COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS1

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer‐induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial‐to‐solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.     

Evidence for high iron requirements of colonial Phaeocystis antarctica at low irradiance

We have carried out field and laboratory experiments to examine the iron requirements of colonial Phaeocystis antarctica in the Ross Sea. In December 2003, we performed an iron/light-manipulation bioassay experiment in the Ross Sea polynya, using an algal assemblage dominated by colonial Phaeocystis antarctica, collected from surface waters with an ambient dissolved Fe concentration of ∼0.4 nM. Results from this experiment suggest that P. antarctica growth rates were enhanced at high irradiance (∼50% of incident surface irradiance) but were unaffected by iron addition, and that elevated irradiance mediated a significant decrease in cellular chlorophyll a content. We also conducted a laboratory iron dose–response bioassay experiment using a unialgal, non-axenic strain of colonial P. antarctica and low-iron (<0.2 nM) filtered seawater, both collected from the Ross Sea polynya in December 2003. By using rigorous trace-metal clean techniques, we performed this dose–response iron-addition experiment at ∼0°C without using organic chelating reagents to control dissolved iron levels. At the relatively low irradiance of this experiment (∼20 μE m⁻² s⁻¹), estimated nitrate-specific growth rate as a function of dissolved iron concentration can be described by a Monod relationship, yielding a half-saturation constant with respect to growth of 0.45 nM dissolved iron. This value is relatively high compared to reported estimates for other Antarctic phytoplankton. Our results suggest that seasonal changes in the availability of both iron and light play critical roles in limiting the growth and biomass of colonial Phaeocystis antarctica in the Ross Sea polynya.     

Effects of iron concentration on pigment composition in Phaeocystis antarctica grown at low irradiance

Interpretation of photosynthetic pigment data using iterative programs such as CHEMTAX are widely used to examine algal community structure in the surface ocean. The accuracy of such programs relies on understanding the effects of environmental parameters on the pigment composition of taxonomically diverse algal groups. Phaeocystis antarctica is an important contributor to total autotrophic production and the biogeochemical cycling of carbon and sulfur in the Southern Ocean. Here we report the results of a laboratory culture experiment in which we examined the effects of ambient dissolved iron concentration on the pigment composition of colonial P. antarctica, using a new P. antarctica strain isolated from the southern Ross Sea in December 2003. Low-iron (<0.2 nM dissolved Fe) filtered Ross Sea seawater was used to prepare the growth media, thus allowing sub-nanomolar iron additions without the use of EDTA to control dissolved iron concentrations. The experiment was conducted at relatively low irradiance (∼20 μE m⁻² s⁻¹), with P. antarctica primarily present in the colonial form—conditions that are typical of the southern Ross Sea during austral spring. Relative to the iron-limited control treatments (0.22 nM dissolved Fe), iron addition mediated a decrease in the ratio of 19′-hexanoyloxyfucoxanthin to chlorophyll a, and an increase in the ratio of fucoxanthin to chlorophyll a. Our results also suggest that the ratio of 19′-hexanoyloxyfucoxanthin to chlorophyll c₃ (Hex:Chl c₃ ratio) may be a characteristic physiological indicator for the iron-nutritional status of colonial P. antarctica, with higher Hex:Chl c₃ ratios (>3) indicative of Fe stress. We also observed that the ratio of fucoxanthin to 19′-hexanoyloxyfucoxanthin (Fuco:Hex ratio) was highly correlated (r ² = 0.82) with initial dissolved Fe concentration, with Fuco:Hex ratios <0.05 measured under iron-limited conditions (dissolved Fe <0.45 nM). Our results corroborate and extend the results of previous experimental studies, and, combined with pigment measurements from the southern Ross Sea, are consistent with the hypothesis that the interconversion of fucoxanthin and 19′-hexanoyloxyfucoxanthin by colonial P. antarctica is used as a photo-protective or light-harvesting mechanism, according to the availability of dissolved iron.     

