The carbon:chlorophyll a ratio (C:chl a or θ) is a sensitive indicator of physiological state in microalgae. The dependence of θ on photon flux density (PFD or I) and temperature in exponentially growing nutrient-sufficient microalgae can be described by an empirical equation with four coefficients. C:chl a increases linearly with increased light level at constant temperature and decreases exponentially with increased temperature at constant light level. Both the slope (ɛ) and intercept (θ0) of linear regressions of θ on photon flux density increase at low temperature. The intercept, θ0, increases from 6 to 40 g of carbon per gram of chlorophyll a (g C g chl a−1) between 30 and 0 °C and ɛ increases by over an order of magnitude from 004 to 1·9 g C g chl a−l m2 s μol photon−1 over the same temperature range.
Low-temperature chlorosis can be interpreted as an adaptive response in the allocation of cell resources between temperature-independent biophysical reactions involved in light-harvesting and temperature-dependent biochemical reactions. This response also reduces the potential for photoinhibitory damage at high light levels which can be exacerbated by low temperatures.
The range of values for θ in nature has not been adequately determined because of difficulty in separating phytoplankton from detritus, bacteria and microzooplankton. Based on the laboratory observations summarized in this paper, it would appear that use of a single value of θ for phytoplankton is inappropriate for ecological studies. For example, at a PFD of 50 μmol m−2 s−1, θ increases from 10 to 130 g C g chl a−1 between 30 and 0 °C under nutrient-sufficient conditions.
These conclusions are based on observations for eight diatoms, two green algae, one euglenoid and two cyanobacteria for which the appropriate data are available. In contrast to these groups, the dinofiagellates contain substantially less chlorophyll a The available data indicate that θ is about three times larger in dinoflagellates than in other algae under comparable PFDs at 20 °C. There are insufficient data available, however, to evaluate the light and temperature dependence of θ in dinoflagellates.