Ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA, during 1972-1977

Plankton dynamics at a station in lower Narragansett Bay, RIare compared for six summer and fall seasons, 1972–1977.In four of these years, initiation of the summer pulse of thectenophore Mnemiopsis leidyi was accompanied by a rapid declinein zooplankton abundance and a summer phytoplankton bloom. Terminationof the phytoplankton bloom coincided with depleted ctenophoreabundance and increased zooplankton biomass in two of the years.Yearly variations in the summer abundance of the diatom Skeletonemacostatum were positively related to the magnitude of the ctenophorepulse. The magnitude of ctenophore population was related tothe zooplankton biomass present at the start of the pulse. Theserelationships, the timing and magnitude of the plankton eventssuggest that M. leidyi regulated summer zooplankton and phytoplanktondynamics. Ctenophores may control phytoplankton blooms indirectlythrough their predation on herbivorous zooplankton and directlyby the nutrient excretion accompanying such grazing. This evidencethat a planktonic carnivore two trophic steps removed from thephytoplankton regulates the latter's dynamics in NarragansettBay is analogous to reported regulation of benthic algal (kelp)dynamics by the sea otter, lobster and various crabs throughtheir predation on herbivorous sea urchins. The factors responsiblefor the seasonal decrease in ctenophores remain unresolved;ctenophore predators on Mnemiopsis are absent in NarragansettBay. Infection by the vermiform larval anemone, Edwardsia lineata,grazing by the butterfish, Peprilus triacanthus, and changesin food availability, temperature and salinity likewise do notexplain this disappearance.     

Seasonal variations of tintinnids (Ciliophora: Oligotrichida) in Narragansett Bay, Rhode Island, U.S.A.

Tintinnids exhibit a bimodal peak of abundance in NarragansettBay, with a minimum in late spring and a lesser peak in latesummer-early autumn, depending on location. Thirty-three speciesin eight genera were identified, with the fauna dominated bythe genus Tintinnopsis. Tintinnid abundance was not obviouslycorrelated with salinity, temperature, or nanoplankton chlorophyllconcentration. Patterns of change in seasonal abundance canserve as a framework for posing additional questions of therole tintinnids play in marine food webs.     

Observations on the valve structure of marine species of the diatom genus Cocconeis Ehr.

The valve structure of three marine diatom species, Cocconeis molesta var. crucifera, C. dirupta and C. pellucida was studied by transmission and scanning electron microscopy. In all Cocconeis examined, the areolae are occluded by hymenes located near the internal openings in the raphid valve and near the internal openings in the araphid valves.     

Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

The contribution of nanoplankton (< 10 µm fraction)to winter – spring (1977 – 78) and summer (1978,1979) phytoplankton nitrogen dynamics in lower NarragansettBay was estimated from ammonium, nitrate and urea uptake ratesmeasured by ¹⁵N tracer methods. During the winter – spring,an average of 80% of chlorophyll a and nitrogen uptake was associatedwith phytoplankton retained by a 10 µm screen. In contrast,means of 51 – 58% of the summer chlorophyll a standingcrops and 64 – 70% of nitrogen uptake were associatedwith cells passing a 10 µm screen. Specific uptake ratesof winter – spring nanoplankton populations were consistentlylower than those of the total population. Specific uptake ratesof fractionated and unfractionated summer populations were notsignificantly different. Ammonium uptake averaged between 50and 67% of the total nitrogen uptake for both the total populationand the < 10µm fraction. The total population and the10 µm fraction displayed similar preferences for individualnitrogen species. Though composed of smaller cells, flagellatedominated nanoplankton assemblages may not necessarily takeup nitrogen at faster rates than diatom dominated assemblagesof larger phytoplankters in natural populations. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia     

Toxic Alexandrium peruvianum (Balech and de Mendiola) Balech and Tangen in Narragansett Bay,Rhode Island (USA)

Phytoplankton monitoring in Wickford Cove, Rhode Island, US (41°34′10.13″N, 71°26′45.76″W), located in Narragansett Bay, detected an unusual species of Alexandrium in the spring of 2009. Thecal plate analysis using brightfield and SEM microscopy revealed a plate morphology consistent with that of Alexandrium peruvianum (Balech and de Mendiola) Balech and Tangen. Molecular analyses indicated that the sequences of the SSU, ITS1, 5.8S, ITS2 and LSU through the D region of the 18S gene were similar to those of A. peruvianum from North Carolina. Toxin analyses of cells brought into culture revealed saxitoxins, gymnodimine and fast-acting spiroimines were present in the cultured clone. Saxitoxins detected included GTX 2, GTX3, B1, STX, C1 and C2. Also present in the Wickford cove isolates of A. peruvianum were 12-methyl gymnodimine and 13-desmethyl spirolide C. A. peruvianum was detected at four sites in lower Narragansett Bay: at two sites in Wickford and two sites in Jamestown, RI. A. peruvianum was observed in the spring of 2009, 2010, 2011 and 2012 at maximum abundance levels ranging from tens of cells per liter to 14,000 cells L⁻¹. The discovery of A. peruvianum in Rhode Island coastal waters, with its potential threat to public health, is notable as it appears to be an emergent bloom species globally. The presence of A. peruvianum in Narragansett Bay is the third confirmed observation of this species on the Atlantic coast of North America. Monitoring efforts in the southern New England region should incorporate measures to detect the presence of A. peruvianum toxins.     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

