Biology subjects

Chaffin, J. D.

Publications and source records attributed to Chaffin, J. D..

4 recordsLinked to original sources

bioRxiv2026

The diel light cycle modulates microcystin content in continuous cultures and natural communities of Microcystis: Implications for bloom monitoring

Harmful algal blooms dominated by Microcystis spp. threaten fisheries, recreation, and drinking-water supplies because many strains produce the hepatotoxin microcystin. Monitoring programs compare microcystin concentrations among samples collected at different times of day, yet the extent that diel variation affects these estimates remains poorly resolved. We tested whether microcystin concentrations varied over a diel cycle under laboratory conditions and evaluated the effect of short-term sunlight exposure on a natural bloom community. Axenic Microcystis aeruginosa PCC 7806 was maintained in cyclostats with controlled nutrients and temperature under a 16 h light:8 h dark cycle. Microcystin concentrations normalized to culture volume (g L-1) and biovolume (ag m-E-3 were significantly higher (p [≤] 0.05) at the end of the dark period (64 {+/-} 8 g MC L-1; 630 {+/-} 40 ag MC m-3) than at the end of the light period (45 {+/-} 7 g MC L-1; 390 {+/-} 60 ag MC m-3), and this pattern was reproducible across consecutive days. Cell abundance and photosynthetic efficiency remained constant, indicating variation was not explained by changes in growth or photosynthetic performance. In parallel, water from a Lake Erie Microcystis bloom was exposed to sunlight or paired dark controls, and microcystin concentrations declined ~19% after just 60 min of sunlight exposure (p = 0.048). Together, these results demonstrate that microcystin measurements can vary predictably across the diel cycle and that light contributes to daytime declines. Although the underlying mechanism(s) remain unresolved, time of day should be considered, or standardized, when monitoring microcystins.

microbiology↗

bioRxiv2025

The ornithine-arginine cycle supported a toxic, metalimnic Planktothrix rubescens bloom

Planktothrix rubescens is distinct from other cyanobacterial harmful algal bloom (cHAB) genera: the crimson-red cHAB thrives in the cold, low-light, nutrient-limited metalimnion. Studies have attributed this ecological success to buoyancy regulation, low-light adaptations, and the uptake of nitrogen-rich amino acids. Yet, it remains to be mechanistically determined how this cHAB maintains physiological nutrient quotas in the metalimnion due to limited in situ molecular studies. We employed metagenomics and metabolomics to investigate a toxigenic P. rubescens bloom in Meads Quarry (Knoxville, TN, USA) observed in two separate years. Our results suggest a perennial, metalimnic P. rubescens population may exist, with spring turnover facilitating seasonal migration to the epilimnion. Although P. rubescens dominated the epilimnion and metalimnion, intracellular metabolite pools grouped by depth and suggested depth-discrete partitioning of the arginine deiminase-mediated ornithine-arginine cycle (OAC, i.e., urea cycle). While the arginine influx driving the OAC is unclear, we hypothesize this input is provided via the uptake of urea or nitrogen-rich amino acids. Further, we demonstrate arginine deiminase (argE) is broadly distributed in Planktothrix genera and known microcystin producers, suggesting argE-mediated arginine recycling via the OAC may influence the fitness of toxigenic cHAB genera which require ample nitrogen to synthesize microcystins. Cumulatively, our results serve as a case study to provide insight on the metabolic pathways driving the ecological success of metalimnic P. rubescens blooms. On a broader scale, this work strengthens the case that alternative nitrogen metabolism - including urea utilization, amino acid uptake, and the OAC - is a driver of toxigenic cyanobacterial blooms in fresh waters.

bioinformatics↗

bioRxiv2024

Mitomycin C eliminates cyanobacterial transcription without detectable lysogen induction in a Microcystis-dominated bloom in Lake Erie

Although evidence indicates that viruses are important in the ecology of Microcystis spp., many questions remain. For example, how does Microcystis exist at high, bloom-associated cell concentrations in the presence of viruses that infect it? The phenomenon of lysogeny and associated homoimmunity offer possible explanations to this question. Virtually nothing is known about lysogeny in Microcystis, but a metatranscriptomic study suggests that widespread, transient lysogeny is active during blooms. These observations lead us to posit that lysogeny is important in modulating Microcystis blooms. Using a classic mitomycin C-based induction study, we tested for lysogeny in a Microcystis-dominated community in Lake Erie in 2019. Treated communities were incubated with 1 mg L-1 mitomycin C for 48 h alongside unamended controls. We compared direct counts of virus-like-particles (VLPs) and examined community transcription for active infection by cyanophage. Mitomycin C treatment did not increase VLP count. Mitomycin C effectively eliminated transcription in the cyanobacterial community, while we detected no evidence of induction. Metatranscriptomic analysis demonstrated that the standard protocol of 1 mg L-1 was highly-toxic to the cyanobacterial population, which likely inhibited induction of any prophage present. Follow-up lab studies indicated that 0.1 mg L-1 may be more appropriate for use in freshwater cyanobacterial studies. These findings will guide future efforts to detect lysogeny in Microcystis blooms. ImportanceHarmful algal blooms dominated by Microcystis spp. occur throughout the worlds freshwater ecosystems leading to detrimental effects on ecosystem services that are well documented. After decades of research, the scientific community continues to struggle to understand the ecology of Microcystis blooms. The phenomenon of lysogeny offers an attractive, potential explanation to several ecological questions surrounding blooms. However, almost nothing is known about lysogeny in Microcystis. We attempted to investigate lysogeny in a Microcystis bloom in Lake Erie and found that the standard protocols used to study lysogeny in aquatic communities are inappropriate for use in Microcystis studies, and perhaps freshwater cyanobacterial studies more broadly. This work can be used to design better methods to study the viral ecology of Microcystis blooms.

microbiology↗

bioRxiv2023

Declines in ice cover induce light limitation in freshwater diatoms

The rediscovery of diatom blooms embedded within and beneath Lake Erie ice cover (2007-2012) ignited an intense interest in psychrophilic adaptations and winter limnology. Subsequent studies determined ice plays a vital role in winter diatom ecophysiology, as diatoms partition to the underside of ice thereby fixing their location within the photic zone. Yet, climate change has led to widespread ice decline across the Great Lakes, with Lake Erie presenting a nearly ice-free state in several recent winters. It has been hypothesized the resultant turbid, isothermal water column will induce light limitation amongst winter diatoms, serving as a detrimental competitive disadvantage. Here, we conducted a physiochemical and metatranscriptomic survey of the winter Lake Erie water column (2019-2020) that spanned spatial, temporal, and climatic gradients to investigate this hypothesis. We determined ice-free conditions decreased diatom bloom magnitude and altered diatom community composition. Diatoms increased the expression of various photosynthetic genes and iron transporters, suggesting they are attempting to increase their quantity of photosystems and light-harvesting components (a well-defined indicator of light limitation). Notably, we identified two gene families which serve to increase diatom fitness in the turbid ice-free water column: proton-pumping rhodopsins (a second means of light-driven energy acquisition) and fasciclins (a means to "raft" together to increase buoyancy and co-locate to the surface to optimize light acquisition). With large-scale climatic changes already underway, our observations provide insight into how diatoms respond to the dynamic ice conditions of today and shed light on how they will fare in a climatically altered tomorrow.

ecology↗