Comparative assessment of algaecide performance on freshwater phytoplankton: Understanding differential sensitivities to frame cyanobacteria management.

Cyanobacterial bloom represent a growing threat to global water security. With fast proliferation, they raise great concern due to potential health and socioeconomic concerns. Algaecides are commonly employed as a mitigative measure to suppress and manage cyanobacteria. However, recent research on algaecides has a limited phycological focus, concentrated predominately on cyanobacteria and chlorophytes. Without considering phycological diversity, generalizations crafted from these algaecide comparisons present a biased perpective. To limit the collateral impacts of algaecide interventions on phytoplankton communities it is critical to understand differential phycological sensitivities for establishing optimal dosage and tolerance thresholds. This research attempts to fill this knowledge gap and provide effective guidelines to frame cyanobacterial management. We investigate the effect of two common algaecides, copper sulfate (CuSO4) and hydrogen peroxide (H2O2), on four major phycological divisions (chlorophytes, cyanobacteria, diatoms, and mixotrophs). All phycological divisions exhibited greater sensitivity to copper sulfate, except chlorophytes. Mixotrophs and cyanobacteria displayed the highest sensitivity to both algaecides with the highest to lowest sensitivity being observed as follows: mixotrophs, cyanobacteria, diatoms, and chlorophytes. Our results suggest that H2O2 represents a comparable alternative to CuSO4 for cyanobacterial control. However, some eukaryotic divisions such as mixotrophs and diatoms mirrored cyanobacteria sensitivity, challenging the assumption that H2O2 is a selective cyanocide. Our findings suggest that optimizing algaecide treatments to suppress cyanobacteria while minimizing potential adverse effects on other phycological members is unattainable. An apparent trade-off between effective cyanobacterial management and conserving non-targeted phycological divisions is expected and should be a prime consideration of lake management.

Phototactic microswimmers in pulsatile flow: Toward a novel harvesting method.

Phototactic behavior is coupled with pulsatile flow features to reveal the advantages of pulsation for separating motile algae cells in a double Y-microchannel. The underlying mechanism is as follows: during half of the pulsation cycle, when the flow rate is low, the phototactic microswimmers are mainly redirected by the external stimulation (light); while, during the rest of the cycle, the flow effects become dominant and the microswimmers are driven toward the desired outlet. The results show that in the absence of light source, the pulsatile flow has no advantage over the steady flow for separation, and the microswimmers have no preference between the exit channels; the separation index (SI) is around 50%. However, when the light is on, SI increases to 65% and 75% in the steady and pulsatile flows, respectively. Although the experiments are conducted on the well-known model alga, Chlamydomonas reinhardtii, a numerical simulation based on a simple model demonstrates that the idea can be extended to other active particles stimulated by an attractive or repulsive external field. Thus, the potential applications can go beyond algae harvesting to the control and enhancement of separation processes without using any mechanical component or chemical substance.

Cyanobacteria: Model Microorganisms and Beyond.

In this review, the general background is provided on cyanobacteria, including morphology, cell membrane structure, and their photosynthesis pathway. The presence of cyanobacteria in nature, and their industrial applications are discussed, and their production of secondary metabolites are explained. Biofilm formation, as a common feature of microorganisms, is detailed and the role of cell diffusion in bacterial colonization is described. Then, the discussion is narrowed down to cyanobacterium Synechocystis, as a lab model microorganism. In this relation, the morphology of Synechocystis is discussed and its different elements are detailed. Type IV pili, the complex multi-protein apparatus for motility and cell-cell adhesion in Synechocystis is described and the underlying function of its different elements is detailed. The phototaxis behavior of the cells, in response to homogenous or directional illumination, is reported and its relation to the run and tumble statistics of the cells is emphasized. In Synechocystis suspensions, there may exist a reciprocal interaction between the cell and the carrying fluid. The effects of shear flow on the growth, doubling per day, biomass production, pigments, and lipid production of Synechocystis are reported. Reciprocally, the effects of Synechocystis presence and its motility on the rheological properties of cell suspensions are addressed. This review only takes up the general grounds of cyanobacteria and does not get into the detailed biological aspects per se. Thus, it is substantially more comprehensive in that sense than other reviews that have been published in the last two decades. It is also written not only for the researchers in the field, but for those in physics and engineering, who may find it interesting, useful, and related to their own research.

A novel thin-film temperature and heat-flux microsensor for heat transfer measurements in microchannels.

Temperature and heat-flux measurement at the microscale for convective heat-transfer studies requires highly precise, minimally intrusive sensors. For this purpose, a new generic temperature and heat-flux sensor was designed, calibrated and tested. The sensor allows measurement of temperature and heat flux distributions along the direction of flow. It is composed of forty gold thermoresistances, 85 nm thick, deposited on both sides of a borosilicate substrate. Their sensitivities are about 37.8 µV K(-1), close to those of a K-type wire thermocouple. Using a thermoelectrical model, temperature biases due to the Joule effect were calculated using the current crossing each thermoresistance and the heat-transfer coefficient. Finally, heat-transfer measurements were performed with deionized water flowing in a straight PDMS microchannel for various Reynolds numbers. The Nusselt number was obtained for microchannels of 50 to 10 µm span. The results were found to be in good agreement with classical Nu-Re macroscopic correlations.