 1 of 2 groups. In Group A (153 eyes), the lid margin was compressed and scrubbed for 360 degrees 5 times with a dry sterile cotton-tipped applicator immediately after ocular sterilization and before povidone-iodine irrigation of the conjunctival sac. Group B (153 eyes) had identical sterilization but no lid scrubbing. Samples from the lid margin, liquid in the collecting bag, and aqueous humor were collected for bacterial culture. Primary outcome measures included the rate of positive bacterial culture for the above samples. The species of bacteria isolated were recorded.

Results: Group A and Group B each comprised 153 eyes. The positive rate of lid margin cultures was 54.24%. The positive rate of cultures for liquid in the collecting bag was significantly higher in Group A (23.53%) than in Group B (9.80%) (P=.001).The bacterial species cultured from the collecting bag in Group B were the same as those from the lid margin in Group A. The positive culture rate of aqueous humor in both groups was 0%.

Conclusion: Mechanical compression of the lid margin immediately before and during cataract surgery increased the risk for bacterial contamination of the surgical field, perhaps due to secretions from the lid margin glands.

Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Metabolism of mineral-sorbed organic matter depends upon microbial lifestyle in fluvial ecosystems

In fluvial ecosystems mineral erosion, carbon (C) and nitrogen (N) fluxes are linked via organo-mineral complexation, where dissolved organic molecules bind to mineral surfaces. Biofilms and suspended aggregates represent major aquatic microbial lifestyles whose relative importance changes predictably through fluvial networks. We tested how organo-mineral sorption affects aquatic microbial metabolism, using organo-mineral particles containing a mix of ¹³C, ¹⁵N-labelled amino acids. We traced ¹³C and ¹⁵N retention within biofilm and suspended aggregate biomass and its mineralisation. Organo-mineral complexation restricted C and N retention within biofilms and aggregates and also their mineralisation. This reduced the efficiency with which biofilms mineralise C and N by 30 % and 6 %. By contrast, organo-minerals reduced the C and N mineralisation efficiency of suspended aggregates by 41 % and 93 %. Our findings show how organo-mineral complexation affects microbial C:N stoichiometry, potentially altering the biogeochemical fate of C and N within fluvial ecosystems.

Microbial transformations in phosphonate biosynthesis and catabolism, and their importance in nutrient cycling

Phosphorus cycling in the biosphere has traditionally been thought to involve almost exclusively transformations of the element in its pentavalent oxidation state. Recent evidence, however, suggests that a significant fraction of environmental phosphorus may exist in a more reduced form. Most abundant of these reduced phosphorus compounds are the phosphonates, with their direct carbon–phosphorus bonds, and striking progress has recently been made in elucidating the biochemistry of microbial phosphonate transformations. These advances are now presented in the context of their contribution to our understanding of phosphorus biogeochemistry and of such diverse fields as the productivity of the oceans, marine methanogenesis and the discovery of novel microbial antimetabolites.

Organophosphonates revealed: new insights into the microbial metabolism of ancient molecules.

Organophosphonates are ancient molecules that contain the chemically stable C–P bond, which is considered a relic of the reducing atmosphere on primitive earth. Synthetic phosphonates now have a wide range of applications in the agricultural, chemical and pharmaceutical industries. However, the existence of C–P compounds as contemporary biogenic molecules was not discovered until 1959, with the identification of 2-aminoethylphosphonic acid in rumen protozoa. Here, we review advances in our understanding of the biochemistry and genetics of microbial phosphonate metabolism, and discuss the role of these compounds and of the organisms engaged in their turnover within the P cycle.