Changes in photosynthesis in response to combined irradiance and temperature stress in cyanobacterial surface waterblooms

Buoyant cyanobacteria, previously mixed throughout the water column, float to the lake surface and form a surface waterbloom when mixing subsides. At the surface, the cells are exposed to full sunlight, and this abrupt change in photon irradiance may induce photoinhibition; at the same time, temperature rises as well. This study investigated the damaging effects of this increase in temperature as well as the ecologically more relevant combination of both an increased temperature and a high photon irradiance. Analysis of surface blooms with oxygen microelectrodes showed that integrated oxygen contents that are dependent on the balance of photosynthetic oxygen evolution and respiratory oxygen uptake decreased when temperature was raised about the lake temperature. Gross rates of photosynthesis were unaffected by temperatures up to of 35 degrees C; hence, a moderate increase in temperature mainly stimulated oxygen uptake. Preincubation of cells of the cyanobacterium Anabaena flos-aquae (Lyngb.) de Brebisson at temperatures up to 35 degrees C did not affect the subsequent measurement of rates of net photosynthesis. Another 5 degrees C rise in temperature severely damaged the photosynthetic apparatus. Failure to restore net rates of photosynthesis was coupled to a strong quenching of the ratio of variable to maximum fluorescence, F-v/F-m, that was the result of a rise in F-0. A combination of high temperature and high photon irradiance was more damaging than high temperature alone. In contrast, low photon irradiances offered substantial protection against heat injury of the photosynthetic apparatus . I conclude from this study that because cyanobacteria usually are acclimated to low average irradiance prior to bloom formation, there is a reasonable risk of chronic photoinhibition. The increase in temperature will enhance the photodamage of cells in the top layer of the bloom. Low photon irradiances in subsurface layers will offer protection against heat injury. If the high temperatures extend to the deepest, dark layers of the bloom, damage in those layers is likely to occur

The Principle and Value of the European Multi Lake Survey

On-going global warming and eutrophication are expected to promote cyanobacterial dominance worldwide. Although increased lake temperature and nutrients are well-established drivers of blooms, the mechanisms that determine cyanobacterial biomass are complex, with potentially direct, indirect, and interactive effects. Cyanobacteria can produce toxins that constitute a considerable risk for animal and human health and thus a substantial economic cost if we are to ensure safe drinking water. Such global range phenomena should be studied at a wide spatial scale, to directly compare phytoplankton response in different lake types across contrasting climatic zones. The European Multi Lake Survey (EMLS) sought to harness the power of group science in order to sample lakes across Europe and disentangle the effect of environmental stressors on potentially toxic cyanobacterial blooms. The first EMLS results showed that the distribution of cyanobacterial toxins and the toxic potential in lakes will be highly dependent on direct and indirect effects of temperature. If nutrients are not regulated, then they may interact synergistically with increased lake temperatures to promote cyanobacterial growth more than that of other phytoplankton taxa. Providing continental scale evidence is highly significant for the development of robust models that could predict cyanobacterial or algal response to environmental change

Microprofiles of photosynthesis and oxygen concentration in Microcystis sp. scums

Oxygen microelectrodes were used to monitor oxygen concentration and rates of gross photosynthetic activity in Microcystis sp. scums which were formed and incubated under laboratory conditions. The depth of the photic layer, rate of photosynthesis, oxygen concentration and the location of the transition to anoxia in the scum depended on irradiance levels and colony size. Gross photosynthetic activity never extended below 2.5 mm depth in the scum. At high irradiance levels oxygen concentration in the upper 1.5 mm of the scum decreased and the oxygen concentration peak shifted to greater depth. Oxygen concentrations in scums composed of small colonies ( 300-mu-m) but small colonies showed stronger indications of photoinhibition. In a natural scum small colonies are presumably shielded from inhibitory intensities by larger colonies which will dominate the upper layers. Accumulation of low-light adapted, smaller colonies in deeper layers likely yielded a second peak in photosynthetic activity. In order to systematically discuss scums and scum formation a distinction is made in three different scum types

Photoinhibition and the availability of inorganic carbon restrict photosynthesis by surface blooms of cyanobacteria

When buoyant cyanobacteria form a surface bloom, they may obtain inorganic carbon from the water and the atmosphere. In this study, artificial blooms of cyanobacteria were exposed to different concentrations of CO(2 )in the atmosphere and HCO3- in the water. The conditions and photosynthetic activity within the bloom were measured with microelectrodes sensing O-2, pH, and light. Net rates of photosynthesis increased with the concentration of CO2 in the atmosphere (air = 350 ppm). However, even under 3,500 ppm CO2, reducing the concentration of inorganic carbon in the water reduced rates of photosynthesis. Thus, both air and water acted as sources of inorganic carbon. Bloom formation may give cyanobacteria an advantage in competition for light and inorganic carbon with other groups of phytoplankton; by placing a dense biomass close to the water surface they are able to intercept a large amount of the flux of light and inorganic carbon. The high surface pH produced in the bloom will promote influx of CO2 from the atmosphere by "chemical enhancement." The obvious advantage of being close to the water surface may be offset by the risk of photoinhibition at high photon irradiance. The dense packing of colonies in the bloom furthermore caused a high local demand for inorganic carbon and consequently extreme carbon depletion. This combination of high photon irradiance and carbon limitation proved to be especially detrimental in blooms. When the availability of inorganic carbon was restricted, nonphotochemical quenching of fluorescence increased, showing some removal of excess excitation energy, but not to the extent that photodamage was prevented. Depending on the outcome of several interacting processes, surface bloom formation may be advantageous or deleterious for the cyanobacteria involved. This also depends on the environmental conditions to which cells were acclimated prior to bloom formation. Cyanobacteria have a better chance of withstanding the conditions in surface blooms if the cells were already acclimated to high average irradiance prior to floating to the surface