Marine Benthic Diatoms Research Articles

Evaluation of optimal culture conditions for marine benthic diatom Halamphora coffeaeformis growth and its biochemical properties

Halamphora coffeaeformis is a benthic diatom that exhibits a high-value biochemical composition and can be harnessed for various applications particularly in aquaculture. Despite its potential, significant challenges persist in the optimization of cultivation parameters and scaling up to mass cultivation level. Thus, this study aimed to identify the most suitable substrate, as well as the optimal light intensity, photoperiod, salinity, and pH for the growth of H. coffeaeformis. The biochemical composition of mass-cultured H. coffeaeformis was also analyzed. The findings revealed that polyvinyl chloride (PVC) substrates substantially outperformed bamboo by yielding significantly higher (p < 0.05) growth performance. The maximum specific growth rate (SGR) of H. coffeaeformis was recorded under the following conditions: the light intensity of 120 μmol m−2 s−1 with an 18:6 h light/dark cycle (0.200 day−1), the salinity of 30 ppt (0.161 day−1) and the pH of 9 (0.409 day−1). Under these optimized conditions, the mass culture of H. coffeaeformis at 100 L exhibited the highest ash content (38.47 ± 0.17 % dry weight (DW)), followed by crude protein (26.26 ± 0.56 % DW) containing high-quality amino acids, carbohydrates (20.96 ± 0.81 %% DW) and crude lipids (4.46 ± 0.56 % DW). The lipid profile was particularly notable for its high saturated fatty acid (SFA) content and sufficient levels of polyunsaturated fatty acids (PUFA), while maintaining the lowest fiber content (0.07 ± 0.01 % DW). These findings underscore the potential of H. coffeaeformis for large-scale aquaculture applications and highlight the importance of fine-tuning cultivation parameters to maximize its biochemical yields.

Widespread Production of Polyunsaturated Aldehydes by Benthic Diatoms of the North Pacific Ocean's Salish Sea.

Diatoms are key primary producers across marine, freshwater, and terrestrial ecosystems. They are responsible for photosynthesis and secondary production that, in part, support complex food webs. Diatoms can produce phytochemicals that have transtrophic ecological effects which increase their competitive fitness. Polyunsaturated aldehydes (PUAs) are one class of diatom-derived phytochemicals that are known to have allelopathic and anti-herbivory properties. The anti-herbivory capability of PUAs results from their negative effect on grazer fecundity. Since their discovery, research has focused on their production by pelagic marine diatoms, and their effects on copepod egg production, hatching success, and juvenile survival and development. Few investigations have explored PUA production by the prolific suite of benthic marine diatoms, despite their importance to coastal trophic systems. In this study, we tested eight species of benthic diatoms for the production of the bioactive PUAs 2,4-heptadienal, 2,4-octadienal, and 2,4-decadienal. Benthic diatom species were isolated from the Salish Sea, an inland sea within the North Pacific ecosystem. All species were found to be producers of at least two PUAs in detectable concentrations, with five species producing all three PUAs in quantifiable concentrations. Our results indicate that production of PUAs from Salish Sea benthic diatoms may be widespread, and thus these compounds may contribute to benthic coastal food web dynamics through heretofore unrecognized pathways. Future studies should expand the geographic scope of investigations into benthic diatom PUA production and explore the effects of benthic diatoms on benthic consumer fecundity.

Marine benthic diatom Psammodictyon panduriforme (MACC 35) as a potent producer of long chain hydrocarbons and fatty acids of biofuel significance

Marine diatoms are promising feedstock for biofuel industries as a source of hydrocarbons and fatty acids. In the present study, eleven marine diatom isolates from the west coast of India were analysed for hydrocarbons and lipids. The comparative analyses indicated that Psammodictyon panduriforme (MACC 35), a diatom isolate from the water samples collected from 1000 m depth, with a hydrocarbon yield of 37.27 mg g−1 L−1 and 2.1 % of the total biomass of 0.56 gL−1 was a promising species followed by Biddulphia sp. (MACC 6) with hydrocarbon yield of 25.4 mg g−1 L−1 and 1.3 % of the total biomass of 0.48 g L−1. Hydrocarbon analyses by gas chromatography with high resolution mass spectrometry (GCMS) showed that the long chain hydrocarbons found in the majority of the marine diatoms were undecane 4,7 dimethyl, hexadecane, octadecane,3-ethyl-5-(2-ethyl-butyl), tetradecane, tetracosane, eicosane-2 methyl, heptacosaneand hexacosane. The fatty acid profile showed the presence of stearic acid, 9-octadecenoic acid, myristic acid, palmitic acid, and arachidic acid with excellent biodiesel properties. Hydrocarbons of high fuel quality can be produced by optimizing hydrocarbon production in the potent marine diatom Psammodictyon panduriforme and catalytic cracking of long chain hydrocarbons to short chain alkanes with high octane ratings.

