Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

Diatoms from surface sediments of Amurskiy Bay,Sea of Japan

The results of a study of diatoms from surface sediments (0–1 cm) of Amurskiy Bay are presented for the first time. The specific composition (221 species and intraspecific taxa) and ecological structure of the diatom flora were determined. The diatom species composition of phytoplankton, periphyton, and surface sediments is compared.     

Bacteria may induce the secretion of mucin‐like proteins by the diatom Phaeodactylum tricornutum

Benthic diatoms live in photoautotrophic/heterotrophic biofilm communities embedded in a matrix of secreted extracellular polymeric substances. Closely associated bacteria influence their growth, aggregation, and secretion of exopolymers. We have studied a diatom/bacteria model community, in which a marine Roseobacter strain is able to grow with secreted diatom exopolymers as a sole source of carbon. The strain influences the aggregation of Phaeodactylum tricornutum by inducing a morphotypic transition from planktonic, fusiform cells to benthic, oval cells. Analysis of the extracellular soluble proteome of P. tricornutum in the presence and absence of bacteria revealed constitutively expressed newly identified proteins with mucin‐like domains that appear to be typical for extracellular diatom proteins. In contrast to mucins, the proline‐, serine‐, threonine‐rich (PST) domains in these proteins were also found in combination with protease‐, glucosidase‐ and leucine‐rich repeat‐domains. Bioinformatic functional predictions indicate that several of these newly identified diatom‐specific proteins may be involved in algal defense, intercellular signaling, and aggregation.     

基于硅藻功能群特征揭示海西海近现代生态环境变化

依据硅藻的形态、生理与生态特点而定义的功能群分类,可以更好地描述对湖泊环境胁迫的响应特征.本研究识别了海西海表层沉积物硅藻群的空间分布模式,并通过多指标分析探讨其与环境因子的关系.结果表明: 主成分分析和冗余分析,水深和沉积物总氮含量是硅藻功能群空间分布异质性的主要驱动因子;水深8 m左右可能是海西海硅藻出现功能群分异的阈值,与热力分层深度等湖泊水文特征相对应.上述表层沉积物硅藻功能群空间分布结果为沉积物化石记录的解释提供了依据.沉积物钻孔分析显示,近百年来海西海硅藻功能群呈现明显的阶段性波动,与年均气温、沉积物总氮含量、中值粒径等指标变化相对应.气候变暖、富营养化和水位上升是驱动海西海硅藻功能群长期变化的主要环境因子,协同促进了功能群D、P、MP占优势.硅藻功能群多样性变化的长期特征表明气候变暖和营养盐富集促进了硅藻功能群多样性的增加,而海西海1957年筑坝和1990年水库扩容两次典型水文调控显著增加了湖泊水深,导致硅藻功能群多样性降低.     

Benthic diatom community composition in three regions of the Great Barrier Reef,Australia

Despite their ecological importance, very little is known about the taxonomy and ecology of benthic diatoms in coral-reef ecosystems. Diatom densities and community compositions were investigated in three distinct regions of the Great Barrier Reef (GBR): (a) Wet Tropics (WT), (b) Princess Charlotte Bay (PCB), and (c) the Outer Shelf (OS). About 209 taxa were observed in the GBR sediments studied, with an average abundance of 2.55 × 10⁶ cells ml⁻¹ in the upper 1 cm of sediment. Total diatom abundances were about twice as high in inshore reefs of PCB and WT compared with OS reefs. A redundancy analysis (RDA) of diatom composition clearly grouped the three regions separately but showed little influence of grain size, nitrogen and organic carbon content of the sediments. The only distinct correlates were inorganic carbon and the distance to the mainland associated with OS communities. Analysis of similarity (ANOSIM) of diatom community composition revealed significant differences between all three regions. Indicator values showed that most highly abundant taxa occurred in all regions. However, several taxa were clearly identified as characteristic of particular regions. It is hypothesised that variations in nutrient and light availability are the most likely explanation for the observed differences in community composition.     

