Biology subjects

Hellweger, F. L.

Publications and source records attributed to Hellweger, F. L..

4 recordsLinked to original sources

bioRxiv2023

Cool temperature acclimation in toxigenic Microcystis aeruginosa PCC 7806 and its non-toxigenic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26{degrees} C to 19{degrees} C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 {Delta}mcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19{degrees} C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents. ImportanceThrough comparisons of a microcystin-producing wildtype strain M. aeruginosa PCC 7806 and a non microcystin-producing mutant, M. aeruginosa PCC 7806{Delta} mcyB, our observations highlight defensive (microcystin production) vs active (production of degradation enzymes and increased electron sinks) strategies for dealing with cold-temperature induced oxidative stress as well as associated physiological changes. This work increases our understanding of microcystins intracellular function, and the role it may play in bloom ecology. In combination with other studies, this work begins to experimentally establish a mechanistic foundation to better understand cold-to-warm seasonal transitions from toxigenic to non-toxigenic strains frequently observed in situ.

microbiology↗

bioRxiv2023

Genome reduction occurred in early Prochlorococcus with an unusually low effective population size

In the oligotrophic sunlit ocean, the most abundant free-living planktonic bacterial lineages evolve convergently through genome reduction. The cyanobacterium Prochlorococcus responsible for 10% global oxygen production is a prominent example. The dominant theory known as genome streamlining posits that they have extremely large effective population sizes (Ne) such that selection for metabolic efficiency acts to drive genome reduction. Because genome reduction largely took place anciently, this theory builds on the assumption that their ancestors Ne was similarly large. Constraining Ne for ancient ancestors is challenging because experimental measurements of extinct organisms are impossible and alternatively reconstructing ancestral Ne with phylogenetic models gives large uncertainties. Here, we develop a new strategy that leverages agent-based modeling to simulate the change of Ne proxy for ancient ancestors, the genome-wide ratio of radical to conservative nonsynonymous nucleotide substitution rate (dR/dC), in response to the change of Ne. Surprisingly, this proxy shows expected increases with decreases of Ne only when Ne falls to about 10k - 100k or lower, magnitudes characteristic of Ne of obligate endosymbiont species where drift drives genome reduction. We therefore conclude that drift, rather than selection, is the primary force that drove Prochlorococcus genome reduction.

genetics↗

bioRxiv2022

Differences in the regulatory strategies of marine oligotrophs and copiotrophs reflect differences in motility

Aquatic bacteria frequently are divided into lifestyle categories oligotroph or copiotroph. Oligotrophs have proportionately fewer transcriptional regulatory genes than copiotrophs and are generally non-motile/chemotactic. We hypothesized that the absence of chemotaxis/motility in oligotrophs prevents them from occupying nutrient patches long enough to benefit from transcriptional regulation. We first confirmed that marine oligotrophs are generally reduced in genes for transcriptional regulation and motility/chemotaxis. Next, using a non-motile oligotroph (Ca. Pelagibacter st. HTCC7211), a motile copiotroph (Alteromonas macleodii st. HOT1A3), and [14C]L-alanine, we confirmed that L-alanine catabolism is not transcriptionally regulated in HTCC7211 but is in HOT1A3. We then found that HOT1A3 took 2.5-4 min to initiate L-alanine oxidation at patch L-alanine concentrations, compared to <30s for HTCC7211. By modeling cell trajectories, we predicted that, in most scenarios, non-motile cells spend <2 min in patches, compared to >4 mins for chemotactic/motile cells. Thus, the time necessary for transcriptional regulation to initiate prevents transcriptional regulation from being beneficial for non-motile oligotrophs. This is supported by a mechanistic model we developed, which predicted that HTCC7211 cells with transcriptional regulation of L-alanine metabolism would produce 12% of their standing ATP stock upon encountering an L-alanine patch, compared to 880% in HTCC7211 cells without transcriptional regulation.

microbiology↗

bioRxiv2021

SAR11 Cells Rely on Enzyme Multifunctionality to Transport and Metabolize a Range of Polyamine Compounds

In the ocean surface layer and cell culture, the polyamine transport protein PotD of SAR11 bacteria is often one of the most abundant proteins detected. Polyamines are organic cations produced by all living organisms and are thought to be an important component of dissolved organic matter (DOM) produced in planktonic ecosystems. We hypothesized that SAR11 cells transport and metabolize multiple polyamines and use them as sources of carbon and nitrogen. Metabolic footprinting and fingerprinting were used to measure the uptake of five polyamine compounds (putrescine, cadaverine, agmatine, norspermidine, and spermidine) in two SAR11 strains that represent the majority of SAR11 cells in the surface ocean environment, Ca. Pelagibacter st. HTCC7211 and C. P. ubique st. HTCC1062. Both strains transported all five polyamines and concentrated them to micromolar or millimolar intracellular concentrations. Both strains could use most of the polyamines to meet their nitrogen requirements, but we did not find evidence of use as carbon sources. We propose potABCD transports cadaverine, agmatine, and norspermidine, in addition to its usual substrates of spermidine and putrescine, and that spermidine synthase, speE, is reversible, catalyzing the breakdown of spermidine and norspermidine, in addition to its usual biosynthetic role. These findings provide support for the hypothesis that enzyme multifunctionality enables streamlined cells in planktonic ecosystems to increase the range of DOM compounds they oxidize. ImportanceGenome streamlining in SAR11 bacterioplankton has resulted in a small repertoire of genes, yet paradoxically they consume a substantial fraction of primary production in the oceans. Enzyme multifunctionality is hypothesized to be an adaptation that increases the range of organic compounds oxidized by cells in environments where selection favors genome minimization. We provide experimental support for this hypothesis by demonstrating that SAR11 cells use multiple polyamine compounds and propose that a small set of multifunctional genes catalyze this metabolism. We also report polyamine uptake rates can exceed metabolism, resulting in high intracellular concentrations of these nitrogen-rich compounds and an increase in cell size. Increases in cytoplasmic solute concentrations during transient episodes of high nutrient exposure has previously been observed in SAR11 cells and may be a feature of their strategy for maximizing the share of labile DOM acquired when in competition with other cell types.

microbiology↗