Vallodiscus gen. nov., a new fossil resting spore morpho-genus related to the marine diatom genus Chaetoceros (Bacillariophyceae)

A new fossil marine diatom resting spore morphogenus, Vallodiscus Suto gen. nov., is described using samples from Deep Sea Drilling Project Site 338 in the Norwegian Sea, Sites 436 and 438 in the north‐west Pacific Ocean and the onland Newport Beach Section, California. Vallodiscus is characterized by a single ring of veins along the epivalve margin and a hypovalve covered with circular depressions of several sizes with gentle elevation. The morpho‐genus bears three new species and one new combination: Vallodiscus simplexus Suto sp. nov., Vallodiscus complexus Suto sp. nov., Vallodiscus lanceolatus Suto sp. nov. and Vallodiscus chinchae (Mereschkowsky) Suto comb. nov.     

Population genetic analyses are consistent with the introduction of Ceramium secundatum (Ceramiaceae,Rhodophyta) to Narragansett Bay,Rhode Island,USA

During ongoing DNA barcode (COI‐5P) surveys of the macroalgal flora along the northwest Atlantic coast, we discovered a population of Ceramium secundatum in Narragansett Bay, Rhode Island, USA. This species is regarded as common and widespread in the northeast Atlantic, ranging from Norway to Morocco, but until now has not been reported from the western Atlantic. Several lines of evidence suggest that C. secundatum may be introduced to Narragansett Bay: (1) despite extensive collecting, specimens have only been obtained from a limited geographic range in the northwest Atlantic; (2) three other nonindigenous seaweed species are reportedly introduced in this region, which is thought to be a consequence of shipping; and (3) this species is introduced to South Africa and New Zealand. To investigate this suspected introduction, we applied population genetic analyses (using the cox2‐3 spacer) to compare the Narragansett Bay C. secundatum population to native populations in the Republic of Ireland and the United Kingdom. Collectively, analyses of biogeographical and molecular data indicate that C. secundatum is likely introduced to Narragansett Bay. The implications of this discovery are discussed.     

Experimental evaluation of herbivory in the ctenophore Mnemiopsis leidyi relevant to ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA

Herbivory of Mnemiopsis leidyi and its interactions with phytoplanktonand non-gelatinous zooplankton were examined in small-scalemicrocosm experiments. Clearance rates for M. leidyi incubatedwith phytoplankton were generally negative, but ranged up to4.5 1 ctenophore^–1 day^–1 when the large (80 µmø) diatom Ditylum brightwelli was offered as a food source.These highest ingestion rates would provide Mnemiopsis withonly 21 % of its daily carbon requirements for respiration.Mean shrinkage of M. leidyi was 8.2–51% when incubatedwith phytoplankton. Although M. leidyi neither fed activelyon phytoplankton, nor satisfied its nutritional needs on sucha diet, the chain-forming diatom Skeletonema costatum becameentangled in mucus strands and balls produced by M. leidyi inthe absence of zooplankton. Attachment onto mucus occurred atphytoplankton concentrations commonly observed in NarragansettBay and may be important in the formation of "marine snow" duringsummer M. leidyi pulses; phytoplankton sinking rate and the"package size" available to herbivores would also be affected.The experiments support our previous hypothesis based on fieldobservations in Narragansett Bay that M. leidyi indirectly regulatesphytoplankton abundance there during the summer bloom as a consequenceof predation on zooplankton. The extent to which M. leidyi influencedphytoplankton dynamics in the microcosms was dependent on therelative abundance and physiological state of the three trophiclevels. A food web diagram for M. leidyi is presented.     

Ploidy Distribution of the Harmful Bloom Forming Macroalgae Ulva spp. in Narragansett Bay,Rhode Island,USA, Using Flow Cytometry Methods

Macroalgal blooms occur worldwide and have the potential to cause severe ecological and economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have biphasic life cycles with separate haploid and diploid phases. In this study, we used flow cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom formation. Both haploid gametophytes and diploid sporophytes were present for both species. Sites ranged from a relative overabundance of gametophytes to a relative overabundance of sporophytes, compared to the null model prediction of √2 gametophytes: 1 sporophyte. We found significant differences in cell area between ploidy levels for each species, with sporophyte cells significantly larger than gametophyte cells in U. compressa and U. rigida. We found no differences in relative growth rate between ploidy levels for each species. Our results indicate the presence of both phases of each of the two dominant bloom forming species throughout the bloom season, and represent one of the first studies of in situ Ulva life cycle dynamics.     

