RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) TO NITRATE, PHOSPHATE, OR IRON STARVATION¹

Abstract

The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient‐starved cells, the ratios of light‐harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a‐specific light absorpition coefficient (a*) and the functional absorption cross‐section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self‐shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light‐harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo‐protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (Fv/Fm) of dark‐adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in Fv/Fm were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient‐starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate‐starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.

Synoptic study of variations in the fluorescence based maximum quantum efficiency of photosynthesis across the North Atlantic Ocean

“…Only in nutrient-rich estuaries have similarly high values been observed in nature (Geider et al 1993a;unpubl.). Values of FJF, approaching 0.6 occur in coastal waters (Kolber et al 1990;Geider et al 1993a), but F,IF, rarely exceeds 0.5 in offshore ocean surface waters (Falkowski et al 199 1;Geider et al 1993a;Greene et al 1994;Kolber et al 1994). Thus, our values of 0.22-O.…”

“…9) across the North Atlantic. Because the maximum FJF,,, value routinely measured in healthy phytoplankton cultures is 0.65 (Kolber et al 1988;Geider et al 1993b), we estimate the range of (6, values to be 34-86% of the maximum possible. The geographical distribution of FJF,,, values followed the two-basin division already used above.…”

“…Kolber et al 1990;Falkowski et al 199 1;Geider et al 1993a). Nutrient limitation (N, P, Fe) produces increases of F&h1 and F&h1 and decreases of F,IF,, growth rate, and the maximum quantum efficiency of photosynthesis in both batch and chemostat phytoplankton cultures (e.g.…”

“…Fluorescence properties have been shown to be correlated with the more conventional measurements of the photosynthesis-light curve (Greene et al 199 1) and photosynthetic gas exchange (Kolber and Falkowski 1993). In particular, the ratio of variable to maximum fluorescence (FJF,) is typically correlated with the maximum quantum efficiency of photosynthesis (see Geider et al 1993a). In recent fieldwork where 14C-based productivity parameters were compared to fluorescence-based estimates, a significant positive correlation between a!…”

“…Welschmeyer and Lorenzen 198 1;Cleveland and Perry 1987;Kolber et al 1988) as well as in natural assemblages in the open ocean (e.g. Cleveland et al 1989;Kolber et al 1990;Geider et al 1993a). Diel variations in cy, the initial slope of the photosynthesis-irradiance curve, may also represent variations in 4, in coastal (MacCaull and Platt 1977;Cot& and Platt 1983) and oceanic (Platt et al 1992) populations.…”