Comparison of different small-scale cultivation methods towards the valorization of a marine benthic diatom strain for lipid production

Marine benthic diatoms have the capacity to produce quality biomass and bioactive compounds for various commercial applications. Amphora sp. NCC169 is one of such species that have high-value lipid production. However, mass-production of Amphora sp. NCC169 in traditional suspension photobioreactor is challenged by its sensitivity to stirring and turbulence. The aim of this study is to compare the biomass and lipid productivity of Amphora sp. NCC169 cultures between a Fernbach flask and a low-maintenance, laboratory-scale culture system adapted from previously described vertically oriented porous substrate bioreactor (PSBR) designs. Results revealed that cells in the PSBR could achieve significantly higher biomass productivity (Pbiomass = 0.51 ± 0.05 g·m−2·day−1) and lipid productivity (Plipid = 0.10 ± 0.03 g·m−2·day−1) than those in Fernbach flasks after 20 days of cultivation (Pbiomass = 0.29 ± 0.01 g m−2 day−1; Plipid = 0.07 ± 0.01 g·m−2·day−1). Cellular photosynthetic efficiency remained favorable in both culture conditions (Fv/Fm > 0.5) for the duration of the experiments.

Evaluation the toxic effect of copper ions on the condition indices of benthic diatom Actinocyclus subtilis (W.Gregory) Ralfs 1861 in the experiment

Introduction. Pollution of marine coastal areas lead to the relevance of environmental monitoring including application of biotesting methods based on- the cultures of unicellular algae. Microalgae have different species-specific resistance to pollutants that expands application of different species as bioindicators of marine pollution.&#x0D; The aim of the study was to determine the threshold concentration of copper ions (Cu2+) for the survival and increase in the cells number of benthic diatom Actinocyclus subtilis (W.Gregory) Ralfs 1861 (Bacillariophyta) under the wide range of toxicant concentrations during 10-day toxicological experiments.&#x0D; Material and methods. The response of strain culture of the benthic diatom A. subtilis to various concentrations of copper sulfate (ranged from 16 to 1024 μg/l in terms of Cu2+ ions) was studied. In accordance with the previously developed protocol, the following indices were evaluated: alterations in the absolute number and proportion (%) of alive cells in the test-culture, as well as the specific growth rate in the number of A. subtilis cells at different concentrations of toxicant. Counting of alive and dead cells was carried out by micrographs taken for 12–15 random viewing fields under Nikon Eclipse inverted light microscope.&#x0D; Results. It was found that in the control and at concentration of copper ions 16 μg/l, the increase in the absolute number of cells in culture is described by sigmoid response curve. At the control еhe exponential growth phase occurs on days 5–7 and at concentration of 16 µg/l on days 3–5 of the experiment. The threshold concentration of copper ions (32 μg/l) which is critical for the survival of A. subtilis was determined, which is 3–7 times lower than threshold level for other benthic diatom species. At concentration of 32 µg/l, the phases of acceleration and exponential growth on the abundance curve are absent. The proportion of living cells in the culture decreases to 80% of the control level on day 3 and to 39% by day 10. At Cu2+ concentrations of 64 µg/l and above, sharp inhibition and death of culture is observed as early as 1–3 days. A positive specific growth rate of A. subtilis culture was revealed in the period of 1–5 days at copper concentration of 16 and &#x0D; 32 µg/l, and at concentration of 64 µg/l and higher the culture dies off. Negative values of the specific growth rate for all concentrations of the toxicant within the period of 5–10 days were obtained.&#x0D; Limitations. By the results of 10-day experiments the effect of 8 concentrations of copper sulfate on the culture of marine benthic diatom A. subtilis was studied. Three replicates in each concentration and exposure time were measured (1350 measurements in total), which is sufficient sampling for statistically reliable determination of the threshold values of copper ion toxicity for given test object.&#x0D; Conclusion. Considering the results obtained, the benthic diatom A. subtilis is highly sensitive to copper ions impact and can be recommended as new test-object for toxicology, as well as for application in monitoring of marine water areas subject to technogenic pollution.