Examination of Ulva bloom species richness and relative abundance reveals two cryptically co-occurring bloom species in Narragansett Bay,Rhode Island

Blooms caused by the green macroalga Ulva pose a serious threat to coastal ecosystems around the world. Despite numerous studies of the causes and consequences of these blooms, we still have a limited understanding of Ulva bloom species richness and abundance due to difficulties in identifying Ulva species using morphological features. Along the northeastern U.S. coastline, all blooms of distromatic Ulva blades were previously identified as Ulva lactuca. Recent molecular sequencing, however, discovered the presence of additional distromatic Ulva species. Therefore, in order to determine the relative abundance of Ulva species within blooms, we conducted monthly surveys at four Narragansett Bay, RI, sites representing a gradient of bloom severity. We found that the biomass of Ulva within blooms was a mix of Ulva compressa and Ulva rigida, not U. lactuca as previously reported. In contrast, sites not impacted by blooms that were located near the mouth of Narragansett Bay were dominated by U. lactuca. We also observed spatial and temporal differences in Ulva and total macroalgal diversity between bloom-impacted sites, indicating that Ulva bloom composition can be radically different between similar sites within close proximity. We discuss our results in the context of Ulva blooms worldwide, highlighting the need to definitively determine bloom species composition in order to fully understand bloom dynamics.     

The occurrence of eukaryotic type III glutamine synthetase in the marine diatom Chaetoceros compressum

Glutamine synthetase (GS) has been described as one of the oldest functioning genes and thus a good molecular clock protein. GS is diverged into three distinct forms, type I (GSI), type II (GSII) and type III (GSIII), the last type of which is a member of the most recently discovered family among GSs and thus has been reported from a limited number of prokaryotes. In the present study, we determined the full-length sequence of GSIII from the marine diatom Chaetoceros compressum. The 3′ untranslated region of the diatom GSIII gene was composed of a polyadenylation signal followed by a poly (A)⁺ tail, clearly demonstrating that its mRNA is transcribed from the eukaryotic genome. We also screened available genome databases and identified full-length GSIII sequences from 5 eukaryotic species. These eukaryotic GSIIIs specifically contained regions A–D and a long additional sequence flanking region V toward the C-terminal site, both being specific to GSIII. Phylogenic analysis revealed that eukaryotic GSIIIs are not within a monophyletic relationship with the possible occurrence of lateral gene transfer in GSIII during evolution.     

The role of external carbonic anhydrase in photosynthesis during growth of the marine diatom Chaetoceros muelleri

Carbon dioxide concentrating mechanisms (CCMs) act to improve the supply of CO₂ at the active site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase. There is substantial evidence that in some microalgal species CCMs involve an external carbonic anhydrase (CA_ext) and that CA_ext activity is induced by low CO₂ concentrations in the growth medium. However, much of this work has been conducted on cells adapted to air‐equilibrium concentrations of CO₂, rather than to changing CO₂ conditions caused by growing microalgal populations. We investigated the role of CA_ext in inorganic carbon (C_i) acquisition and photosynthesis at three sampling points during the growth cycle of the cosmopolitan marine diatom Chaetoceros muelleri. We observed that CA_ext activity increased with decreasing C_i, particularly CO₂, concentration, supporting the idea that CA_ext is modulated by external CO₂ concentration. Additionally, we found that the contribution of CA_ext activity to carbon acquisition for photosynthesis varies over time, increasing between the first and second sampling points before decreasing at the last sampling point, where external pH was high. Lastly, decreases in maximum quantum yield of photosystem II (F_v/F_m), chlorophyll, maximum relative electron transport rate, light harvesting efficiency (α) and maximum rates of C_i‐ saturated photosynthesis (V_max) were observed over time. Despite this decrease in photosynthetic capacity an up‐regulation of CCM activity, indicated by a decreasing half‐saturation constant for CO₂ (K_0.5CO₂), occurred over time. The flexibility of the CCM during the course of growth in C. muelleri may contribute to the reported dominance and persistence of this species in phytoplankton blooms.     

