Studies of growth rates of some freshwater cryptophyte algae

Cryptophytes are free-living unicellular algae which are important for the productivity and food chain Dynamics of temperate lakes. This study provides fundamental information on the ecophysiology of two freshwater cryptophytes of different cell size, mainly in terms of growth and related factors. This thesis comprises of six chapters, three of which describe light or light-and-temperature experiments with small-scale batch cultures (Chapters. 2 to 4), one depicts a larger scale laboratory experiment simulating natural conditions (Chapter 5) and the two last (Chapters 6 and 7) are based on short-term investigations in situ. The effects of light and temperature on nutrient-saturated growth and cellular composition (chlorophyll a, proteins, carbohydrates) were studied in batch cultures. With the help of mathematical models, the physiological basis for interspecific differences of growth response was determined (Chapter 2). The cryptophyte strain L315 appeared to be a cold-water species as its optimum temperature was ca. 19°C. The strain L485 was more adapted to warm-water conditions with its optimum of ca. 24.5 °C. In respect of their growth response to irradiance, L485 can be said to be a stenotopic and L315 a eurytopic strain, as L485 shows photoinhibition soon after saturation point, whereas L315 tolerates a much wider range of irradiance. The role of changes in cellular composition is discussed. In order to explain the observed growth differences the effects of light and temperature on gross photosynthesis, respiration and hence net productivity were studied (Chapter 3). The observed respiration/photosynthesis ratios were high, as in L485 and L315 respiration accounted for 17-77 % and 14-81 % of gross photosynthesis, respectively. Under optimum conditions the respiration/Pmax for L485 was 17 % and for L315 58 %. The response of cryptophytes to chromatic light was studied by means of quantitative epifluorescence microscopy and it was found that in comparison to blue-green algae cryptophytes L485 and L315 do not gain such great adaptational advantages in terms of growth by chromatic adaptation (Chapter 4). The modest role of chromatic adaptation is discussed. The role of diel vertical migrations (DVM) in the growth of cryptophytes was studied in 4 m tall experimental columns (Chapter 5). Results revealed that by migrating into cooler, nutrient rich. hypolimnion flagellated cryptophytes can increase their growth rate under conditions where resources (light and nutrients) are spatially separated for prolonged time periods. This study also emphasizes the need for more detailed DVM studies in situ. Finally, the pattern and timing of nuclear and cellular division in two Cryptomonas species in situ was studied by means of mitotic index technique (Chapter 6) and DNA quantification (Chapter 7). The nuclear division of Cryptomonas L485 (Chapter 6) appeared to be well phased, but as in this division pattern mitosis and cytokinesis were totally overlapping, it was impossible to calculate in situ growth rates. Field observations (Chapter 7) revealed that DNA quantification by means of epifluorescence microscopy is possible from a natural cryptophyte population, but as the Cryptomonas sp. population under scrutiny was not well phased, growth rate calculation could not be carried out. The survival strategies of Cryptophytes L485 and L315 in terms of r vs. K strategies are discussed in Chapter 8. It is pointed out that, although the habitats occupied by these strains as well as some of their morphological and physiological features indicate that L485 is probably a r-strategist and L315 a K-strategist, it is not possible to draw final conclusions on the basis of this study. Light and temperature, i.e. the factors mostly studied in this thesis, are presumably not the environmental factors of greatest selective importance for these cryptophytes in natural competitive situations.