11/5/2023 0 Comments Dead cellsUsing the Internet as an analogy, we could say that cell-bound EEA would be the equivalent of a “wired” Internet connection, whereas cell-free EEA would be a “wireless” connection (i.e., would still provide the end product – data transfer/hydrolysate – even if not physically connected to the hardware/cell). The total EEA is the result of the combination of cell-associated and cell-free enzymes ( Figure Figure1 1). The Living and the ‘Living Dead’: Cell-Attached versus Cell-Free EEAĮxtracellular enzymes exist in two forms cell-bound (i.e., cell-attached), and dissolved (i.e., cell-free, operationally defined as passing through a 0.22 μm filter). Nevertheless, extracellular enzymatic activities (EEAs) are found from epipelagic to bathypelagic waters, commonly observing an increasing ratio of EEA to cell abundance with depth ( Hoppe and Ullrich, 1999 Hoppe et al., 2002 Baltar et al., 2009). Yet, not all high molecular weight DOM can be transported by this mechanism (i.e., biochemical and microbiological studies suggest that the binding/hydrolysis/transport is very selective for specific polysaccharides), and that mechanism still involve extracellular hydrolysis – but the binding proteins hang onto the pieces, such that they are transported into the cell with no loss to the external environment ( Cuskin et al., 2015). However, an alternative polysaccharide uptake mechanism of bacteria was recently revealed, which allows them to directly incorporate large molecular weight DOM compounds ( Cuskin et al., 2015 Reintjes et al., 2017). For that purpose they use extracellular enzymes so due to the central role of those enzymes they are referred to as the ‘gatekeepers’ of the C cycle ( Arnosti, 2011). But, that food selectivity comes at a price: heterotrophic prokaryotes will need to hydrolyze most of those molecules into subunits small enough to be incorporated, because most molecules need to be smaller than 600 Da to pass through the prokaryotic cell wall ( Weiss et al., 1991). According to the “size-reactivity” model, heterotrophic microbes preferentially degrade high molecular weight dissolved organic matter (DOM) because it tends to be more bioavailable than the low molecular weight DOM ( Benner and Amon, 2015). When it comes to the consumption of organic matter for transformation and recycling, microbes seem to have a preference for specific types of organic matter. Thus, if we aim to understand what the future of marine biogeochemical cycling is going to be, we need to understand what the fate of microbes, and their enzymes, will be. We live in a time of change, and anthropogenic impacts can alter the structure and functioning of marine microbial communities, and consequentially, the role of the ocean in the global biogeochemical cycles. These tiny organisms have the set of core genes coding for the enzymes of the major reactions responsible for transforming energy and matter into (usable) substrates essential for life ( Falkowski et al., 2008). Microbes are the engines driving Earth’s biogeochemical cycles ( Falkowski et al., 2008). The marine environment plays a critical role in global biogeochemical cycles ( Daily, 2003 Harley et al., 2006 Hutchins and Fu, 2017). Importance of Microbes and Their Extracellular Enzymatic Activities (EEA) This perspective article advocates the need to go “beyond the living things,” studying the response of cells/organisms to different stressors, but also to study cell-free enzymes, in order to fully constrain the future and evolution of marine biogeochemical cycles. This article aims to place cell-free EEA into the wider context of hydrolysis of organic matter, deal with recent studies assessing what controls the production, activity and lifetime of cell-free EEA, and what their fate might be in response to environmental stressors. In contrast, cell-free enzymes belong to a kind of “living dead” realm because they are not attached to a living cell but still are able to perform their function away from the cell and as such, the factors controlling their activity and fate might differ from those affecting cell-attached enzymes. Since cell-attached EEAs are linked to the cell, their fate will likely be linked to the factors controlling the cell’s fate. Although we are learning more about how microbial diversity and function (including total EEA) will be affected by environmental changes, little is known about what factors control the importance of the abundant cell-free enzymes. Cell-free enzymes make up a substantial proportion (up to 100%) of the total marine EEA. The total EEA is the sum of cell-bound (i.e., cell-attached), and dissolved (i.e., cell-free) enzyme activities. Microbial extracellular enzymatic activities (EEAs) are the “gatekeepers” of the carbon cycle. Microbes are the engines driving biogeochemical cycles.
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