Abstract
AbstractCell division of the coccolithophore Emiliania huxleyi and other phytoplankton typically becomes entrained to diel light/dark cycles under laboratory conditions, with division occurring primarily during dark phases and production occurring during light phases. Under these conditions, increases in cell and biomass concentrations deviate from exponential functions on time scales < 24 h. These deviations lead to significant diel variations in common measurements of phytoplankton physiology such as cellular quotas of particulate organic and inorganic carbon (POC, PIC) and their production rates. Being time‐dependent, only the temporal mean of the various values during the day are comparable between experiments. Deviations from exponential growth furthermore imply that increases in cell and biomass concentrations cannot be expressed by the daily growth rate μ24 h (typically determined from daily increments in cell concentrations). Consequently, conventional calculations of production as the product of a cellular quota (e.g., POC quota) and μ24 h are mathematically incorrect. To account for this, we here describe short‐term changes in cell and biomass concentrations of fast‐dividing, dilute‐batch cultures of E. huxleyi grown under a diel light/dark cycle using linear regression. Based on the derived models, we present calculations for daily means of cellular quotas and production rates. Conventional (time‐specific) measurements of cellular quotas and production differ from daily means by up to 65% in our example and, under some circumstances, cause false “effects” of treatments. Intending to reduce errors in ecophysiological studies, we recommend determining daily means—mathematically or by adjusting the experimental setup or sampling times appropriately.
Highlights
When the dark phase is shorter than the length of two division sequences, cell division inevitable extends into the light phase
Summary and conclusions: Here, we show that the increases in cell and biomass concentrations in E. huxleyi cultures with phased division and production on time scales < 24 h can be well described with stepwise linear functions
The deviations from exponential growth and the strong diel variations in cellular quotas implicate that cellular pool sizes and production rates calculated by conventional calculations can be erroneous
Summary
In phytoplankton cultures that grow under diel light/dark cycles, cell division of most phytoplankton becomes phased, that is, cell division occurs in restricted periods of the day only (e.g., Nelson and Brand 1979; Harding et al 1981). Cellular POC or PIC production rates, for example, are commonly used as measures for photosynthetic carbon production or calcification, respectively (Raven and Crawfurd 2012; Meyer and Riebesell 2015). Such physiological cell parameters are commonly estimated from single measurements during the day, applying equations derived from exponential growth (for details, refer to the “Discussion” section). In cultures that are entrained to diel light/dark cycles, these equations do not apply and can introduce errors that can lead to misleading conclusions
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