In order to estlrnate In n t u bactenal exoproteolysis and its contribution to bactenal secondary production (BSP), we studied microbial activities (BSP and kinetlcs of exoproteolytic activity) in relation to d~ssolved comblned amino acids (DCAA) dunng the survey of a n Atlantic marine pond In thls pond, high concentrations of DCAA (mean = 4 27 pM), probably generated by zooplankton grazing, allowed large bacterial biomass development and high production (max = 10" cells I-' and 18 8 X log cells 1-' d-' respectively) Exoproteolytic enzyme activity (mean V, = 55 9 pM d-l) was tightly coupled to bactenal production, suggesting that protein utlllzation was essential for bacterial growth Calculation, according to the Michaelis-Menten equation of the dissolved protein In sltu hydrolysis rates using the lunetic parameters (V, and K,,,) and the ambient concentratlon of DCAA proved to be a better est~rnation of the actual rate of hydrolytic activlty than the potential activlty based on V,,, only The rates found with thls calculat~on agreed wlth those obtalned from different environments with other methods (DCAA and labelled protelns uptake) Furthermore our estimates of In sltu proteolys~s were consistent with BSP (nitrogen fluxes onglnating from dissolved protein hydrolysis averaging 56% of the N bactenal production), and as a result, these estimates seem to provide a realistic estimation of the actual dissolved protein hydrolysis rates KEY WORDS: Bacterioplankton . In situ exoproteolytic hydrolysis rate . Kinet~c parameters . DCAA Manne pond
A method was developed to analyse virus-like particles (VLPs) in seston-rich waters and to quantify their dynamics in a coastal marsh of the Bay of Biscay, French Atlantic coast. The method combined clarification and concentration steps with electron microscopy to obtain information on particle abundance, type and size distribution (e.g. presence of tailed phages, Fuselloviridae, etc.). The mean recovery rates of T2-phages using this method were 71 to 79%, higher than other published rates. The transmission electron microscopy (TEM) counts were validated with T2 plaque lysis assay and epifluorescent (DAPI-stained) particle counting: the TEM method was valid for environmental particle concentrations above 1 to 2 × 10 6 VLP ml -1; TEM counts were lower than T2-plaque counts (TEM/lysis median = 0.293) but higher than DAPI counts (TEM/DAPI median = 2.39). The method was used to evaluate the coupling between viral and bacterial dynamics in a marsh pond during 2 months. The VLP abundance varied from 1 to 30 × 10 6 ml -1 and the viral population was dominated by small particles (20 to 64 nm). Tailed phages, identified as bacteriophages, were always less abundant than non-tailed VLPs (4 to 23% of total virus), yet their dynamics were better linked with bacterial development than those of total virus. Our results demonstrate that the best way to characterise bacterial lysis from virus in seston-rich coastal environments would be to study the dynamics of tailed phages and virus size-classes rather than the commonly applied total VLPs. KEY WORDS: Virus enumeration · Seston-rich water · TEM · DAPI · Virus-bacteria dynamics
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