Stable carbon isotope fractionation by marine phytoplankton in response todaylength, 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-re plete conditions at a high Light intensity in batch cultures. Compared to e xperiments under continuous Light, all species exhibited a significantly hi gher instantaneous growth rate (mu(i)), defined as the rate of carbon fixat ion during the photoperiod, when cultivated at 12:12 h, 16:8 h, or 18:6 h l ight:dark (L/D) cycles. Isotopic fractionation by the diatoms Skeletonema c ostatum, Asterionella glacialis, Thalassiosira punctigera, and Coscinodiscu s wailesii (Group I) was 4 to 6 parts per thousand lower in a 16:8 h L/D cy cle than under continuous light, which we attribute to differences in mu(i) . In contrast, epsilon(p) in Phaeodactylum tricornutum, Thalassiosira weiss flogii, and in the dinoflagellate Scrippsiella trochoidea (Group II) was la rgely 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 fra ctionation apparently depend on the factor controlling growth rate. We sugg est that a general relationship between epsilon(p) and mu(i)/[CO2,aq] may n ot 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 c hanges in daylength has a similar or even greater effect on isotopic fracti onation 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 2 4 and 48 h time series the algal cells became progressively enriched in C-1 3 during the day and the first hours of the dark period, followed by C-13 d epletion in the 2 h before beginning of the following Light period. The dai ly amplitude of the algal isotopic composition (delta(13)C), however, was l ess than or equal to 1.5 parts per thousand, which demonstrates that diurna l variation in delta(13)C is relatively small.