Phytoplankton isotopic composition

Laws, E. A., B. N. Popp, R. R. Bidigare, M. C. Kennicutt S. A. Macko, 1995. Dependence of phytoplankton isotopic composition on growth rate and [C02laq Theoretical considerations and experimental results. Geochim. Cosmochim. Acta59 1 13 1-1 13 8. 

Even more complex is C-isotope fractionation in aquatic plants. Factors that control the of phytoplankton include temperature, availability of C02(aq), light intensity, nutrient availability, pH and physiological factors such as cell size and growth rate (Laws et al. 1995, 1997 Biigare et al. 1997 Popp et al. 1998 and others). In particular the relationship between C-isotope composition of phytoplankton and concentration of oceanic dissolved CO2 has been subject of considerable debate because of its potential as a palaeo-C02 barometer (see discussion). 

One of the most important groups of all living matter is marine phytoplankton. Natural oceanic phytoplankton populations vary in 8 C-value by about 15%c (Sackett et al. 1973 Wong and Sackett 1978). Rau et al. (1982) demonstrated that different latitudinal trends in the carbon isotope composition of plankton exist between the northern and the southern oceans south of the equator the correlation between latitude and plankton C-content is significant, whereas a much weaker relationship exists in the northern oceans. 

Laws EA, Popp BN, Bidigare RR, Kennicutt MC, Macko SA (1995) Dependence of phytoplankton carbon isotopic composition on growth rate and C02aq theoretical considerations and experimental results. Geochim Cosmochim Acta 59 1131-1138 Laws EA, Bidigare RR, Popp BN (1997) Effect of growth rate and CO2 concentration on carbon isotope fractionation by the marine diatom Phaeodactylum tricornutum. Limnol Oceanogr 42 1552-1560... 

Analyses of carbon isotopic composition were performed as an independent measure of potential food sources for zooplankton. The 813C values of terrestrial detritus and of littoral zone emergents such as Carex ranged from -26.0 to -29.2%o (Table I 49, 92). By comparison, the zooplankton were quite depleted in 13C their 813C values were more similar to phytoplankton and ranged from -32.1%o in Daphnia to -41.0%o in Cyclops (Table I). 

The relatively smaller 15N enrichments of Cyclops compared to phytoplankton and the other zooplankton in the leucine experiment are consistent with the idea that Cyclops feeds on microheterotrophs that are less labeled with 15N than the phytoplankton. As with the 15N, Cyclops was less enriched in 13C than phytoplankton. But the 13C proved less useful as a tracer because phytoplankton took up the label and because the actual isotopic compositions of bacteria and microheterotrophs were unknown. 

The carbon isotopic composition of phytoplankton has been shown to be strongly affected by the />( (>2 of surface waters in the ocean (Rau et al., 1989, 1992). Moreover, fractionation of carbon isotopes by phytoplankton is also correlated with cell growth rate, cell size, cell membrane permeability, and CO2 (aq) (Laws et al., 1995 Rau et al., 1997). 

Figure 23. Isotopic compositions of amino acids, expressed as fractionations relative to alanine, from natural populations of estuarine diatoms and marine phytoplankton analyzed by Keil and Fogel (2001). For comparison, the heavy gray line depicts the average of the analyses reported in Figure 14. The range of evalues is identical to that in Figure 14. The horizontal scale covers only the amino acids analyzed by Keil and Fogel (2001).

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