A weighted—averaging regression and calibration model for inferring total phosphorus concentration from diatoms in British Columbia (Canada) lakes

1. The relationship between surficial sediment diatom taxa (Bacillariophyceae) and measured limnological variables in forty-six British Columbia lakes was explored using canonical correspondence analysis (CCA). Lake-water total phosphorus concentration (TP), maximum lake depth, conductivity, and calcium concentration each accounted for independent and statistically significant directions of variation in the distribution of diatom taxa. 2. Weighted-averaging (WA) models were developed to infer lake-water TP from the relative abundances of 131 diatom taxa in the surficial sediments of thirty-seven lakes. WA regression and calibration with classical deshrinking provided the best model for TP reconstructions. 3. Our quantitative inference model has two major advantages over existing multiple linear-regression models: (i) inferences are based on the responses of individual taxa to TP, and do not involve grouping the taxa into a small number of ecological categories; and (ii) the model assumes that diatoms respond to TP in a unimodal, rather than a linear, fashion. 4. The WA model can now be used to infer past lake-water TP, within the range 5–28νgr1^?1, from diatoms preserved in the sediments of British Columbia lakes. The model can provide quantitative estimates of the onset, rate, and magnitude of lake eutrophication in response to natural processes and human disturbances.     

Diatoms and their preservation in the sediments of Lake Neuchâtel (Switzerland) as evidence of past hydrological changes

Core LNO3 is 1136 cm long and includes uninterrupted sediments from the Oldest Dryas to historic time which have been submitted for geological, geochemical, palynological and diatom analysis. Thirty samples have been investigated for diatoms, focussing especially on their preservation, which changed during major hydrological events. In the lower part of the core (Oldest Dryas to beginning of Atlantic), with low carbonate, but high clastic silicate content, the diatoms are rare but are well preserved. In the upper well carbonated part of the core (Atlantic to historical periods), the diatoms are abundant but show traces of dissolution.Between 1136 and 909 cm (Oldest Dryas) the flora is made up of benthic species principally of the genus Fragilaria. This flora is often the first to develop after the last glaciation. Between 879 and 569 cm (Bølling to beginning of Atlantic), littoral and terrestrial species adapted to eutrophic conditions are dominating. They suggest a strong fluvial impact on sedimentation, which corroborates the hypothesis of past inflows of the River Aar into the lake. Between 529 cm and 1 cm (Atlantic to historical periods) planktonic species of the genera Cyclotella, Aulacoseira or Stephanodiscus are dominating. This suggests that the upper well carbonated sediments were mainly formed by pelagic precipitation, without influence of the River Aar. The trophic level of the lake seems to have decreased, then increased again during prehistoric deforestation and development of agriculture in the watershed.     

Late Cretaceous marine arthropods relied on terrestrial organic matter as a food source: Geochemical evidence from the Coon Creek Lagerstätte in the Mississippi Embayment

The Upper Cretaceous Coon Creek Lagerstätte of Tennessee, USA, is known for its extremely well‐preserved mollusks and decapod crustaceans. However, the depositional environment of this unit, particularly its distance to the shoreline, has long been equivocal. To better constrain the coastal proximity of the Coon Creek Formation, we carried out a multiproxy geochemical analysis of fossil decapod (crab, mud shrimp) cuticle and associated sediment from the type section. Elemental analysis and Raman spectroscopy confirmed the presence of kerogenized carbon in the crabs and mud shrimp; carbon isotope (δ¹³C) analysis of bulk decapod cuticle yielded similar mean δ¹³C values for both taxa (?25.1‰ and ?26‰, respectively). Sedimentary biomarkers were composed of n‐alkanes from C₁₆ to C₃₆, with the short‐chain n‐alkanes dominating, as well as other biomarkers (pristane, phytane, hopanes). Raman spectra and biomarker thermal maturity indices suggest that the Coon Creek Formation sediments are immature, which supports retention of unaltered, biogenic isotopic signals in the fossil organic carbon remains. Using our isotopic results and published calcium carbonate δ¹³C values, we modeled carbon isotope values of carbon sources in the Coon Creek Formation, including potential marine (phytoplankton) and terrestrial (plant) dietary sources. Coon Creek Formation decapod δ¹³C values fall closer to those estimated for terrigenous plants than marine phytoplankton, indicating that these organisms were feeding primarily on terrigenous organic matter. From this model, we infer that the Coon Creek Formation experienced significant terrigenous organic matter input via a freshwater source and thus was deposited in a shallow, nearshore marine environment proximal to the shoreline. This study helps refine the paleoecology of nearshore settings in the Mississippi Embayment during the global climatic shift in the late Campanian–early Maastrichtian and demonstrates for the first time that organic δ¹³C signatures in exceptionally preserved fossil marine arthropods are a viable proxy for use in paleoenvironmental reconstructions.     

