Effects of zooplankton carcasses degradation on freshwater bacterial community composition and implications for carbon cycling

Abstract

Summary Non‐predatory mortality of zooplankton provides an abundant, yet, little studied source of high quality labile organic matter (LOM) in aquatic ecosystems. Using laboratory microcosms, we followed the decomposition of organic carbon of fresh 13C‐labelled Daphnia carcasses by natural bacterioplankton. The experimental setup comprised blank microcosms, that is, artificial lake water without any organic matter additions (B), and microcosms either amended with natural humic matter (H), fresh Daphnia carcasses (D) or both, that is, humic matter and Daphnia carcasses (HD). Most of the carcass carbon was consumed and respired by the bacterial community within 15 days of incubation. A shift in the bacterial community composition shaped by labile carcass carbon and by humic matter was observed. Nevertheless, we did not observe a quantitative change in humic matter degradation by heterotrophic bacteria in the presence of LOM derived from carcasses. However, carcasses were the main factor driving the bacterial community composition suggesting that the presence of large quantities of dead zooplankton might affect the carbon cycling in aquatic ecosystems. Our results imply that organic matter derived from zooplankton carcasses is efficiently remineralized by a highly specific bacterial community, but does not interfere with the bacterial turnover of more refractory humic matter.

Zooplankton carcasses stimulate microbial turnover of allochthonous particulate organic matter

“…1 ) and a very efficient utilisation of daphnia-derived OM (Fig. 2 ) supported our previous findings that daphnia carcasses are a bioavailable source of OM [ 37 ], which can easily be accessed and utilised by the microbial community. Trends in respiration were consistent with the microbial communities initially utilising OM derived from daphnia carcasses and progressively shifting their energy demands towards the less bioavailable leaf material.…”

“…Zooplankton carcasses represent hotspots of OM turnover, providing the microbial community with a growth structure (i.e., the chitin exuviae) facilitating attachment and colonisation, and a diversity of nutrient-rich OM compounds [ 32 , 35 , 36 ]. Seasonal fluctuations in zooplankton-derived OM trigger shifts in microbial community composition [ 37 ], but whether it can stimulate the degradation of less bioavailable, leaf-derived OM remains unknown.…”

Microbial Processing of Jellyfish Detritus in the Ocean

“…By coupling taxonomic profiling of our metagenomic data with microscopy-based tracking of individual metabolically active bacterial populations of predominant MAGs, we show that the DOM fraction of A. aurita detritus can be degraded by a simple consortium composed of only three dominant gammaproteobacterial populations, Pseudoalteromonas, Alteromonas, and Vibrio (Supplementary Figure S4). This suggests that an abundant source of high quality and bioavailable DOM reduces the biodiversity of bacteria by favoring a small number of copiotrophs dominating the community (Kolmakova et al, 2019). These opportunistic populations accounted for >90% of all metabolically active (both respiring and HPG incorporating) bacteria in the jellyfish-degrading community and rapidly consumed almost the entire pool of jellyfish proteins (>98%), amino acids (∼70%) and jelly-DOC within ∼1.5 d, indicating a rapid turnover of jellyfish-DOM, including soluble proteins (Figures 1, 3 and Supplementary Figure S5, and Supplementary Table S5).…”

The importance of jellyfish–microbe interactions for biogeochemical cycles in the ocean

“…The detritosphere of both phytoplankton (Pomeroy and Deibel 1980) and crustacean and noncrustacean (rotifiers) zooplankton (Fukami et al 1985;Tang et al 2009;Bickel and Tang 2010;Bickel et al 2014) was studied. However, most studies focused on the succession of bacterioplankton populations over seasonal cycles including the dynamics after the decay of phytoplankton blooms (Teeling et al 2012(Teeling et al , 2016Buchan et al 2014;Needham and Fuhrman 2016), while bacterioplankton succession patterns following the decay of zooplankton blooms are scarcely reported (Bickel et al 2014;Kolmakova et al 2019). The microbiome of gelatinous zooplankton (Tinta et al 2019;Jaspers et al 2019 and references therein) was investigated and to some extent also the changes of the ambient bacterioplankton community composition during gelatinous zooplankton blooms (Riemann et al 2006;Dinasquet et al 2012aDinasquet et al ,b, 2013 were studied.…”