Effective resource exploitation under variable environmental conditions is one of the most important causes of intra- and interspecific morphological diversity in phytoplankton (Naselli-Flores and Barone 2000; Naselli-Flores et al. 2007). Available literature on morphological variability of phytoplankton indicates that environmental and biological constraints such as nutrients, light availability, and grazing pressure from herbivores influence phytoplankton morphology (Kagami and Urabe 2001; O'Farrell et al. 2007). It follows therefore that morphological traits adapted by phytoplankton are a reflection of changes and regularities of seasonal and/or environmental patterns. They not only show regular periodicities in weather patterns, but also reflect effects of perturbations or other disturbances in ecosystem (Naselli-Flores et al. 2007).
Arthrospira fusiformis (Voronichin) Komárek and Lund is a filamentous cyanobacterium that forms almost unialgal blooms in soda lakes of the East African Rift Valley (Vareschi 1978). These lakes are characterized by high levels of carbonate and bicarbonate contents and a pH of up to 11 (Vonshak 1997; Oduor and Schagerl 2007a). A. fusiformis is the main food source of the Lesser Flamingos, Phoeniconaias minor Saint-Hilaire (Vareschi and Vareschi 1984) linking Arthrospira abundance directly to the high number of these birds in African saline–alkaline lakes (Krienitz and Kotut 2010; Kaggwa et al. 2013). Lesser Flamingos are a big tourist attraction in Lakes Nakuru and Bogoria in Kenya, which has economic importance for local people (Harper et al. 2003; Schagerl and Oduor 2008; Krienitz and Kotut 2010). At times, the dominance of A. fusiformis suddenly crashes and the lake shifts toward an unstable pelagic community of different organisms, which cause high degrees of food insecurity for top-level consumers like fish and flamingos (Krienitz and Kotut 2010; Krienitz et al. 2013). A. fusiformis is commercially sold as “Spirulina platensis” for dietary supplement because of its high content of essential fatty acids, vitamins, proteins, and minerals (Jassby 1988; Tokuşogulu and Ünal 2003; Mühling et al. 2005; Zieliñska and Chojnacka 2009).
Arthrospira fusiformis strains have been observed to occur in a varied range of saline habitats which shows its ability to adapt to freshwater alkaline conditions as well as saline–alkaline and even hypersaline environments (Dadheech et al. 2010). In both natural and culture conditions, it shows high morphological variability (Mühling et al. 2003; Ballot et al. 2004; Wang and Zhao 2005). The main morphological feature of A. fusiformis is the patterned arrangement of its multicellular cylindrical trichome in an open helix. Trichomes are composed of cylindrical cells that undergo binary fission in a single plane, perpendicular to the main axis. Cell diameter ranges from about 3–12 μm, though occasionally it may reach up to 16 μm. The helix pitch typically ranges from 10–70 μm and its diameter from 20–100 μm. These two parameters which define the shape of the helix architecture are highly dependent on growth and environmental conditions (Vonshak and Tomaselli 2000).
Under laboratory conditions, Kebede (1997) detected differences in the length of its trichomes and degree of helicity when cultured at varying salinity levels expressing the physiological stress to which the cells were subjected to. The author observed that long trichomes occurred at the lowest salinity level (13 g L−1) while very short but closely coiled trichomes were dominating in Cl− rich and highly saline media (55–68 g L−1). Additionally, very loose helices were distinctive for cultures grown in SO42− rich media. The helix feature in A. fusiformis shows high variability (Mühling et al. 2003; Wang and Zhao 2005) which probably is determined at the genetic level and induced by various environmental factors, hence the concept of ‘plasticity genes’ (Schlichting and Pigliucci 1993). This refers to the regulatory loci that directly respond to a specific environmental stimulus by triggering a specific series of morphogenic changes (Pigliucci 1996).
In the shallow African saline–alkaline lakes, it has already been observed that there are large temporal fluctuations in A. fusiformis biomass (Oduor and Schagerl 2007a; Schagerl and Oduor 2008; Krienitz and Kotut 2010). Even though such shifts in A. fusiformis biomass may go along with morphological changes, no comprehensive field study has been done on the morphological variability of A. fusiformis so far. In this study, we sought to address this gap by assessing the temporal morphological changes of A. fusiformis and identifying key environmental and biological variables that were responsible for these changes. Such shifts in morphology of the dominant primary producer probably have significant impacts on the food web structure, as grazing might be promoted or hindered by certain morphological features. Additionally, the study allowed evaluating the potential of A. fusiformis morphology as a reliable indicator of the biological stability in soda lakes.