Sedimentary diatom concentrations and accumulation rates as predictors of lake trophic state

Diatom concentrations in surface sediments are positively correlated with limnetic chlorophyll a concentrations in Florida (USA) lakes. Using this relationship, I examine models that provide quantitative inferences for trophic state in historical applications.The best model predicts chlorophyll a trophic state index (TSI) values from log-transformed diatom concentrations and explains approximately half the variance in the dependent variable. Diatom accumulation rates are not better than sedimentary diatom concentrations as predictors of TSI. The entire diatom assemblage is as sensitive an indicator of TSI as are the planktonic diatoms alone. A model that considers the ecological preferences of specific taxa was found to be a better predictor than the model based on total diatom concentration.The sedimentary diatom concentration model provides a useful method for assessing historical changes in primary productivity, except in lakes where factors (e.g., silica limitation, blue-green bacterial inhibition) limited diatom production, or post-depositional changes removed sedimentary diatoms. TSI inferences are presented for sediment cores from two Florida lakes, one of which demonstrates a problematic application, and the other of which does not.     

Diatoms in biotechnology: modern tools and applications

Diatoms have played a decisive role in the ecosystem for millions of years as one of the foremost set of oxygen synthesizers on earth and as one of the most important sources of biomass in oceans. Previously, diatoms have been almost exclusively limited to academic research with little consideration of their practical uses beyond the most rudimentary of applications. Efforts have been made to establish them as decisively useful in such commercial and industrial applications as the carbon neutral synthesis of fuels, pharmaceuticals, health foods, biomolecules, materials relevant to nanotechnology, and bioremediators of contaminated water. Progress in the technologies of diatom molecular biology such as genome projects from model organisms, as well as culturing conditions and photobioreactor efficiency, may be able to be combined in the near future to make diatoms a lucrative source of novel substances with widespread relevance.     

东海南部陆缘(莆、泉段)全新世沉积硅藻

对东海南部陆缘莆田、泉州地区两口钻井岩芯系统的沉积硅藻研究,共发现33属117种（或变种）硅藻化石,根据剖面硅藻组合特征的变化,结合计算机的对应分析结果划分了硅藻带,建立该区全新世的7个硅藻组合序列,恢复当时古环境演变的7个阶段,填补该区沉积硅藻系统研究的空白,丰富了海陆过渡沉积硅藻的研究。     

CYTOPLASMIC MASSES PRESERVED IN EARLY HOLOCENE DIATOMS: A POSSIBLE TAPHONOMIC PROCESS AND ITS PALEO‐ECOLOGICAL IMPLICATIONS1

In Lake Suigetsu, central Japan, greenish/light‐brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark‐colored (rich in diatom valves, clay, and organic matter) and light‐colored (mainly diatom valves) laminae. The greenish/light‐brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark‐colored laminae. The dark‐colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic‐rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark‐colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule‐containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark‐colored laminae of the Early Holocene muds were abundant in the overlying light‐colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.     

底栖硅藻应用于河流生态系统健康评价的研究进展

硅藻是水生态系统中最重要的初级生产者之一, 不仅为生态系统中其他消费者提供碳源, 也因分布广泛、种类多、世代时间短等特点, 通常被认为是水环境及生态系统良好的指示生物。文章介绍了河流生态系统健康及生物完整性的概念, 综述了硅藻尤其是底栖硅藻在水生态系统中的自身特点及作用; 概述了硅藻在生物监测中广泛应用的前提: 对环境因子的响应和国内外对硅藻及硅藻指数在河流生态监测中的研究进展; 介绍了广泛应用的硅藻指数及基于硅藻的生物完整性指数的原理及构建方法; 指出了硅藻及硅藻指数在生物监测及评价中可能所存在的问题, 并提出了今后我国在这个领域后期需开展的工作。     

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

Oldest known non-marine diatoms (Aulacoseira) from the uppermost Cretaceous Deccan Intertrappean beds and Lameta Formation of India

Non-marine diatoms occur in the Deccan Intertrappean beds (Upper Cretaceous) of Mohgaon-Kalan, Chhindwara District, Madhya Pradesh and Pisdura, Lameta Formation (Upper Cretaceous), Maharashtra, India. This represents the oldest record of non-marine diatoms yet reported and the oldest from the Indian subcontinent. The diatoms were recovered from thin sections of chert and dinosaur coprolites by random fracturing. Solitary forms are the most common but colonial filaments up to five cells were also observed. Based on the morphological characters, the diatoms are identified as Aulacoseira Thwaites. The Lower Cretaceous marine diatom genus Archepyrgus Gersonde and Harwood also resembles Aulacoseira in general morphological characters and it seems that Aulacoseira evolved from Archepyrgus and migrated to the non-marine realm.     

