Biogeochemical control on fluorescent dissolved organic matter dynamics in a large freshwater lake (Lake Biwa, Japan)

Abstract

The dynamics of fluorescent dissolved organic matter (FDOM) in the large monomictic freshwater Lake Biwa (surface area 675 km 2 , maximum depth 104 m) was studied from December 2010 to December 2011. The proteinlike FDOM (FDOM T ) and dissolved organic carbon (DOC) showed epilimnetic accumulation (FDOM T from 4.42 6 0.22 quinine sulfate units [QSU] to 6.30 6 0.04 QSU; DOC from 80.8 6 2.7 mmol L 21 to 102.7 6 3.5 mmol L 21 ) between nutrient-replete winter mixing to nutrient-depleted stratified periods. This accumulation is attributed to the reduced heterotrophic activity following severe P-limitation. The positive correlation between accumulated DOC and FDOM T in the epilimnion and their uniform reduction in the hypolimnion (, 9%) suggest FDOM T as a proxy for semi-labile DOM. The humic-like FDOM (FDOM M ) generally increased with depth, a pattern similar to nutrients and total carbon dioxide (TCO 2 ), but adverse to dissolved oxygen. The significant positive correlations of FDOM M with apparent oxygen utilization (r 5 0.86, p , 0.001), TCO 2 (r 5 0.91, p , 0.001), nitrate (r 5 0.83, p , 0.001), and phosphate (r 5 0.76, p , 0.001) in the deeper layers suggest that FDOM M is formed during hypolimnetic mineralization. We estimated that , 8% of the organic carbon degraded in the hypolimnion is transferred into humic substances. The minor contribution of DOC (6.4%) to hypolimnetic mineralization suggests that production of humic substances is mainly fueled by the mineralization of sinking biogenic particles. The production and consumption of FDOM in freshwater lakes may influence the quality and bioavailability of carbon exported from these systems.

Vertical partitioning of freshwater bacterioplankton community in a deep mesotrophic lake with a fully oxygenated hypolimnion (Lake Biwa, Japan)

“…Another group of nitrifiers, Nitrospira, were also represented in the hypolimnion during the stratification (Figs 4 and 5), yet only sporadic reports are available on Nitrospira inhabiting the oxygenated water columns of freshwater lakes (Bel'kova et al, 2003;Tada and Grossart, 2014;Mukherjee et al, 2016). In Lake Biwa, nitrate accumulation in the hypolimnion throughout the stratified period with a constantly low ammonium concentration (order of nM) was reported (Kim et al, 2006;Thottathil et al, 2013). Although direct evidence is lacking, it is likely that the three nitrifiers were involved in nitrification in the water column of the oxygenated hypolimnion of the lake.…”

“…These results indicated that the thermocline separates the bacterioplankton community. In Lake Biwa, the strongly stratified mid‐summer epilimnion is characterized as the nutrient deficient (Kim et al ., ) and DOC‐rich (Maki et al ., ; Thottathil et al ., ) clear water phase. Such a severe condition may have selected groups that were acclimatized to this environment and resulted in characteristic communities in the epilimnion from August to October (Fig.…”

The Broad Habitat Spectrum of the CL500-11 Lineage (Phylum Chloroflexi), a Dominant Bacterioplankton in Oxygenated Hypolimnia of Deep Freshwater Lakes

“…On the other hand, CL500-11 MAGs harbor a high number of transporters for di- and oligopeptide, as well as transporters for spermidine/putrescein, and branched-chain amino acids ( Denef et al, 2016 ; Mehrshad et al, 2018 ). Such nitrogen-rich, protein-like DOM can be originated from remnants of bacterial cell walls ( McCarthy et al, 1998 ; Nagata et al, 2003 ) and membranes ( Tanoue et al, 1995 ), and consumption of such DOM by the microbial community in the oxygenated hypolimnion have been suggested by DOM profiling by stoichiometry ( Kim et al, 2006 ) and by excitation and emission matrices ( Thottathil et al, 2013 ; Hayakawa et al, 2016 ) in Lake Biwa. One CL500-11 MAG also included a transporter for N -acetylglucosamine, a breakdown product of bacterial cell walls ( Denef et al, 2016 ; Mehrshad et al, 2018 ).…”

Genome-resolved viral and cellular metagenomes revealed potential key virus-host interactions in a deep freshwater lake

“…In addition, we have previously unveiled the vertical stratification of bacterioplankton communities in deep freshwater lakes, where the oxygenated hypolimnion i.e., the deep aerobic water layer below the thermocline, which generally constitutes a volumetrically significant part of deep temperate, oligo-mesotrophic lakes was dominated by specific lineages that are uncommon in the epilimnion, including members of Chloroflexi (Okazaki et al, 2013, Planctomycetes, Nitrospira and Thaumarchaeota (Okazaki and Nakano, 2016;Okazaki et al, 2017). These lineages are likely responsible for important biogeochemical processes in the oxygenated hypolimnion, such as remineralization or conversion of semi-labile organic matter (Maki et al, 2010;Thottathil et al, 2013), nitrification (Small et al, 2013; However, despite a growing number of viromic studies in freshwater systems (Skvortsov et al, 2016;Moon et al, 2017;Silva et al, 2017;Arkhipova et al, 2018), viruses infecting other typical freshwater bacterioplankton lineages, such as LD12 (Ca. Fonsibacter) (Henson et al, 2018) and Limnohabitans (Kasalický et al, 2013) have yet to be characterized.…”