van Maria Leeuwe

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). I. INTRACELLULAR DMSP CONCENTRATIONS

Iron is essential for phytoplankton growth, as it is involved in many metabolic processes. It controls photosynthesis as well as many enzymatic processes. As such, iron affects the cell’s energy supply and contributes to the assimilation of carbon and nitrogen. To determine whether iron limitation would result in energy stress or induced nitrogen deficiency, an Antarctic Phaeocystis sp. (Prymnesiophyceae) strain was studied for its biochemical composition, with the main emphasis on intracellular production of dimethylsulfoniopropionate (DMSP). DMSP is suggested to replace nitrogen containing solutes under conditions of nitrogen deficiency. Batch cultures of Antarctic Phaeocystis sp. were grown under iron‐rich and iron‐poor conditions and simultaneously subjected to high and low light intensities. Iron depletion induced chlorosis and suppressed growth rates as well as the maximum yield of the cultures; these effects were reinforced by low light intensities. Cell volumes were strongly reduced under iron‐limited conditions. However, this reduction in cell volume was accompanied by a reduced DMSP content only in cultures experiencing low light intensities. Under high light conditions, no reduction of DMSP was observed; hence, intracellular DMSP concentrations increased. These observations are discussed relative to carbon and nitrogen metabolism and the biosynthetic pathway of DMSP. It is argued that under high light, low iron conditions, the cells were bordering on nitrogen deficiency induced by iron limitation, whereas under low light, low iron conditions, the cells were energy limited resulting in overall suppressed metabolic rates. Between treatments, DMSP to chlorophyll‐a ratios varied by a factor of 5, demonstrating the dependence of this parameter on the physiological state of the cell.

Nutrient anomalies in Fragilariopsis kerguelensis blooms, iron deficiency and the nitrate/phosphate ratio (A. C. Redfield) of the Antarctic Ocean

Abstract During seasonal development of blooms in the Polar Frontal region, concentrations of nitrate and phosphate decreased in surface waters. In blooms of Fragilariopsis kerguelensis at the southern rim (49–50°S) of the Polar Frontal region the dissolved ratio NO 3 PO 4 increased from the winter value of ∼14 to 15.8 (18 October 1992) to as high as 25 (23 November 1992). Ambient dissolved Fe in these blooms was subnanomolar compared to ∼1.1–1.9 nM in the overall Polar Frontal region. Blooms more northerly in the Polar Frontal region were dominated by other diatoms and higher dissolved Fe (> 1 nM), and showed only very modest NO 3 PO 4 anomalies. From nutrient inventories the biogenic pools (PON and DON) and export of settling biogenic debris would have N P ratios as low as 4.4–6.1 compared to ∼14 in deep Antarctic waters. Such shifts are consistent with decreasing availability of Fe for nitrate reduction, but also may be due to intrinsically low N P in Fragilariopsis kerguelensis cells. Moreover, a low ratio DON DOP in dissolved organic matter and enhanced recycling of N versus P cannot be excluded either. Triplicate mesocosm (20 l) experiments were performed with a diatom-dominated community in ambient seawater (initial Fe = ∼0.9 nM) collected at the Polar Front during early spring. Three other triplicates were enriched with 2 nM Fe to total Fe ∼ 2.9 nM. During the incubations, the Fe-enriched experiments showed assimilation at near-perfect Redfield N P ratios of ∼15 and a virtually near-zero intercept. The untreated incubations showed significantly lower uptake ratios at ∼13 and non-zero intercepts, suggesting leftover nitrate after all phosphate was utilised. At initial Fe = ∼0.9 nM, the Fe-containing algal enzymes for reduction of nitrate appeared to be impaired, hence nitrate assimilation was less efficient. The observed N P fractionation in Fragilariopsis kerguelensis blooms at the Polar Frontal region, in combination with the local formation of AAIW flowing northward, might help maintain the lower N P ratio at ∼14 in Antarctic waters, as compared to a ∼15 as an average value for the other oceans. The functionality of Fe in C-fixation, nitrate assimilation as well as N2 fixation may partly explain the large variability of the NO 3 PO 4 ratio in this and other ocean basins (Fanning, 1992; Journal of Geophysical Research, 97, 5693–5712), as well as recently reported variations in the extended C/N/P ratio.

