Abstract
Metacaspases are distant homologs of metazoan caspase proteases, implicated in stress response, and programmed cell death (PCD) in bacteria and phytoplankton. While the few previous studies on metacaspases have relied on cultured organisms and sequenced genomes, no studies have focused on metacaspases in a natural setting. We here present data from the first microbial community-wide metacaspase survey; performed by querying metagenomic and metatranscriptomic datasets from the brackish Baltic Sea, a water body characterized by pronounced environmental gradients and periods of massive cyanobacterial blooms. Metacaspase genes were restricted to ~4% of the bacteria, taxonomically affiliated mainly to Bacteroidetes, Alpha- and Betaproteobacteria and Cyanobacteria. The gene abundance was significantly higher in larger or particle-associated bacteria (>0.8 μm), and filamentous Cyanobacteria dominated metacaspase gene expression throughout the bloom season. Distinct seasonal expression patterns were detected for the three metacaspase genes in Nodularia spumigena, one of the main bloom-formers. Clustering of normalized gene expression in combination with analyses of genomic and assembly data suggest functional diversification of these genes, and possible roles of the metacaspase genes related to stress responses, i.e., sulfur metabolism in connection to oxidative stress, and nutrient stress induced cellular differentiation. Co-expression of genes encoding metacaspases and nodularin toxin synthesis enzymes was also observed in Nodularia spumigena. The study shows that metacaspases represent an adaptation of potentially high importance for several key organisms in the Baltic Sea, most prominently Cyanobacteria, and open up for further exploration of their physiological roles in microbes and assessment of their ecological impact in aquatic habitats.
Highlights
Caspases are proteases that initiate targeted protein degradation and the execution of apoptotic programmed cell death (PCD) pathways in metazoan tissues (Elmore, 2007; McIlwain et al, 2013)
We will refer to these putative proteases as “metacaspases.” Recently, computational analysis of fully sequenced prokaryotic genomes showed that ∼19% of sequenced bacteria carry metacaspase genes, in particular Alphaproteobacteria, Deltaproteobacteria, and Cyanobacteria (Asplund-Samuelsson et al, 2012)
The present study on metacaspases in the Baltic Sea is based on three sets of samples
Summary
Caspases are proteases that initiate targeted protein degradation and the execution of apoptotic programmed cell death (PCD) pathways in metazoan tissues (Elmore, 2007; McIlwain et al, 2013). At the turn of the century, proteins displaying sequence homology to caspases were identified in non-metazoan eukaryotes such as protists, plants, and fungi, and in bacteria (Aravind et al, 1999; Uren et al, 2000) Such caspase homologs are classified and named according to their subdomain organization, activation mechanism, and proteolytic target specificity (Asplund-Samuelsson, 2015). The highest metacaspase frequency (up to 12 metacaspase genes per genome) was found in filamentous multicellular Cyanobacteria capable of cell differentiation, phylogenetically belonging to Clade 1 Cyanobacteria (Larsson et al, 2011), such as the genera Trichodesmium, Nostoc, and Anabaena This is in contrast to small, marine, and globally wide-spread unicellular cyanobacteria of the genera Synechococcus and Prochlorococcus, which largely lack these genes (Bidle and Falkowski, 2004; Jiang et al, 2010; Asplund-Samuelsson et al, 2012). Other bacteria commonly carry only one or two metacaspase genes, if any
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call