The contribution of single and colonial cells of Phaeocystis pouchetii to spring and summer blooms in the north-eastern North Atlantic

Studies of the phytoplankton ecology in different localities in north-Norwegian fjords, the White Sea and the Barents Sea were carried out in spring and early summer to investigate the contribution of single and colonial stages of Phaeocystis pouchetii to phytoplankton abundance. Three different types of flagellated and four colonial cells were observed in all localities. P. pouchetii was rare under the ice of the Barents and White Seas, but their abundance increased rapidly during ice retreat. Single cell C dominated over colonial cell C, often by 50 times or more. The highest share of colonial cells was encountered in April in northern Norwegian fjords, in May in the Barents Sea and in May–June in the White Sea. At times the single cell dominated the total P. pouchetii biomass in Balsfjord (April 1999, 2001) with hardly any colonies present. In the White Sea colonies of P. pouchetii were less abundant than in the other regions. Cell carbon of P. pouchetii colonies appears never to be as dominating in the north-eastern North Atlantic as P. globosa blooms in coastal regions such as the southern North Sea. However, the lobal matrix of P. pouchetii colonies appears to be less solid than that of P. globosa and partly dissolution of the colony matrix during handling and storage of fixes samples induces uncertainty about the absolute numbers of P. pouchetii colonial cell counts. Despite of that, single cells of P. pouchetii seem to dominate significantly over colonial cell biomass at most sites and during some years and in some regions colonial cells seem rare. We speculate that top-down regulation of Phaeocystis spp. blooms possibly determines the ratio between single and colonial cells.     

STUDIES ON TRANSPARENT EXOPOLYMER PARTICLES (TEP) PRODUCED IN THE ROSS SEA (ANTARCTICA) AND BY PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE)1

The distribution and production of transparent exopolymer particles (TEPs) were studied quantitatively both in cultures of Phaeocystis antarctica Karsten (Prymnesiophyceae) and in natural phytoplankton assemblages in the Ross Sea, Antarctica. TEP production in culture was a function of growth rate and photosynthetic activity and was strongly influenced by photon flux density. The concentrations of TEP measured during a bloom, dominated by P. antarctica, were higher than those produced by coastal diatom blooms and were correlated with chlorophyll a (Chl a), being low at Chl a levels below 3 μgL^?1 but increasing rapidly at greater Chl a concentrations. Because higher chlorophyll hek are dominated 4 larger P. antarctica colonies, this relationship suggests that TEP was produced primarily by sloughing and disintegration of the colonial matrix. TEP concentrations (both absolute and relative to Chl a) increased as the bloom's biomass increased. Vertical distributions of TEP and Chl a showed TEP: chlorophyll maxima at the bottom of the water column at most stations. Because TEP and floc formation are tightly coupled, we suggest that mucous flocs derived from TEP, rather than intact P. antarctica colonies, are the dominant component of aggregates and subsequent organic carbon vertical flux.     

Summer and spring trophic niche of larval and juvenile <Emphasis Type="Italic">Pleuragramma antarcticum</Emphasis> in the Western Ross Sea,Antarctica

The aim of this paper is to contribute to the knowledge on the feeding habits of larvae and juvenile Pleuragramma antarcticum in the western Ross Sea. In summer, the diet of P. antarcticum postlarvae (8–17 mm) was dominated by calanoid eggs (35.5%), Limacina (32.1%) and tintinnids (17.6%), while the principal food of juveniles consisted mainly of copepods (98.2%), with Oncaea curvata being the most abundant (85.1%) and the most frequently consumed prey. The food composition of P. antarcticum postlarvae (24–29 mm), collected in spring, suggest that they fed actively under the sea ice. Stephos longipes, Harpacticus furcifer and Paralabidocera antarctica sea ice copepods represent, in all their different developmental stages, the most abundant biomass food in Terra Nova Bay in this period. Our results therefore suggest that the diet of younger Pleuragramma specimens shifted in prey composition from the first summer to the following spring. This study draws attention to the key role of the copepod, P. antarctica, in the food web of Terra Nova Bay. This article belongs to a special topic: Five articles coordinated by L. Guglielmo and V. Saggiomo appear in this issue of Polar Biology and are a result of a workshop on Sea-ice communities in Terra Nova Bay (Ross Sea) held in August 2007 in Capo Calavà, Messina, Italy.     

