Maria A. van Leeuwe

Low dissolved Fe and the absence of diatom blooms in remote Pacific waters of the Southern Ocean

The remote waters of the Pacific region of the Southern Ocean are the furthest away from any upstream and upwind continental Fe sources. This prime area for expecting Fe limitation of the plankton ecosystem was studied (March–April 1995) along a north–south transect at ∼89°W. At the end of the austral summer the upper wind-mixed layers were in the order of ∼100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The dissolved Fe was found at low concentrations ranging from 0.05 nM near the surface to 0.5 nM in deeper waters. Along the transect (52°S–69°S), the dissolved iron was enhanced in the Polar Front, as well as near the Antarctic continental margin (0.6–1.0 nM). In between, the southern ACC branch was depleted with iron; here the concentrations in surface waters were quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM (October 1992) and 0.31 nM (November 1992) averages in early spring in the southern ACC part of Atlantic 6°W sections [de Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importance of iron for phytoplankton spring blooms and CO2 drawdown in the Southern Ocean. Nature 373, 412–415; Loscher, B.M., de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143–188.]. First, the lower ∼0.21 nM in March–April 1995 may partly be due to continuation of the seasonal trend where the phytoplankton growth, albeit modest, was removing Fe from the surface waters. Secondly, the 89°W Pacific stations are further downstream continental or seafloor sources than the Atlantic 6°W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. Indeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 89°W is consistent with the distance from sources and the late summer. It also would explain the very low abundance of phytoplankton (Chl a) in the region and the conspicuous absence of plankton blooms. In the subAntarctic waters north of the Polar Front there are no diatoms, let alone diatom blooms, due to low availability of silicate. Thus, it appears the biological productivity is suppressed due to iron deficiency, in combination with the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms.

Responses of Southern Ocean phytoplankton to the addition of trace metals

Trace metal enrichment experiments were performed under ultraclean conditions with natural oceanic plankton populations in the Southern Ocean along 6°W in October–November 1992. Five Fe-enrichment experiments were conducted, as well as a further eight experiments with single or combined addition of Fe, Mn, Co and Zn. Water was incubated from the Polar Frontal region at the beginning and again later in the middle of a bloom, from the Antarctic Circumpolar Current south of the Front, from the sea-ice edge area and from the ice-covered northern Weddell Sea. Growth responses of phytoplankton to the addition of Mn (2.5 nM), Co (0.4 nM) or Zn (2 nM) were not very clear (slight enhancement), which may partly have been due to low initial biomass. Growth enhancements by Fe (2 nM) were more pronounced, albeit only where initial biomass was relatively high and where distinct growth in the controls was observed simultaneously. Overall, rates of chlorophyll a increased and final yields increased, relative to the controls, by about 10% to more than 50%. Additions of 5 and 10 nM Fe yielded about 10% higher specific growth rates than additions of 2 nM. Combining Fe with one or more of the other trace metals did not result in significantly higher yields or growth rates as with Fe alone, even at high initial biomass. Moreover, the accumulation of cellular 55Fe over time was neither enhanced nor suppressed by Mn, Co, and Zn, further suggesting that co-limitation by the other metals did not occur. Fe is evidently the most important trace metal controlling phytoplankton development. Microzooplankton grazing was determined (dilution method) at the end of one 9 day experiment where a very clear response by phytoplankton was observed following addition of 2 nM Fe. Gross production and grazing rates (as chlorophyll) were 0.71 and 0.34 day−1, respectively, in the control and 0.89 and 0. 30 day−1 in the Fe enrichment. Apparently only algal production was stimulated by Fe enrichment. We conclude that in the Southern Ocean biomass build-up by phytoplankton is limited by decreasing availability of trace metals, notably Fe, under conditions of sufficient light and low grazing pressure.

NUTRIENT LIMITATION AND HIGH IRRADIANCE ACCLIMATION REDUCE PAR AND UV-INDUCED VIABILITY LOSS IN THE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE)1

The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.

