Abstract
Extracellular enzyme activities (EEA) are crucial components of microbial food web interactions and biogeochemical cycles in aquatic ecosystems. They also represent relevant biological traits in the ecophysiology of phytoplankton and other components of microbial plankton. To assess species-specific and (sub-)population-level characteristics of phytoplankton EEA at the single-cell level and close-to-in-situ conditions solely the enzyme labelled fluorescence (ELF)-based substrates have been used, because they become fluorescent and precipitate around the enzyme activity location upon enzymatic cleavage. However, the enzyme-labelled fluorescence alcohol (ELFA) standard is no longer commercially available, hence standard curves cannot be run anymore and single-cell phosphatase activity (SCPA) is no longer quantifiable. Therefore, we introduce a simple protocol for an ELFA standard do it yourself (DIY) production to enable quantifying microplankton SCPA again. This protocol is based on fluorescence measurements easily available to environmental enzyme activity laboratories, and it circumvents any need for chemical synthesis equipment and knowledge. The method is based on a controlled reaction of the ELF-phosphate (ELFP) substrate with commercially available alkaline phosphatase, which efficiently turns all the substrate into ELFA product. The ELFA product was dried out and dissolved again in dimethyl sulfoxide (DMSO) for storage. The ELFA concentration of that standard-to-be ELFA solution in DMSO was determined by linear regression between a low concentration dilution series of ELFA solution measured fluorimetrically and parallel measurements of a series of phosphatase-catalysed reactions at an overlapping ELFP concentration range. Finally, the fluorescence- and concentration-stable ELFA solution in DMSO with a known concentration constitutes the ELFA standard that is necessary to quantify bulk (fluorimeter) and single-cell (microscope and flow cytometer) phosphatase activity in microplankton.
Highlights
Enzyme activities are the means by which microorganisms accelerate chemical reactions within and around them to match the tempo of their biological demands of matter and energy
With bacteria accounting for a big share of enzyme activities in the environment, phytoplankton has been identified as the main contributor to phosphatase activity in many aquatic ecosystems (e.g. Refs. [1, 2])
We ran reactions at pH 7.9 because it is within the alkaline phosphatase stability range and it is safely below the threshold of pH 8 above which phosphatases might hydrolyse ELFP into enzyme-labelled fluorescence alcohol (ELFA) but precipitation and fluorescence of ELFA is importantly reduced by phenolic ionization [11]
Summary
Enzyme activities are the means by which microorganisms accelerate chemical reactions within and around them to match the tempo of their biological demands of matter and energy. Hydrolysis of organic molecules in the environment to make them bioavailable and redox transformation of inorganic nitrogen species are examples of microbial enzyme-catalysed reactions with an impact on biogeochemical cycles. Enzyme activities play a crucial role in microbial food web interactions by linking the organic matter size continuum to different microbes. With bacteria accounting for a big share of enzyme activities in the environment, phytoplankton has been identified as the main contributor to phosphatase activity in many aquatic ecosystems Enzyme kinetics and phosphatase activity is an important phytoplankton trait that might be worth
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