TWO DISTINCT COLONIAL MORPHOTYPES OF AMPHORA COFFEAEFORMIS (BACILLARIOPHYCEAE) CULTURED ON SOLID MEDIA1

When cultured on different types of solid media, the marine-fouling diatom Amphora coffeaeformis (Ag.) Kütz. consistently formed two distinct colonial morphotypes named tight and fuzzy. Tight colonies were comprised mainly of small, morphologically distorted, nonmotile cells, whereas morphologically normal and highly motile cells formed the fuzzy colonies. Cells from tight colonies were less adherent to glass, grew more slowly in liquid media, and had a slightly decreased viability on plates with copper than cells from fuzzy colonies. Whereas the protein profiles of the two types of cells were nearly identical in polyacrylamide gels stained with Coomassie blue, cells from tight colonies produced a significantly lower amount of a protease-resistant, low M_r polysaccharide or glycoconjugate as detected in silver-stained gels. The frequency of appearance of the fuzzy and tight morphotypes was not influenced by the mode of nutrition or the type of substratum to which the algal cells adhered. However, certain formulations of solid medium and the presence of growth-inhibitory concentrations of copper in agar plates favored the formation of tight colonies. Due to their frequencies and patterns of appearance, it was clear that the two naturally formed morphotypes were not the consequence of spontaneous mutations, genetic rearrangement, or selection of stable natural variants, and we have hypothesized that they were linked to a normal physiological behavior. The tight colonial morphotype was used as a valuable marker to screen for true motility/adhesion mutants within an ultraviolet-mutagenized population of A. coffeaeformis. Seven mutants were isolated that were non-motile on agar plates, poorly adherent to glass, and distinguished from naturally formed cells from tight colonies by their inability to form fuzzy colonies upon subculture on solid media.     

Factors controlling the colonial structure of <Emphasis Type="Italic">Pediastrum tetras</Emphasis> (Chlorophyceae)

Accounting for morphological plasticity in phytoplankton populations is relevant for taxonomy, systematic/evolutionary, and ecological studies. In this work, the green alga Pediastrum tetras (Ehrenberg) Ralfs was used to describe the variation in population size structure over its growth cycle and to analyze responses to changes in biotic and abiotic factors. Pediastrum cultures reached a final stable concentration in approximately 10 days. This density (8 × 10⁵ cells ml⁻¹) remained stable for at least another 13 days and the intrinsic growth rate was 0.24 ± 0.01 day⁻¹. In the exponential phase, the relative number of single cells and the proportion of large cells (with vesicles inside) within colonies increased. When density peaked, a relative increase of single cells as well as small cells in new colonies took place. Finally, during the stationary phase, the trend reversed: fewer single cells and a larger cell size (without vesicles) were observed. Results indicated that nutrient supply could affect population structure, diminishing the proportion of eight-cell colonies. Daphnia magna Straus significantly reduced the Pediastrum population density due to predation, and this led to a significant decrease in the density of the largest colonies. In addition, info-chemicals induced a slight increase in the density of the largest colonies compared to the control treatment. Our study suggests a possible trade-off in P. tetras colonial size in natural environments: during the stationary growth period in a lake, Pediastrum populations tend to increase in size for efficient use of nutrients, while they decrease in size in the presence of herbivores. Handling editor: J. Padisak     

Biochemical,morphological, and genetic variations in <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> due to colony disaggregation

Microcystis aeruginosa commonly occurs as large colonial morph under natural conditions, but disaggregates and exists as single cells in laboratory cultures. To demonstrate the adaptive changes, differentiation of carbohydrates, pigments, and protein between colonial and disaggregated M. aeruginosa were examined. Their morphological and ultrastructural characteristics were subsequently observed by scanning electron microscopy and transmission electron microscopy. Results showed that chlorophyll a and phycocyanin in cells, soluble carbohydrate produced in the culture medium, and total carbohydrate in cells and sheath of colonial M. aeruginosa are significantly higher (p < 0.05) compared with those in disaggregated cells. No significant change was found in protein concentration per cell (p > 0.05) between them. Their morphological and ultrastructural characteristics were evidently different, and by morphological criteria they could be separated into two morphotypes. In addition, the genetic diversity of 16S–23S internal transcribed spacer of them were examined and compared with reference strains of M. aeruginosa. The alignment of two sequences revealed that genetic identity level was extremely high (96.94%) and no significant difference was found in the nucleotide diversity (0.014 ± 0.008). This suggested that similar genotypes could present distinct morphotypes in M. aeruginosa. The tree topologies and analysis of molecular variance of the two sequences and reference sequences from GenBank database indicated that the genotypes of M. aeruginosa strains were not always related to their localities and exhibit heterogeneity within a species.     

Spawning of the colonial coral <Emphasis Type="Italic">Cladocora caespitosa</Emphasis> (Anthozoa,Scleractinia) in the Southern Adriatic Sea

Data on sexual reproduction of scleractinian coral species living in temperate zones, particularly in the Mediterranean Sea, are quite scarce. This paper describes sexual reproduction of the colonial coral Cladocora caespitosa from Veliko jezero (Mljet Island) in the Adriatic Sea. Spawned orange eggs and white sperm bundles were observed on the coral bank of C. caespitosa two nights before the full moon (20 June 2005) coinciding with increasing water temperature and correlated with the lunar cycle. Spawning was observed during five nights, involving about 30% of the colonies from the coral bank. Different colonies on the bank released only one type of gamete during the reproductive period. The diameter of the sperm bundles ranged from 100 to 200 μm (average 163 μm; SD = 47.08), while the female gametes diameter ranged from 300 to 500 μm (average 416 μm; SD = 73.12).     

