The role of morphological changes in Microcystis adaptation to nutrient availability at the colonial level

Potential gap in understanding cyanoHABs: Light-dependent morphological variations in colonial cyanobacterium Microcystis

Colony formation is a crucial characteristic of Microcystis, a cyanobacterium known for causing cyanobacterial harmful algal blooms (cyanoHABs). It has been observed that as Microcystis colonies grow larger, they often become less densely packed, which correlates with a decrease in light penetration. The objective of this study was to investigate the effects of light limitation on the morphological variations in Microcystis, particularly in relation to the crowded cellular environment. The results indicated that when there was sufficient light (transmittance = 100 %) to support a growth rate of 0.11±0.01 day-1, a significant increase in colony size was found, from 466±15 μm to 1030±111 μm. However, under light limitation (transmittance = 50 % - 1 %) where the growth rate was lower than 0, there was no significant improvement in colony size. Microcystis in the light limitation groups exhibited a loose cell arrangement and even the presence of holes or pores within the colony, confirming the negative correlation between colony size and cell arrangement. This pattern is driven by regional differences in growth within the colony, as internal cells have a significantly lower frequency of division compared to peripheral cells, due to intra-colony self-shading (ICSS). The research demonstrates that Microcystis can adjust its cell arrangement to avoid excessive self-shading, which has implications for predicting and controlling cyanoHABs. These findings also contribute to the understanding of cyanobacterial variations and can potentially inform future research on the diverse phycosphere.

Unveiling a classical mutant in the context of the GH3 β-glucosidase family in Neurospora crassa

Classical fungal mutant strains obtained by mutagenesis have helped to elucidate fundamental metabolic pathways in the past. In the filamentous fungus Neurospora crassa, the gluc-1 strain was isolated long ago and characterized by its low level of β-glucosidase activity, which is essential for the degradation of cellulose, the most abundant biopolymer on Earth and the main polymeric component of the plant cell wall. Based on genomic resequencing, we hypothesized that the causative mutation resides in the β-glucosidase gene gh3-3 (bgl6, NCU08755). In this work, growth patterns, enzymatic activities and sugar utilization rates were analyzed in several mutant and overexpression strains related to gluc-1 and gh3-3. In addition, different mutants affected in the degradation and transport of cellobiose were analyzed. While overexpression of gh3-3 led to the recovery of β-glucosidase activity in the gluc-1 mutant, as well as normal utilization of cellobiose, the full gene deletion strain Δgh3-3 was found to behave differently than gluc-1 with lower secreted β-glucosidase activity, indicating a dominant role of the amino acid substitution in the point mutated gh3-3 gene of gluc-1. Our results furthermore confirm that GH3-3 is the major extracellular β-glucosidase in N. crassa and demonstrate that the two cellodextrin transporters CDT-1 and CDT-2 are essential for growth on cellobiose when the three main N. crassa β-glucosidases are absent. Overall, these findings provide valuable insight into the mechanisms of cellulose utilization in filamentous fungi, being an essential step in the efficient production of biorefinable sugars from agricultural and forestry plant biomass.

Insights from colony formation: The necessity to consider morphotype when assessing the effect of antibiotics on cyanobacteria

Colonial cyanobacteria have been identified as the primary contributor to the global occurrence of cyanobacterial harmful algal blooms (cyanoHABs), which are further intensified by the presence of "pseudo-persistent" antibiotics. Nevertheless, the impact of antibiotics on the growth and size of colonial cyanobacteria remains unclear. In this study, the response of cyanobacterium Microcystis to varying doses of antibiotics was assessed (0, 0.1, 0.5, 1, 10, and 50 μg L-1) by comparing the unicellular and colonial morphotypes. Interestingly, the morphological structure of cyanobacteria plays a significant role in their reaction to antibiotics. In comparison to the unicellular morphotype, the colonial morphotype exhibited a greater promotion in growth rate (11 %-22 %) to low doses of antibiotics and was less inhibited (-121 %--62 %) under high doses. Furthermore, antibiotics may affect the size of cyanobacterial colonies by disrupting the secretion of algal organic matter, which also exhibited a two-phase pattern. This work sheds light on the significance of methodology research involving both unicellular and colonial cyanobacteria. Future research and lake management should prioritize studying the morphological traits of cyanobacteria under different levels of antibiotic exposure. This approach may lead to novel strategies for predicting cyanoHABs under antibiotic pollution more effectively.

Colonial morphology weakens the response of different inorganic carbon uptake systems to CO2 levels in Microcystis population

Rising atmospheric CO2 concentration negatively impacts aquatic ecosystems and may induce evolutionary changes in the CO2-concentrating mechanism (CCM) of cyanobacteria. As the most notorious freshwater cyanobacteria, Microcystis strains have high phenotypic plasticity to form colonies and blooms in lakes and reservoirs worldwide. However, phenotypic plasticity of Microcystis responses to elevated CO2 is still a major open question. Here, we studied how Microcystis strains with two genotype of inorganic carbon uptake systems, bicA and sbtA, and different colonial morphology response to 200 ppm, 400 ppm, and 800 ppm CO2 levels. The results revealed that sbtA genotypes showed significantly higher specific growth rates, Chl a concentration, and photosynthetic efficiency at 200 ppm CO2, whereas higher specific growth rates, Chl a concentration, and photosynthetic efficiency were found in bicA genotype at 800 ppm CO2. The highest values of specific growth rates, Chl a concentration, Fv/Fm, and maximal net photosynthesis (Pm) were observed in unicellular morphology, followed by small colony and large colonial morphology at all CO2 levels. The values of K0.5 (DIC), K0.5 (CO2), and K0.5 (HCO3-) in the large colonials increased with rising CO2 levels, but these values significantly decreased in the unicellular and small colonials. ANOSIM analysis indicated that colonial morphology reduced significantly inter-group differences between bicA and sbtA genotypes at all CO2 treatments. These results suggest that colonial morphology of Microcystis can weakens the response of different inorganic carbon uptake systems to CO2 levels. Moreover, phenotypic and genotypic plasticity is likely to broaden strongly the fitness of Microcystis from rising atmospheric CO2.