Summary
Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In ...this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B‐DOM), the quality of which (as enrichment in humic or protein‐like substances) differed between strains. B‐DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B‐DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B‐DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.
The Mediterranean region is undergoing an increase in the frequency and intensity of extreme rainfall events, resulting in terrestrial runoffs that can affect aquatic environments in coastal regions. ...The goal of this study was to investigate the effects of terrestrial runoff on natural coastal planktonic assemblages. For this purpose, an
in situ
mesocosm experiment was conducted in May 2021 in the Mediterranean Thau Lagoon. A terrestrial runoff event was simulated in duplicate mesocosms by adding natural forest soil that was left to maturate naturally for two weeks in river water. After the addition of maturated soil, the abundance and diversity within the planktonic food web, from viruses to metazooplankton, were monitored for 18 days. The addition of maturated soil to the terrestrial runoff treatment greatly depressed the light availability in the mesocosms and potentially enhanced flocculation and sedimentation in the mesocosms, resulting in an immediate negative effect on phytoplankton, decreasing the chlorophyll-
a
(Chl-
a
) concentration by 70% for 12 days. Afterward, remineralized nutrient in the terrestrial runoff treatment induced a subsequent positive effect on phytoplankton, which resulted in a diatom bloom and an increase in picophytoplankton and cyanobacteria abundance toward the end of the experiment. Overall, the Chl-
a
concentration was 30% lower in the terrestrial runoff treatment over the 18 days of experiment, whereas bacteria were 15% more abundant than in the control. This suggests that over the course of the experiment, the addition of maturated soil favoured bacteria instead of phytoplankton at the base of the planktonic food web. The addition of the maturated soil was detrimental for all protozooplankton groups and mixotrophic dinoflagellates, but seemed to favour metazooplankton, notably mollusk larvae, copepod nauplii, and rotifers. This implies that in the terrestrial runoff treatment, the preferential pathway for biomass transfer was through the direct consumption of bacteria and/or phytoplankton by metazooplankton. Therefore, in Thau Lagoon, after a terrestrial runoff, the transfer of biomass within the planktonic food web would potentially be more efficient by promoting direct transfer from the base to the top of the food web, subsiding intermediate trophic levels such as protozooplankton.
The proteome of the marine bacterium Photobacterium angustum S14 was exposed to UVB and analyzed by the implementation of both the post-digest ICPL labeling method and 2D-DIGE technique using ...exponentially growing cells. A total of 40 and 23 proteins were quantified in all replicates using either the ICPL or 2D-DIGE methods, respectively. By combining both datasets from 8 biological replicates (4 biological replicates for each proteomics technique), 55 proteins were found to respond significantly to UVB radiation in P. angustum. A total of 8 UVB biomarkers of P. angustum were quantified in all replicates using both methods. Among them, the protein found to present the highest increase in abundance (almost a 3-fold change) was RecA, which is known to play a crucial role in the so-called recombinational repair process. We also observed a high number of antioxidants, transport proteins, metabolism-related proteins, transcription/translation regulators, chaperonins and proteases. We also discuss and compare the UVB response and global protein expression profiles obtained for two different marine bacteria with trophic lifestyles: the copiotroph P. angustum and oligotroph Sphingopyxis alaskensis.
Spatial increases and temporal shifts in outbreaks of gelatinous plankton have been observed over the past several decades in many estuarine and coastal ecosystems. The effects of these blooms on ...marine ecosystem functioning and particularly on the dynamics of the heterotrophic bacteria are still unclear. The response of the bacterial community from a Mediterranean coastal lagoon to the addition of dissolved organic matter (DOM) from the jellyfish
Aurelia aurita
, corresponding to an enrichment of dissolved organic carbon (DOC) by 1.4, was assessed for 22 days in microcosms (8 l). The high bioavailability of this material led to (i) a rapid mineralization of the DOC and dissolved organic nitrogen from the jellyfish and (ii) the accumulation of high concentrations of ammonium and orthophosphate in the water column. DOM from jellyfish greatly stimulated heterotrophic prokaryotic production and respiration rates during the first 2 days; then, these activities showed a continuous decay until reaching those measured in the control microcosms (lagoon water only) at the end of the experiment. Bacterial growth efficiency remained below 20 %, indicating that most of the DOM was respired and a minor part was channeled to biomass production. Changes in bacterial diversity were assessed by tag pyrosequencing of partial bacterial 16S rRNA genes, DNA fingerprints, and a cultivation approach. While bacterial diversity in control microcosms showed little changes during the experiment, the addition of DOM from the jellyfish induced a rapid growth of
Pseudoalteromonas
and
Vibrio
species that were isolated. After 9 days, the bacterial community was dominated by
Bacteroidetes
, which appeared more adapted to metabolize high-molecular-weight DOM. At the end of the experiment, the bacterial community shifted toward a higher proportion of
Alphaproteobacteria
. Resilience of the bacterial community after the addition of DOM from the jellyfish was higher for metabolic functions than diversity, suggesting that jellyfish blooms can induce durable changes in the bacterial community structure in coastal lagoons.