Comparative Investigation of Marine Benthic Diatom Assemblages with Morphological and Metabarcoding Approaches Along the Coasts of Istanbul Metropolis

Comparative Investigation of Marine Benthic Diatom Assemblages with Morphological and Metabarcoding Approaches Along the Coasts of Istanbul Metropolis

EVALUATION OF THE TOLERANCE THRESHOLD OF THE MARINE BENTHIC DIATOM PLEUROSIGMA AESTUARII (BRéB. IN KüTZ.) W. SMITH, 1853 (BACILLARIOPHYTA) UNDER THE IMPACT OF COPPER (II) IONS

Copper compounds accumulate in marine bottom sediments as the result of human activity and, being highly toxic, affect microphytobenthos. Evaluation of the tolerance ranges of benthic diatoms to copper pollution is important for biotesting and assessment of coastal marine environment. This work is aimed to reveal the dynamics of growth and cell death of marine benthic diatom Pleurosigma aestuarii (Bréb. in Kütz.) W. Smith, 1853 under the impact of a wide range of copper (II) ions concentrations over the course of a 10-day experiment in clonal culture, as well as to identify a tolerance threshold for this species, critical for its survival when exposed to the toxicant. The study has focused on assessment of the changes in the proportion (%) of alive cells, absolute cell number and specific growth rate of the culture at different exposure durations and concentrations of Cu2+ ions (32-1024 μg/L) and was meant to evaluate the applicability of this species as a new test object for ecotoxicology. In the control culture and in the cultures exposed to Cu2+ in the range of concentrations 32-256 μg/L, the proportion of alive cells did not change over the course of the experiment (95-99 %). At Cu2+ concentration 320 μg/L, the proportion of alive cells decreased to 23 % on the 3rd day and to 10 % on the 5th day, which provided a basis to consider this value as a threshold one for P. aestuarii survival. At Cu2+ concentrations 384 μg/L and higher, up to the maximum one (1024 μg/L), drastic inhibition of the culture was recorded as early as on the 1st day, and on the days 3-5, all the cells died. The increase in the absolute cell number in the concentration range 32- 256 μg/L were consistent with the dose-response sigmoid model. Over the timespan of days 1-7, the cell number increased by 3-5 times, reaching its maximum, and then it decreased by 10-12 % by the 10th day. At Cu2+ concentrations 320 μg/L and higher, the increase in the cell number was strongly suppressed since the 1st day. Within the 32-256 μg/L range, the test culture is characterized by positive specific cell growth rate for the period up to 7 days; at the threshold concentration 320 µg/L and higher, this test parameter becomes negative. Thus, P. aestuarii should be recommended as a new appropriate test object both for toxicological experiments and for monitoring of coastal marine environment affected by technogenic pollution.

Contributions to the Diatom Flora of the Aegean Sea and the Mediterranean Sea from the Turkish Coasts with Remarks on Rare Taxa

This study reports on newly recorded and several rare marine benthic diatoms from the Turkish coasts of the Aegean Sea. Altogether 510 species were observed from 6 locations throughout the Aegean Sea, and taxonomic and biogeographic remarks concerning thirty-four new recorded species are presented. Amongst the represented taxa, Amicula, Auricula, Cyclophora, Minidiscus, Oestrupia and Ralfsiella were observed for the first time in Turkish coastal waters. Biddulphia seychellensis, Cyclophora tabellariformis, Grammatophora maxima, G. gibberula, Amphicocconeis mascarenica, Cocconeis maxima, Amicula specululum, Caloneis egena, Diploneis dalmatica, D. droopii, Mastogloia affinis, M. linearis, M. mauritana and Oestrupia powelii var. vidovichii, which were rarely recorded from the coasts of the oceans and the Mediterranean Sea, were examined by light microscopy (LM) and scanning electron microscopy (SEM). Also, the findings revealed that the geographic range of A. mascarenica, B. seychellensis, C. tabellariformis and Psammodictyon corpulentum, which were distributed in warmer marine waters, has expanded to the temperate waters of the Aegean Sea. Additionally, the recently described species Hyalosira septata, from the loggerhead sea turtle carapace and the Turkish Mediterranean Sea coast was observed in the Aegean Sea. This study contributes to the diatom knowledge of marine waters from the understudied Aegean Sea coasts of Türkiye, enhancing the biogeography of some rare and well-established taxa.

