Scientists have found that a coastal ecosystem that experiences periodic phytoplankton blooms appears to have two distinct bacterial extracellular hydrolytic systems linked to shifts in bacterial community structure.
The team at University of the Basque Country linked changes in kinetic parameters to shifts in bacterial community structure and phytoplankton groups during a three-year study.
Their findings were published in ‘Kinetic modulation of bacterial hydrolases by microbial community structure in coastal waters” in Environmental Microbiology and Environmental Microbiology Reports, which are both Applied Microbiology International publications.
Corresponding author Dr Naiara Abad said the team had performed a time-series analysis of frequently studied hydrolases - including leucine aminopeptidase, β- and α-glucosidases - in a coastal ecosystem that suggested the existence of two distinct bacterial extracellular hydrolytic systems operating at different substrate concentrations and whose seasonal characteristics were linked to the shifts in bacterial community structure and phytoplankton groups.
They examined the waters near the coastal station in Armintza in the southeastern part of the Bay of Biscay.
”Heterotrophic bacteria play a key role in marine systems by synthesising and releasing extracellular enzymes that hydrolyse high-molecular-weight organic matter from various sources, including phytoplankton production, viral lysis, grazing, and terrestrial inputs. Investigating their kinetic parameters is crucial to understand the relationship between enzyme expression and organic matter availability,” she said.
“In our coastal station, seasonal phytoplankton blooms intermittently alter the supply of organic matter and promote the replacement of specific members of the bacterial community. Therefore, we proposed that these environmental disruptions will ultimately lead to changes in the kinetic parameters of bacterial hydrolases.”
The team made use of various mathematical models to fit enzymatic data and determine the kinetic parameters of bacterial hydrolases, revealing the existence of two enzymatic systems that operated concurrently throughout the study period, with one being more prevalent at low substrate concentrations and the other at high substrate concentrations.
They also constructed ecological networks by examining both contemporaneous and time-delayed correlations.
Shift in communities
The results demonstrated that the springtime eukaryotic bloom maximises resource availability, leading to the growth of bacterial groups such as Bacteroidetes, Gammaproteobacteria, and Roseobacter that produce hydrolases that are responsive to high substrate concentrations. In contrast, the summertime cyanobacterial bloom favours the proliferation of bacterial members with lower hydrolytic capacity, such as SAR11.
“Our findings are innovative because kinetic studies are uncommon in marine environments, and we successfully connected changes in kinetic parameters to shifts in bacterial community structure and phytoplankton groups during our three-year investigation,” Dr Abad said.
“Moreover, our study highlights the significance of employing complex models to determine the kinetic parameters of bacterial hydrolases in natural ecosystems. This challenges the conventional belief that enzymatic reactions in natural ecosystems fit the Michaelis-Menten equation.
“As a next step, we could isolate and purify hydrolases from natural samples obtained during ecologically significant periods of the year to further expand upon our discovery of concurrent enzymatic systems. Such an approach could enhance our ability to estimate the affinity of hydrolases with greater accuracy, including the application of the sigmoidal model.”
This work was supported by projects co-financed by the Ministry of Science and Innovation of the Spanish Government and European FEDER funds, European Union—NextGenerationEU funds through the UPV/EHU, the Basque Government and by the University of Basque Country.