Stable carbon isotope fractionation by marine phytoplankton in response to daylength, growth rate, and CO2 availability

Stable carbon isotope fractionation (epsilon(p)) of 7 marine phytoplankton species grown in different irradiance cycles was measured under nutrient-replete conditions at a high Light intensity in batch cultures. Compared to experiments under continuous Light, all species exhibited a significantly higher instantaneous growth rate (mu(i)), defined as the rate of carbon fixation during the photoperiod, when cultivated at 12:12 h, 16:8 h, or 18:6 h light:dark (L/D) cycles. Isotopic fractionation by the diatoms Skeletonema costatum, Asterionella glacialis, Thalassiosira punctigera, and Coscinodiscus wailesii (Group I) was 4 to 6 parts per thousand lower in a 16:8 h L/D cycle than under continuous light, which we attribute to differences in mu(i). In contrast, epsilon(p) in Phaeodactylum tricornutum, Thalassiosira weissflogii, and in the dinoflagellate Scrippsiella trochoidea (Group II) was largely insensitive to daylength-related differences in instantaneous growth rate. Since other studies have reported growth-rate dependent fractionation under N-limited conditions in P. tricornutum, mu(i)-related effects on fractionation apparently depend on the factor controlling growth rate. We suggest that a general relationship between epsilon(p) and mu(i)/[CO2,aq] may not exist. For 1 species of each group we tested the effect of variable CO2 concentration, [CO2,aq], on isotopic fractionation. A decrease in [CO2,aq] from ca 26 to 3 pmol kg(-1) caused a decrease in epsilon(p) by less than 3 parts per thousand. This indicates that variation in mu(i) in response to changes in daylength has a similar or even greater effect on isotopic fractionation than [CO2,aq] in some of the species tested. in both groups epsilon(p) tended to be higher in smaller species at comparable growth rates. In 24 and 48 h time series the algal cells became progressively enriched in C-13 during the day and the first hours of the dark period, followed by C-13 depletion in the 2 h before beginning of the following Light period. The daily amplitude of the algal isotopic composition (delta(13)C), however, was less than or equal to 1.5 parts per thousand, which demonstrates that diurnal variation in delta(13)C is relatively small.