Identification and preventative treatment of overwintering cyanobacteria in sediments : a literature review

Abstract

Freshwaters can experience growths of toxin-producing cyanobacteria or harmful algal blooms (HABs). HAB-producing cyanobacteria can develop akinetes, which are thick-enveloped quiescent cells akin to seeds in vascular plants or quiescent colonies that overwinter in sediment. Overwintering cells produce viable “seed beds” for HAB resurgences and preventative treatments may diminish HAB intensity. The purpose of this literature review was to identify (1) environmental factors triggering germination and growth of overwintering cells, (2) sampling, identification, and enumeration methods, and (3) feasibility of preventative algaecide treatments. Conditions triggering akinete germination (light ≥0.5 μmol m-2s-1, temperature 22-27℃) differ from conditions triggering overwintering Microcystis growth (temperature 15-30℃, nutrients, mixing). Corers or dredges are used to collect surficial (0-2 cm) sediment layers containing overwintering cells. Identification and enumeration via microscopy are aided by dilution, sieving, or density separation of sediment. Grow-out studies simulate environmental conditions triggering cell growth and provide evidence of overwintering cell viability. Lines of evidence supporting algaecide efficacy for preventative treatments include (1) field studies demonstrating scalability and efficacy of algaecides against benthic algae, (2) data suggesting similar sensitivities of overwintering and planktonic Microcystis cells to a peroxide algaecide, and (3) a mesocosm study demonstrating a decrease in HAB severity following preventative treatments. This review informs data needs, monitoring techniques, and potential efficacy of algaecides for preventative treatments of overwintering cells.

Survival and Development Strategies of Cyanobacteria through Akinete Formation and Germination in the Life Cycle

“…The decrease in water temperatures from late autumn to winter leads to the occurrence of cell senescence and the formation of akinetes, which settle into the sediment. The akinetes overwinter in the sediment and germinate again the following spring, thus repeating the cycle of growth [56,90]. This year-round life cycle of akinete germination, development into vegetative cells, and subsequent akinete formation in temperate regions with large annual differences in water temperature demonstrates the connection between the water column (vegetative cell) and the sediment (akinetes) [56,90].…”

“…Although some cyanobacterial akinetes have been shown to germinate in the dark, albeit to a very small extent, in most experiments, akinetes did not germinate in the dark. Given that the germination of akinetes in the field begins in the sediment, which is subject to dilution and extinction of light through the water column, it is ecologically plausible to assume that germination occurs under low light conditions [56]. Further, within the range of low light intensities, an increase in light intensity is linked to an increase in photosynthesis in germinated cells, which may facilitate akinete germination.…”

“…The akinetes overwinter in the sediment and germinate again the following spring, thus repeating the cycle of growth [56,90]. This year-round life cycle of akinete germination, development into vegetative cells, and subsequent akinete formation in temperate regions with large annual differences in water temperature demonstrates the connection between the water column (vegetative cell) and the sediment (akinetes) [56,90]. This seasonal variation in the life cycle of cyanobacteria also allows us to recognize akinete formation and germination as a temporally (winter vs. spring) and spatially (water column vs. sediment) separated phenomena.…”

“…On the other hand, akinetes that have been deposited in shallow sediment environments provided with adequate light and temperature might have the potential to germinate readily into vegetative cells [45,57]. Shallow sediment areas, such as the littoral zone, can be hot spots for akinetes as well as other harmful cyanobacteria (e.g., Microcystis) to germinate or become active, enter the water column and develop into blooms [56,91]. From an annual life cycle perspective, this indicates that akinete formation and germination are seasonally decoupled phenomena but that they may be synchronized during exponential growth periods such as summer.…”

“…On the other hand, akinetes that have been deposited in shallow sediment environments provided with adequate light and temperature might have the potential to germinate readily into vegetative cells [45,57]. Shallow sediment areas, such as the littoral zone, can be hot spots for akinetes as well as other harmful cyanobacteria (e.g., Microcystis) to germinate or become active, enter the water column and develop into blooms [56,91].…”