Distribution of marine benthic diatoms on the coasts of the sea of Marmara and their responses to environmental variables

The Sea of Marmara, located in northwest Turkey, comprises challenging environmental conditions and is an important bridge between the Black Sea and the Aegean Sea. In this study, we assessed the trophic situation of the coasts of the Sea of Marmara using the benthic diatom composition and their relation to the environmental variables in the spring and autumn of 2019 and 2020. A total of 120 samples were collected and 452 species were observed; Navicula Bory and Nitzschia Hassal were represented with the highest numbers of taxa. Analysis of Similarity (ANOSIM) revealed that diatom communities differed significantly between the spring-autumn periods and years. Canonical Correspondence Analysis (CCA) showed that diatoms grouped under water, temperature, salinity, dissolved oxygen, and TIN influence. TRIX observations revealed a highly mesotrophic-eutrophic status along the coasts. Our study contributes to the knowledge of diatom diversity, distribution and community changes in spring and autumn on the coasts Sea of Marmara. Our findings suggest that marine benthic diatoms could be used as eutrophication indicators in the coastal waters for long-term monitoring with the support of environmental parameters.

A New Insight Into Amicula, a Genus of Tiny Marine Benthic Diatoms With the Description of Two New Tropical Species and the Largest Mitogenome Known for a Stramenopile

The current article focuses on the morphological and molecular characterization of the often inconspicuous genus Amicula. This recently erected genus from brackish and marine sediments was monotypic but here we describe two new tropical species: Amicula micronesica sp. nov. and Amicula vermiculata sp. nov. Once considered an incertae sedis genus regarding its higher rank taxonomy, its position among the family Diploneidaceae is proposed here by molecular phylogenetics. The complete plastid and mitochondrial genomes of Amicula micronesica sp. nov. are also presented here. It appears that the 177614-bp long mitogenome is the biggest yet recorded among stramenopiles, due to its invasion by 57 introns. Moreover, it utilizes the genetic code 4 for translation instead of the code 1 usually found among diatoms.

Algal extracellular organic matter pre-treatment enhances microalgal biofilm adhesion onto microporous substrate

Adhesive biocoating has microstructure composed of biomolecules to entrap viable cells in a stabilized matrix over exposed surfaces. Although marine benthic diatoms are a common group of algae excreting substantial amount of extracellular polymeric substances (EPS), studies regarding the utilization of these EPS are scarce. Using the soluble EPS derived from Navicula incerta and pre-deposition of it as a thin conditioning layer on microporous polyvinylidene fluoride (PVDF) membranes, the pre-coated surface was used to investigate the cell binding affinity of three marine microalgae, namely Amphora coffeaeformis, Cylindrotheca fusiformis and Navicula incerta. Microalgae actively engaged themselves on the pre-coated membranes which was 10 times greater than the initial cell adhesion degree. Soluble EPS is mainly comprised of polysaccharide while bounded EPS is mainly comprised of protein. On EPS pre-coated membranes, N. incerta released the least amount of bounded polysaccharides (<100 mg m−2) and vice versa for the other two because EPS production is usually maximized to assist cell adhesion onto unfavorable substrates. In stark contrast, when the adaptation period (first 6 h) ended, cells began to secrete more bounded protein for cell growth, and an increasing trend of protein content found in N. incerta has verified its optimal adaptation onto the biocoating itself. On pristine PVDF membranes, the adhesion degree was ranked in ascending order: C. fusiformis, N. incerta and A. coffeaeformis. Interestingly, after the pre-coating process, the order was reported as: A. coffeaeformis, N. incerta and C. fusiformis, but it should be noted that C. fusiformis demonstrated fluctuating cell colonization degree and bounded EPS production over time. In other words, the biofilm's susceptibility was confirmed since the cells latched loosely on the membranes rather than in a biofilm matrix. Biocoating enables uniform cell distribution and firmer biofilm growth, opening the door to vast future applications in environmental bioremediation and sensing.