Morphology and molecular phylogeny of the marine diatom genus Nagumoea (Bacillariophyceae) from Japan

The canal-bearing diatom genus Nagumoea, described based on only morphological evidence, was tentatively assigned to the order Bacillariales, although its phylogenetic position remained unclear. Because three isolates of Nagumoea (SK002, SK024 and SK053) were successfully established from Japanese coasts, we performed their morphological observations and molecular phylogenetic analyses to discuss the phylogeny and taxonomic position of this genus. Strains SK002 and SK024 were identified as Nagumoea africana, whereas SK053 conformed with Nagumoea serrata. There was high interspecific divergence between N. africana and N. serrata in the rbcL sequences (8.03–8.17%), indicating their distinctness. Furthermore, intraspecific variations were detected within N. africana (2.35%) in the rbcL, implying its cryptic diversity. The maximum likelihood and Bayesian phylogenetic trees inferred from the plastid rbcL, psbC and nuclear 18S rDNA genes recovered Nagumoea as monophyletic with strong statistical support and embedded within an unresolved, poorly supported lineage containing Achnanthes, Craspedostauros, Staurotropis and Undatella in the canal-bearing order Bacillariales (= the family Bacillariaceae). Although the constrained tree based on the monophyly of Nagumoea and the other canal-bearing clade (Surirellales and Rhopalodiales) was statistically rejected by the topology tests, the phylogenetic position of Nagumoea with other Bacillarialean members remains equivocal. The possession of two plastids positioned fore and aft, observed in the present study, and lack of keel, typical of the Bacillariales, indicate the possibility of Nagumoea being part of the ingroup of the Bacillariales or its closely related outgroup.     

Evaluation of nitrogen fixation in the diatom genus Rhizosolenia Ehr. in the absence of its cyanobacterial symbiont Richelia intracellularis Schmidt

Aggregates of two species of Rhizosolenia (R. castracanei andR. imbricata var. shrubsolei) have been reported to containendosymbiotic bacteria and to fix nitrogen. The general importanceof this process to the genus is not known, although of greatpotential significance. Clonal cultures of five species of Rhizosolenia(R. alata, R. bergonii, R. calcar-avis, R. imbricata var. shrubsoleiand R. setigera) were examined for nitrogen fixation using acetylenereduction and compared to cyanobacterial controls. Mecososmaggregates of R. imbricata var. shrubsolei were tested, as wellas chains of R. debyana hand-collected using SCUBA. The Rhizosoleniaspecies did not contain the endosymbiotic cyanobacterium Richeliaintracellularis and were not examined for the presence of endosymbioticbacteria. Nitrogen fixation was not found in the Rhizosoleniaspp.; in contrast, the diazotrophic cyanobacterial controlsOscillatoria erythraea and Anabaena sp. reduced acetylene atsignificant rates. The absence of nitrogen fixation in the Rhizosoleniaspp. suggests diazotrophy by endosymbiotic bacteria is not widespreadin the genus Rhizosolenia, nor is it necessary for survivalof these large diatom cells in nutrient-poor, near-shore waters.     

Concerted response of xanthophyll-cycle pigments in a marine diatom, Chaetoceros gracilis, to shifts in light condition

The interconversion rate of diadinoxanthin (DD) cycle under high irradiance and subsequent darkness was analyzed using the cultivated centric marine diatom, Chaetoceros gracilis Schütt. A prompt de‐epoxidation from diadinoxanthin to diatoxanthin (DT) occurred immediately after the onset of higher irradiance. The fist‐order rate constant, k, for this de‐epoxidation was 0.1–0.2 min‐¹ irrespective of the irradiance. The difference in photon fluence rate lead to the difference of the final amount of DT, leaving the rate constant at almost the same value. After turning off the light, epoxidation from DT to DD occurred. The first‐order constant of epoxidation was much slower than that of de‐epoxidation: 0.005 – 0.009 min‐¹. Independent of this epoxidation process, de novo synthesis of DD‐cycle pigments was also observed under the subsequent darkness. Based on these findings, a common nature of the DD‐cycle as a protection mechanism for photo‐systems was demonstrated for C. gracilis.     

湛江湾角毛藻群落的时空分布及其影响因素

2009年2-11月以水采、网采和显微鉴定的方法分别对湛江湾的浮游植物进行了周年的季节调查,分析了湛江湾角毛藻(Chaetoceros)群落的时空分布和影响因素.结果表明:湛江湾角毛藻共有32种,其中有17种为赤潮种,优势种有旋链角毛藻(C.curvisetus)、假弯角毛藻(C.pseudocurvisetus)和牟氏角毛藻(C.muelleri)3种;种类丰富度和细胞丰度在季节分布上表现为春夏季高于秋冬季,而在空间分布上总体表现为湾口高,向内湾逐渐降低.相关分析表明:角毛藻细胞丰度与水温、盐度、pH、叶绿素a浓度及浮游植物细胞丰度呈极显著的正相关,与DIN、SiO32-呈极显著的负相关,与PO43-相关性不显著:角毛藻细胞丰度的季节变化主要取决于水温,由季节更替而产生的种间竞争和浮游动物的摄食压力也是重要的影响因素,而空间变化则受到光照、盐度、pH和高丰度养殖贝类滤食压力的影响.