Photoacclimation in microphytobenthos and the role of xanthophyll pigments

Estuarine microphytobenthos are frequently exposed to excessively high irradiances. Photoinhibition in microalgae is prevented by various photophysiological responses. We describe here the role of the xanthophyll pigments in photoacclimation. The pigment composition of the microphytobenthos was studied in three European estuaries (Barrow, Ireland; Eden, UK; Tagus, Portugal). Using HPLC-analyses, microscale changes in biomass and pigment composition were monitored over short (hourly) and long (seasonal) time scales. In the Barrow estuary, the biomass of microphytobenthos (measured as chlorophyll a) increased significantly in the top 400–500 µm of the sediment surface within 1 h of emersion; simultaneously, the xanthophyll pool size (diadinoxanthin plus diatoxanthin, dd + dt) almost doubled. A more gradual conversion of dd into dt was observed, with the dt:dd ratio increasing from <0.1 at the start of emersion to >0.3 after 3 h emersion. Similar trends in the dt:dd ratio were observed in the surface sediments of the Eden and the Tagus estuaries. Higher ratios were recorded in the Tagus estuary, which may be explained by higher incident irradiance. In addition, seasonal studies carried out in the Eden and Tagus estuaries showed that the xanthophyll pool size increased by 10% in the summer months. The pool size was highest in the Tagus estuary. Concurrently, high values for the de-epoxidation state were recorded, with values for dt/(dt + dd) > 0.35 recorded in the summer. At the Eden, the ratio never exceeded 0.3. The de-epoxidation state was higher in winter than in summer, which was ascribed to the low winter temperatures. During a vertical migration study, a negative relationship between chlorophyll a and the de-epoxidation state was observed. It is suggested that this relationship originates from ‘micro-migration’ within the biofilm. Migration within the euphotic zone may provide an alternative means for cells to escape photodamage. In this paper, we propose that both xanthophyll cycling and ‘micro’-migration play an important role in photoacclimation and it appears that these processes operate in parallel to regulate the photosynthetic response.

Photoacclimation by the Antarctic flagellate Pyramimonas sp. (Prasinophyceae) in response to iron limitation

In this study we tested the hypothesis that iron limitation suppresses photoacclimation in cultures of the Antarctic flagellate Pyramimonas sp. The cultures were exposed to two different irradiances under iron-rich and iron-poor conditions. Light-harvesting capacity was determined by assessing the pigment composition and measuring in vivo absorption spectra. Light utilization efficiency (α) was determined from photosynthesis versus irradiance curves. The quantum yield of photosynthesis (φm) was calculated using α and the absorption spectra. Iron limitation led to commonly observed changes in cells of Pyramimonas, that is, a decrease in cellular pigment content and a reduction in cellular carbon and nitrogen quota. A reduction in αcell followed a decrease in φm and light-harvesting capacity. Interpretation of the effects of iron limitation was different when considered on a carbon basis. Because iron limitation resulted in a decrease in cellular carbon content, the carbon-specific absorption coefficient was not affected. Consequently, the observed decrease in αC was mainly due to the decrease in φm, showing that iron limitation did not control light utilization via pigment synthesis but exerted control on energy transfer. This is supported by the findings that at high irradiance a shift in pigment ratios within the total pool of violaxanthin, antheraxanthin and zeaxanthin towards zeaxanthin, which is indicative of photoacclimation to high irradiance, was observed for iron-replete cells as well as for iron-depleted cells. In contrast to what is generally hypothesized, the effects of iron limitation were not enhanced at low irradiance. Low irradiance led to an increase in the cellular light-harvesting pigment content. This increase was less pronounced in iron-depleted cells than in iron-replete cells. However, looking at the light-harvesting capacity of the cells on a carbon basis, it was found that iron-depleted cells responded similarly to iron-replete cells. We therefore conclude that the light-harvesting capacity was governed by light conditions and not by iron limitation. In addition to the increase in absorption capacity at low irradiance, an increase in light utilization efficiency was measured, again under both iron-rich and iron-poor conditions. Notably, the relative increase in αC was strongest in iron-depleted cells. Photoacclimation was clearly demonstrated by normalizing α to chl a. For iron-replete cells, αchl was highest at high irradiance. In contrast, for iron-depleted cells a chl was highest at low irradiance. We argue that iron-depleted cells can photoacclimate to low irradiance by a reduction in the ‘package effect’ and reducing growth rates.In this study we tested the hypothesis that iron limitation suppresses photoacclimation in cultures of the Antarctic flagellate Pyramimonas sp. The cultures were exposed to two different irradiances under iron-rich and iron-poor conditions. Light-harvesting capacity was determined by assessing the pigment composition and measuring in vivo absorption spectra. Light utilization efficiency (α) was determined from photosynthesis versus irradiance curves. The quantum yield of photosynthesis (φm) was calculated using α and the absorption spectra. Iron limitation led to commonly observed changes in cells of Pyramimonas, that is, a decrease in cellular pigment content and a reduction in cellular carbon and nitrogen quota. A reduction in αcell followed a decrease in φm and light-harvesting capacity. Interpretation of the effects of iron limitation was different when considered on a carbon basis. Because iron limitation resulted in a decrease in cellular carbon content, the carbon-specific absorption coefficient wa...

