Factors controlling abundance of Arthrospira fusiformis

The Great Rift Valley running through Africa offers a number of endorheic saline-alkaline lakes representing some of the most alkaline naturally occurring environments on earth. They feature an enormous phytoplankton biomass dominated by the cyanobacterium Arthrospira fusiformis, which is the basis of life in these lakes. The importance of A. fusiformis for the stability of these ecosystems can not be over-emphasised, because this taxon is the only food source of the lesser flamingos, which are inhabiting the lakes in hundreds of thousends. However, phytoplankton standing crop has been observed to be extremely variable, with no simple explanation available. We will focus on this fundamental question by investigating two types of Kenyan lakes: the very shallow Lake Nakuru, which is occupied by the planktivorous fish Oreochromis and mixes daily and Lake Bogoria, which is fish-free and stratified over longer periods. The project aims will be achieved through the following specific objectives: (i) analyze the fluctuations of physico-chemical variables and meteorological data on weekly intervals, (ii) investigate changes in plankton community structure and (iii) determine the vitality of phytoplankton by means of fluorescence parameters. Furthermore, we (iv) will establish the bacterio- and virioplankton abundance and (v) determine the percentage of A. fusiformis cells infected by cyanophages. Finally, we analyze interrelationship between abiotic and biotic factors simultaneously by multivariate statistics and deduce key processes for Arthrospira occurrence.

Some of the most alkaline lakes naturally occurring are located within the East African Great Rift Valley. They are listed amongst the most productive ecosystems of the world and host highest numbers of heterotrophic prokaryotes and virus-like particles. The enormous phytoplankton biomass is commonly dominated by the filamentous, spirally coiled cyanobacterium Arthrospira fusiformis (formerly called Spirulina platensis), which however shows dramatic breakdowns from time to time with no simple explanation available. We focused on this fundamental question of algae biomass fluctuations and found cyanophage infections coinciding with drops in biomass. Additionally, light supply and consumer number significantly influenced A. fusiformis abundance. Arthrospira shows characteristic types of morphology. Medium-sized cells and large, widely-coiled filaments prevailed most. Based on fluorescence measurements, we found widely-coiled filaments representing high vitality. Percentage of large, widely-coiled filaments was promoted by elevated levels of SRP, wind speed, temperature and conductivity and the opposite for small filaments. Large, narrow-coiled filaments were associated with an increase in mainly A.-grazing zooplankton and cyanophage infections, widely-coiled spirals were promoted by increased turbulences. From this study, we were able to demonstrate for the first time morphological patterns of Arthrospira in nature. Further, we were able to study a trophic cascade effect; such effects occur when the food web is disrupted by loss or significant reduction of one or more of its members. A colourful example on top of a short food chain is Lesser Flamingos, forming characteristic mass occurrences in East African Rift Valley lakes. At irregular intervals, the dominance of their most important food source A. fusiformis is interrupted by sudden breakdowns, thus leading to a drastic drop in Lesser Flamingo numbers. We documented for the first time that breakdown of A. fusiformis is related to cyanophage infection, which further led to a dramatic reduction of the Lesser Flamingo abundance at this lake. Our findings demonstrate how virus infection on the very base of a food chain can affect in a bottom-up cascade the distribution of warm-blooded end-consumers. We anticipate our observations as a starting point for further analyses of virus-mediated cascading effects in aquatic food webs in general. We opine that A. morphology is suitable for indicating the biological stability in soda lakes as its shape shifts are highly conspicuous this outcome might be also of interest for commercial Spirulina farms. Cyanotoxins can be excluded for flamingo mass mortalities during our study period. The key role of Arthrospira in the functioning of the ecosystems was well depicted and the research output significantly contributes to sustainable management of these unique systems.