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). II. PIGMENT COMPOSITION

A strain of Phaeocystis sp., isolated in the Southern Ocean, was cultured under iron‐ and light‐limited conditions. The cellular content of chlorophyll a and accessory light‐harvesting (LH) pigments increased under low light intensities. Iron limitation resulted in a decrease of all light‐harvesting pigments. However, this decrease was greatly compensated for by a decrease in cell volume. Cellular concentrations of the LH pigments were similar for both iron‐replete and iron‐deplete cells. Concentrations of chlorophyll a were affected only under low light conditions, wherein concentrations were suppressed by iron limitation. Ratios of the LH pigments to chlorophyll a were highest for iron‐deplete cells under both light conditions. The photoprotective cycle of diato/diadinoxanthin was activated under high light conditions, and enhanced by iron stress. The ratio of diatoxanthin to diadinoxanthin was highest under high light, low iron conditions.  Iron limitation induced synthesis of 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin at the cost of fucoxanthin. Fucoxanthin formed the main carotenoid in iron‐replete Phaeocystis cells, whereas for iron‐deplete cells 19′‐hexanoyloxyfucoxanthin was found to be the main carotenoid. This shift in carotenoid composition is of importance in view of the marker function of both pigments, especially in areas where Phaeocystis sp. and diatoms occur simultaneously. A hypothesis is presented to explain the transformation of fucoxanthin into 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin, referring to their roles as a light‐harvesting pigment.

Iron enrichment experiments in the Southern Ocean: physiological responses of plankton communities

The physiological responses of plankton to iron enrichment were investigated in experiments performed in 20-1 culture vessels. Natural phytoplankton communities in sea water, with mean ambient Fe concentrations ranging from 0.3-0.4 nM in the Antarctic Circumpolar Current to 1.2-1.9 nM in the Polar Frontal region, were incubated for several days. Upon addition of 2 nM of iron, synthesis of chlorophyll a and nutrient uptake was stimulated. The specific nitrate-uptake rates as determined by 15N-uptake experiments consistently increased, as well as the ratios of chlorophyll a to particulate carbon. Growth rates in iron-enriched bottles were consequently enhanced relative to control bottles. The biochemical composition of the plankton community, indicated by carbon to nitrogen ratios and fatty acid composition, remained unaffected by iron addition. On the basis of 14C incorporation into the major biochemical pools, no changes were observed in the allocation of carbon into proteins, polysaccharides, lipids and low-molecular-weight metabolites in the particulate fraction. Antarctic phytoplankton endures the low ambient iron concentrations by maintaining physiological processes at lower activity rates, whereas the biochemical composition of the plankton remains virtually unaffected.

Pigment distribution in the Pacific region of the Southern Ocean (autumn 1995)

Abstract Phytoplankton distribution patterns are still largely unknown for the Pacific region of the Southern Ocean. Pigment distributions were determined by HPLC on 40-m samples collected from the mixed layer during the ANTXII/4 cruise in March–May 1995 aboard RV “Polarstern”. A transect was covered (90°W, from 51°S to 70°S), crossing the Subantarctic Front in the north, the Polar Front, and the Southern Polar Front in the south. Coinciding with high concentrations of silicate, diatoms dominated in the Antarctic waters south of the Polar Front. North of the Polar Front, silicate concentrations dropped to values less than 10 μM. In this area flagellates (Prymnesiophyceae and green algae) were the dominant phytoplankton group. Nutrient depletion of the surface waters near the Southern Polar Front indicated formerly enhanced productivity. These findings confirmed previous observations by the British Sterna expedition, which described locally elevated chlorophyll a biomass near the southern boundary of the Southern Polar Front. We propose a role for supply of bioavailable iron via the front, and emphasise the importance of frontal systems for phytoplankton productivity in the Southern Ocean.