Estimating the relative abundance of emperor penguins at inaccessible colonies using satellite imagery

Emperor penguin (Aptenodytes forsteri) populations are useful environmental indicators due to the bird’s extreme reliance on sea ice. We used remote sensing technology to estimate relative adult bird abundance at two inaccessible emperor penguin colonies in the Ross Sea, Antarctica. We performed supervised classification of 12 panchromatic satellite images of the seven known Ross Sea colonies. We used regression to predict adult bird counts at the inaccessible colonies by relating the number of pixels classified as “penguin” in the satellite images of the accessible colonies to corresponding known adult bird counts from aerial photographs or ground counts. While our analysis was hampered by excessive guano and shadows, we used satellite imagery to differentiate between relatively small (<3,000 adult birds) and larger colonies (>5,000 adult birds). Remote sensing technology is logistically less intense and less costly than aerial or ground censuses when the objective is to document penguin presence and/or large emperor penguin population changes (e.g., catastrophic changes). Improvements expected soon in the resolution of the satellite images should allow for more accurate abundance estimates.     

Cell differentiation and morphogenesis in the colony-forming choanoflagellate Salpingoeca rosetta

It has been posited that animal development evolved from pre-existing mechanisms for regulating cell differentiation in the single celled and colonial ancestors of animals. Although the progenitors of animals cannot be studied directly, insights into their cell biology may be gleaned from comparisons between animals and their closest living relatives, the choanoflagellates. We report here on the life history, cell differentiation and intercellular interactions in the colony-forming choanoflagellate Salpingoeca rosetta. In response to diverse environmental cues, S. rosetta differentiates into at least five distinct cell types, including three solitary cell types (slow swimmers, fast swimmers, and thecate cells) and two colonial forms (rosettes and chains). Electron microscopy reveals that cells within colonies are held together by a combination of fine intercellular bridges, a shared extracellular matrix, and filopodia. In addition, we have discovered that the carbohydrate-binding protein wheat germ agglutinin specifically stains colonies and the slow swimmers from which they form, showing that molecular differentiation precedes multicellular development. Together, these results help establish S. rosetta as a model system for studying simple multicellularity in choanoflagellates and provide an experimental framework for investigating the origin of animal multicellularity and development.     

Medusae from Mcmurdo Sound,Ross Sea including the descriptions of two new species,Leuckartiara brownei and Benthocodon hyalinus

Seven species of medusae were collected using scuba equipment in neritic waters of McMurdo Sound, Ross Sea, Antarctica. Two species dominated, the narcomedusa Solmundella bitentaculata and the scyphomedusa Diplulmaris antarctica. Two new taxa were found. Leuckartiara brownei n. sp., a pandeid anthomedusa, has gonads with two major longitudinal folds in the interradii and several smaller perradially directed folds. Besides four large perradial tentacles, it has up to 28 short rudimentary tentacles. This is the first report of this genus from the Southern Ocean. Benthocodon hyalinus n. gen., n. sp., a rhopalonematoid trachymedusa, has 8 linear to sinuous gonads on the radial canals. The tubular manubrium is attached to a gastric peduncle. The tentacles are numerous (about 800) and grow successively. This is the second reported trachymedusa from the Ross Sea. Solmundella bitentaculata ate the thecosome pteropod Limacina helicina. Diplulmaris antarctica medusae fed on the gymnosome pteropod Clione antarctica and on L. helicina. The hyperiid amphipod Hyperiella dilatata, mostly females, was seen attached to the exumbrellas of both D. antarctica and S. bitentaculata. Up to 54 amphipods were seen on each D. antarctica medusa. The amphipods did no visible damage to the medusae.     

A NEW CELL STAGE IN THE HAPLOID‐DIPLOID LIFE CYCLE OF THE COLONY‐FORMING HAPTOPHYTE PHAEOCYSTIS ANTARCTICA AND ITS ECOLOGICAL IMPLICATIONS1

Few members of the well‐studied marine phytoplankton taxa have such a complex and polymorphic life cycle as the genus Phaeocystis. However, despite the ecological and biogeochemical importance of Phaeocystis blooms, the life cycle of the major bloom‐forming species of this genus remains illusive and poorly resolved. At least six different life stages and up to 15 different functional components of the life cycle have been proposed. Our culture and field observations indicate that there is a previously unrecognized stage in the life cycle of P. antarctica G. Karst. This stage comprises nonmotile cells that range in size from ～4.2 to 9.8 μm in diameter and form aggregates in which interstitial spaces between cells are small or absent. The aggregates (hereafter called attached aggregates, AAs) adhere to available surfaces. In field samples, small AAs, surrounded by a colony skin, adopt an epiphytic lifestyle and adhere in most cases to setae or spines of diatoms. These AAs, either directly or via other life stages, produce the colonial life stage. Culture studies indicate that bloom‐forming, colonial stages release flagellates (microzoospores) that fuse and form AAs, which can proliferate on the bottom of culture vessels and can eventually reform free‐floating colonies. We propose that these AAs are a new stage in the life cycle of P. antarctica, which we believe to be the zygote, thus documenting sexual reproduction in this species for the first time.     