The pigment composition of Phaeocystis antarctica (Haptophyceae) under various conditions of light, temperature, salinity, and iron

The pigment composition of Phaeocystis antarctica was monitored under various conditions of light, temperature, salinity, and iron. 19′‐Hexanoyloxyfucoxanthin (Hex‐fuco) always constituted the major light‐harvesting pigment, with remarkably stable ratios of Hex‐fuco‐to‐chl a under the various environmental conditions. Increased pigment‐to‐chl a ratios at low irradiance confirmed the light‐harvesting function of Fucoxanthin (Fuco), 19′‐Hexanoyloxy‐4‐ketofucoxanthin (Hex‐kfuco), 19′‐butanoyloxyfucoxanthin (But‐fuco), and chl c2 and c3. Increased pigment‐to‐chl a ratios at high irradiance, low iron concentrations, and to a lesser extent at high salinity confirmed the photoprotective function of diadinoxanthin, diatoxanthin, and ß,ß‐carotene. Pigment ratios were not always according to expectations. The consistent increase in But‐fuco/chl at high temperature, high salinity, and low iron suggests a role in photoprotection rather than in light harvesting. Low Hex‐kfuco/chl ratios at high salinity were consistent with a role as light harvester, but the high ratios at high temperature were not, leaving the function of Hex‐kfuco enigmatic. Dedicated experiments were performed to test whether or not the light‐harvesting pigment Fuco could be converted into its structural relative Hex‐fuco, and vice versa, in response to exposure to light shifts. Rapid conversions could not be confirmed, but long‐term conversions cannot be excluded. New pigment ratios are proposed for chemotaxonomic applications. The ratios will improve pigment‐based diagnosis of algal species in waters dominated by P. antarctica.

Impact of sea-ice melt on dimethyl sulfide (sulfoniopropionate) inventories in surface waters of Marguerite Bay, West Antarctic Peninsula

The Southern Ocean is a hotspot of the climate-relevant organic sulfur compound dimethyl sulfide (DMS). Spatial and temporal variability in DMS concentration is higher than in any other oceanic region, especially in the marginal ice zone. During a one-week expedition across the continental shelf of the West Antarctic Peninsula (WAP), from the shelf break into Marguerite Bay, in January 2015, spatial heterogeneity of DMS and its precursor dimethyl sulfoniopropionate (DMSP) was studied and linked with environmental conditions, including sea-ice melt events. Concentrations of sulfur compounds, particulate organic carbon (POC) and chlorophyll a in the surface waters varied by a factor of 5–6 over the entire transect. DMS and DMSP concentrations were an order of magnitude higher than currently inferred in climatologies for the WAP region. Particulate DMSP concentrations were correlated most strongly with POC and the abundance of haptophyte algae within the phytoplankton community, which, in turn, was linked with sea-ice melt. The strong sea-ice signal in the distribution of DMS(P) implies that DMS(P) production is likely to decrease with ongoing reductions in sea-ice cover along the WAP. This has implications for feedback processes on the regions climate system. This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.