Diatoms and pollen in a trial core from Sandwich Harbour,South West Africa (Namibia)

Sandwich Harbour forms a largely enclosed bay and extensive lagoon system that receives a limited subsurface, freshwater inflow from the Kuiseb River. It has been suggested that the Kuiseb River contributed more freshwater to Sandwich Harbour in the past, and that the river has since been displaced northwards towards Walvis Bay. Pollen and diatom analyses were carried out on a trial core from the central inshore region of the harbour to determine whether sufficient biological material for extensive studies had been preserved, whether the sediments were undisturbed and whether changes in salinity were evident. Pollen was not preserved in sufficient quantities to analyse but diatoms were abundant in the upper portion of the core. Diatom analysis indicated that the sediments were relatively undisturbed. The earliest sediments were dominated by a freshwater soil flora, and this was displaced by a marine littoral flora with brackish and freshwater diatoms increasing in frequency in the uppermost sediments. The increase in brackish forms is correlated with the formation of a sandbar across the harbour at the end of the last century. Further analyses of the present day flora and sediment cores from different parts of the harbour should provide a clear indication of the history and development of the area, especially the role played by the Kuiseb River.     

Problems with the application of diatom-total phosphorus transfer functions: examples from a shallow English lake

1. A diatom‐total phosphorus transfer function has been applied to a sedimentary diatom sequence from Groby Pool, a small shallow lake in Leicestershire, U.K. 2. Extensive aquatic plant records exist for Groby Pool dating back over two centuries. These records, in conjunction with selected aquatic pollen and herbarium diatom data, provide independent, qualitative evidence for the progression of eutrophication and its effects on aquatic plant communities and habitat structure. 3. Before 1800, Groby Pool was probably mesotrophic with clear water and a diverse submerged macrophyte community, but subsequently it experienced considerable nutrient enrichment. Key evidence for this includes: (i) historical plant records indicating the loss of species associated with mesotrophic waters and their replacement by others typical of eutrophic conditions, (ii) a significant increase in the percentage of planktonic diatoms in the lake sediment record (particularly Cyclostephanos dubius) after 1890, and (iii) increases in percentages of Stephanodiscus parvus and Cocconeis placentula in the second half of the twentieth century. 4. Diatom‐inferred total phosphorus (DI‐TP) estimates were inconsistent with the qualitative evidence for eutrophication at Groby Pool. In particular the DI‐TP profile was thought to overestimate phosphorus during the period of dominance by small Fragilaria spp. before 1890, and to misjudge the timing and direction of subsequent changes in nutrient loading. 5. This study highlights some of the problems associated with the application of diatom‐TP transfer functions to sedimentary diatom sequences from shallow lakes. The major problem relates to the frequent dominance of non‐planktonic diatoms in the sediments of these systems, many species of which (particularly small Fragilaria spp.) appear to be more sensitive to changes in habitat availability than to phosphorus. Potential ways of improving diatom‐TP models via altered approaches to sampling are suggested.     

Paleodepth determination from Antarctic benthic diatom assemblages

Analysis of modern surface sediments from fjords in the Vestfold Hills (Antarctica) indicates that 58% of the variation in benthic diatom assemblages can be attributed to changes in environmental parameters with water depth. Attenuation of light through the water column is suggested to account for 45% of the variation, and the decrease in substrate grain size with depth possibly accounts for a further 13%. Cluster analysis and principal component analysis were used to objectively circumscribe five floral zones between the surface and 35 m depth. The depth distribution of the benthic diatoms was then used to interpret the paleodepth of relict fjord (Holocene) sediments exposed around Deep Lake in the Death Valley (Vestfold Hills). Paleodepths measured from the sea-ice bench around Deep Lake combined with data from grain size analysis indicate that the relict fjord sediments have no analogue amongst the modern fjord sediments sampled in the Vestfold Hills. Without a comparable modern habitat on which to model the diatom depth zones, however, this study was unable to accurately determine the paleowater depth at Deep Lake using diatoms. Paleodepth determination will be possible using grain size analysis and diatom data when the substrate and light requirements of benthic diatoms are understood.