Synopsis of a new collection of sea spiders (Arthropoda: Pycnogonida) from the Ross Sea,Antarctica

The biodiversity research expedition TAN0204 with RS Tangaroa to the Ross Sea in 2004 yielded a new collection of 2,687 specimens of pycnogonids. As much as 25 different species encompassing 14 genera and eight families were identified and their records are discussed herein. The collection is archived in the Marine Invertebrate Collection of the New Zealand National Institute of Water and Atmospheric Research (NIWA). The majority (69%) of specimens are from the Nymphon australe group (Nymphonidae), although species richness and abundance varied among the stations sampled. The collection includes several specimens from polymerous taxa; Pentanymphon antarcticum (Nymphonidae), Decolopoda australis (Colossendeidae) and Pentapycnon bouvieri (Pycnogonidae). All species were classified based on morphological characters, and DNA sequences (from the 18S, 12S, 16S and COI regions) for 21 of the representative morphotypes are given. The DNA sequences confirmed the species-level distinctiveness of these morphotypes. No species new to science were identified, although further detailed morphometric and/or molecular analyses may reveal cryptic or sibling species, especially in species such as the highly abundant Nymphon australe group. An erratum to this article can be found at     

Patterns in the distribution of myctophid fish in the northern Scotia Sea ecosystem

The mesopelagic fish community of the northern Scotia Sea was investigated during the austral autumn using multi-frequency acoustics, opening and closing nets and pelagic trawls fished from the surface to 1,000 m. The Family Myctophidae (15 species in 5 genera) dominated the ichthyofauna, with larval notothenids caught over the South Georgia shelf and bathylagids and stomiids abundant in deeper hauls. The biomass of myctophids was estimated to be 2.93 g wet weight 1,000 m⁻³, with Electrona carlsbergi, E. antarctica, Protomyctophum bolini, P. choriodon, Gymnoscopelus braueri, G. fraseri, G. nicholsi and Krefftichthys anderssoni, being the most abundant species. Analysis of community structure indicated a high level of depth stratification within the myctophids, with evidence of diurnal vertical migration in some, but not all, species. Length-frequencies of G. braueri, G. nicholsi, E. antarctica and K. anderssoni were multimodal, suggesting that all life stages may be present in the northern Scotia Sea. In contrast, P. choriodon, P. bolini, G. fraseri and E. carlsbergi had unimodal distributions despite having multi-year lifecycles, indicating that they probably migrate into the region from warmer areas to the north.     

Biogeochemical pools and fluxes of carbon and nitrogen in a maritime tundra near penguin colonies along the Antarctic Peninsula

There is limited information regarding biogeochemical pools and fluxes in maritime tundra ecosystems along the Antarctic Peninsula. To collect baseline information on biogeochemical processes in a tundra ecosystem dominated by two vascular plant species (Colobanthus quitensis and Deschampsia antarctica) at Biscoe Point off the coast of Anvers Island, we measured pools and fluxes of C and N in transplanted tundra microcosm cores, complemented with sampling of precipitation and surface runoff. Snow and snowmelt from the tundra collection site and soil leachates from the cores were enriched with N and dissolved organic carbon compared to precipitation and snowmelt samples collected at Palmer Station, indicating high loading of N and organic matter from the penguin colonies adjacent to the tundra site. Relatively high values of δ¹⁵N in the live and dead biomass of D. antarctica and C. quitensis (5.6–25.1‰) indicated an enrichment of N in this tundra ecosystem, possibly through N inputs from adjacent penguin colonies. Stepwise multiple linear regressions found that ecosystem respiration and gross primary production were best predicted by live biomass of D. antarctica, suggesting a disproportionately high contribution of D. antarctica to CO₂ fluxes. The cores with higher δ¹⁵N and lower δ¹³C in the soil organic horizon exhibited higher CO₂ fluxes. The results suggest that abundant N inputs from penguin colonies and the competitive balance between plant species might play a critical role in the response of tundra ecosystems along the Antarctic Peninsula to projected climate change.