Intra-population strain variation in phosphorus storage strategies of the freshwater cyanobacterium Raphidiopsis raciborskii

Abstract

Several cyanobacteria, including diazotrophic Raphidiopsis raciborskii, can form harmful blooms when dissolved inorganic phosphorus concentrations are very low. We hypothesized that R. raciborskii strains would vary in phosphorus (P) allocations to cell growth and storage, providing resilience of populations to continuously low or variable P supplies. We tested this hypothesis using six toxic strains (producing cylindrospermopsins) isolated from a field population using batch monocultures with and without P and dissolved inorganic nitrogen (DIN). Treatments replete with DIN, irrespective of P addition, had similar exponential growth rates for individual strains. P storage capacity varied 4-fold among strains and was significantly higher in DIN-free treatments than in replete treatments. P was stored by all R. raciborskii strains, in preference to allocation to increase growth rates. P stores decreased with increased growth rate across strains, but weeere not related to the time to P starvation in P-free treatments. The storage capacity of R. raciborskii, combined with strategies to efficiently uptake P, means that P controls may not control R. raciborskii populations in the short term. Intra-population strain variation in P storage capacity will need to be reflected in process-based models to predict blooms of R. raciborskii and other cyanobacteria adapted to low-P conditions.

Different Gene Expression Response of Polish and Australian Raphidiopsis raciborskii Strains to the Chill/Light Stress

“…Similarly, a high tolerance to changes in light intensity has been demonstrated for R. raciborskii [1,23]. The dual roles of light and temperature in the dominance of certain species is not fully understood.…”

“…The successful range expansion of R. raciborskii species occurs through phenotypic plasticity, allowing it to rapidly adapt to new conditions, and physiological attributes that allow it to thrive in inhospitable environments [2]. For example, its high affinity for phosphorus uptake and strain variation in phosphorus storage [23] has allowed its dominance in habitats with phosphate deficiency [24]. Growth under a broad range of temperature and light conditions is also an important capability for successful invasion [18,[25][26][27].…”

Phosphorus storage and utilization strategies of two bloom-forming freshwater cyanobacteria

“…Subcultures (5 ml) were taken for OD750 readings and cell enumeration two to three times a week until the day of 10% photosynthetic yield reduction (table 2), as per methods in Xiao et al [12]. Our previous studies showed that Australian R. raciborskii strains do not have well defined lag, exponential, stationary and decline phases, irrespective of growth media or conditions [12,[24][25][26].…”

“…Subcultures (5 ml) were taken for OD750 readings and cell enumeration two to three times a week until the day of 10% photosynthetic yield reduction (table 2), as per methods in Xiao et al [12]. Our previous studies showed that Australian R. raciborskii strains do not have well defined lag, exponential, stationary and decline phases, irrespective of growth media or conditions [12,[24][25][26]. However, for the purpose of this study, we used the first order rate kinetics [26,27] to calculate the growth rate from cell enumeration, and we defined exponential growth phase as corresponding to period before a substantial decrease in growth rate, suggesting that cells were approaching stationary phase and acknowledging that growth rates were not clearly delineated in a classical definition.…”

“…Cell storage of P is another strategy for dealing with low DIP. Cellular P quotas can vary fourfold among R. raciborskii strains isolated from the same population and are significantly higher in DIN-free treatments than those in replete DIN and P treatments [12]. However, the interplay between P quota, and DIP uptake affinity across strains and species remains poorly understood.…”