A total of 90 bacterial strains were isolated from the sea surface microlayer (i.e., bacterioneuston) and underlying waters (i.e., bacterioplankton) from two sites of the northwestern Mediterranean ...Sea. The strains were identified by sequence analysis, and growth recovery was investigated after exposure to simulated solar radiation. Bacterioneuston and bacterioplankton isolates were subjected to six different exposure times, ranging from 0.5 to 7 h of simulated noontime solar radiation. Following exposure, the growth of each isolate was monitored, and different classes of resistance were determined according to the growth pattern. Large interspecific differences among the 90 marine isolates were observed. Medium and highly resistant strains accounted for 41% and 22% of the isolates, respectively, and only 16% were sensitive strains. Resistance to solar radiation was equally distributed within the bacterioneuston and bacterioplankton. Relative contributions to the highly resistant class were 43% for gamma-proteobacteria and 14% and 8% for alpha-proteobacteria and the Cytophaga/Flavobacterium/Bacteroides (CFB) group, respectively. Within the gamma-proteobacteria, the Pseudoalteromonas and Alteromonas genera appeared to be highly resistant to solar radiation. The majority of the CFB group (76%) had medium resistance. Our study further provides evidence that pigmented bacteria are not more resistant to solar radiation than nonpigmented bacteria.
UVB oxidizes proteins through the generation of reactive oxygen species. One consequence of UVB irradiation is carbonylation, the irreversible formation of a carbonyl group on proline, lysine, ...arginine or threonine residues. In this study, redox proteomics was performed to identify carbonylated proteins in the UVB resistant marine bacterium Photobacterium angustum. Mass-spectrometry was performed with either biotin-labeled or dinitrophenylhydrazide (DNPH) derivatized proteins. The DNPH redox proteomics method enabled the identification of 62 carbonylated proteins (5% of 1221 identified proteins) in cells exposed to UVB or darkness. Eleven carbonylated proteins were quantified and the UVB/dark abundance ratio was determined at both the protein and peptide levels. As a result we determined which functional classes of proteins were carbonylated, which residues were preferentially modified, and what the implications of the carbonylation were for protein function. As the first large scale, shotgun redox proteomics analysis examining carbonylation to be performed on bacteria, our study provides a new level of understanding about the effects of UVB on cellular proteins, and provides a methodology for advancing studies in other biological systems.
Copepods are important suppliers of bioreactive compounds for marine bacteria through fecal pellet production, sloppy feeding, and the excretion of dissolved compounds. However, the interaction ...between copepods and bacteria in the marine environment is poorly understood. We determined the nitrogen and phosphorus compounds excreted by copepods fed with two natural size-fractionated diets (<20-µm and 20–150-µm) in the upwelling zone of central/southern Chile in late summer and spring. We then assessed the biogeochemical response of the bacterial community and its structure, in terms of total and active cells, to enrichment by copepod-excreted dissolved compounds. Results revealed that copepods actively excreted nitrogen and phosphorus compounds, mainly in the form of ammonium and dissolved organic phosphorus (DOP), reaching excretion rates of 2.6 and 0.05 µmol L-1 h-1, respectively. In both periods, the maximum excretion rates were associated with the 20–150-µm size fraction, but particularly during spring, when a higher organic matter quality was observed in excretion products compared to late summer. There were higher excretion rates of dissolved free amino acids (DFAAs) from copepods fed with the <20-µm size fraction, mainly histidine (HIS) in late summer and glutamic acid (GLU) in spring. A shift in the composition of the active bacterial community was observed between periods and treatments, which was associated with the response of the common seawater surface phyla Proteobacteria and Bacteroidetes. The specific bacterial activity (16S rRNA: rDNA) suggested a different response to the two size-fractionated diets. In late summer, Betaproteobacteria and Bacteroidetes were stimulated by the treatment enriched with excretion products derived from the 20–150-µm and <20-µm size fractions, respectively. In spring, Alphaproteobacteria were active in the treatment enriched with the excretion products of copepods fed with the <20-µm size fraction, whereas they were inhibited in the treatment enriched with excretion products in the 20–150-µm size fraction. Our findings indicate that different copepod diets can have a significant impact on the quantity and quality of their excretion compounds, which can subsequently generate shifts in the active bacterial composition. The bacterial response is probably associated with common-opportunistic sea surface microbes that are adapted to rapidly reacting to environmental offers.
Bacterial populations inhabiting the sea surface microlayer from two contrasted Mediterranean coastal stations (polluted vs. oligotrophic) were examined by culturing and genetic fingerprinting ...methods and were compared with those of underlying waters (50
cm depth), for a period of two years. More than 30 samples were examined and 487 strains were isolated and screened. Proteobacteria were consistently more abundant in the collection from the pristine environment whereas Gram-positive bacteria (i.e.,
Actinobacteria and
Firmicutes) were more abundant in the polluted site. Cythophaga-Flavobacter–Bacteroides (CFB) ranged from 8% to 16% of total strains. Overall, 22.5% of the strains showed a 16S rRNA gene sequence similarity only at the genus level with previously reported bacterial species and around 10.5% of the strains showed similarities in 16S rRNA sequence below 93% with reported species. The CFB group contained the highest proportion of unknown species, but these also included
Alpha- and
Gammaproteobacteria. Such low similarity values showed that we were able to culture new marine genera and possibly new families, indicating that the sea-surface layer is a poorly understood microbial environment and may represent a natural source of new microorganisms. Genetic fingerprinting showed, however, no consistent differences between the predominant bacterial assemblages from surface microlayer and underlying waters, suggesting that the presence of a stable and abundant neustonic bacterial community is not a common trait of coastal marine environments.
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