A new insight into Amicula, a genus of tiny marine benthic diatoms with the description of two new tropical species and the largest mitogenome known for a stramenopile

A new insight into Amicula, a genus of tiny marine benthic diatoms with the description of two new tropical species and the largest mitogenome known for a stramenopile

Two new Halamphora (Bacillariophyta) species from the marine coasts off Livingston Island, Antarctica.

During a survey of the marine benthic diatom flora on the coasts off Livingston Island (South Shetland Islands, Maritime Antarctic Region), two Halamphora species that could not be identified based on the currently available literature, were observed. Detailed light and scanning electron microscopy observations and thorough comparison with similar taxa in the literature revealed that both taxa should be described as new species. The first taxon, Halamphorakenderoviana sp. nov., was most likely misidentified in past Antarctic studies, and included within the range of another taxon, Halamphoracoffeaeformis. Analysis of literature data showed that the second new taxon, Halamphoramoncheviana sp. nov., has been previously reported from the Antarctic Continent (but as an unidentified species). The new taxa are compared with similar Halamphora taxa worldwide. Data on their ecology and distribution are also provided.

Diversity and habitat preferences of benthic diatoms from South Bay (Livingston Island, Antarctica)

Background and aims – Despite a long research history, knowledge of Antarctic marine benthic diatoms is fragmentary. This study reports on marine benthic diatoms from South Bay, Livingston Island, focusing on diatoms living on hard substrata, and species distribution across different coastal habitats.Material and methods – Samples were collected from tidal pools (19), intertidal cobbles (9), artificial substrata installed at various depths (10), coastal rocks (2), and bottom sediments at depths &gt; 20 m (2). Species identifications and community analyses were done using LM with additional information obtained using SEM. nMDS based on diatom abundance data was applied to display differences between the samples by habitat/substratum type and sampling month. The significance of the habitat/substratum type and sampling month on diatom communities was checked with PERMANOVA. Similarity/dissimilarity within and between sample groups and their contributing species were explored with SIMPER.Key results – In total, 133 diatom taxa were recorded, of which 110 are benthic. A large number of taxa could not be certainly identified. Taxonomic remarks and ecology and distribution data for some rarely reported species with convoluted taxonomic and nomenclatural histories are given. One new combination is proposed. Diatom communities were influenced by the habitat/substratum type, but not by seasonality. Significant differences existed between communities in tidal pools and those on cobbles, artificial substrata, and sediments, and between those on sediments and artificial substrata. Navicula aff. perminuta dominated on cobbles and often on artificial substrata. Species forming mucilage tubes, tree-like colonies, and chains of cells embedded in mucilage were restricted to tidal pools.Conclusion – Benthic diatom communities from South Bay are highly diverse and species show distinct distributions in the coastal habitats, but the scarce studies and often confusing nomenclature history of the taxa make their identification challenging, and potentially common species for the region remain unknown.

Selection constrains lottery assembly in the microbiomes of closely related diatom species

It is generally recognised that interactions between microalgae and bacteria play an important role in the functioning of marine ecosystems. In this context, increasing attention is paid to the processes that shape microalga-associated microbiomes. In recent years, conflicting evidence has been reported with respect to the relative importance of selective vs neutral processes in the assembly process. Whereas some studies report strong selection imposed by the host, others propose a more neutral, lottery-like assembly model according to which the chance of bacteria becoming part of the microbiome is proportional to their abundance in the environment and not driven by the selectional pressure created by the host. In the present study, we investigated to what degree selective vs neutral assembly processes constrain taxonomic, phylogenetic and functional variation within and between microbiomes associated with 69 isolates belonging to the Cylindrotheca closterium benthic marine diatom complex. The diatom cultures were initiated from non-axenic clonal isolates from different marine environments and geographic locations, and were then reared in a common garden (lab) environment. An important environmental imprint, likely due to in situ lottery dynamics, was apparent in the diatom microbiomes. However, microbiome assembly was also phylogenetically and functionally constrained through selective filtering related to the host microhabitat. Randomised microbiome assembly simulations revealed evidence for phylogenetic overdispersion in the observed microbiomes, reflecting an important role in the assembly process for competition between bacteria on the one hand and predominantly genetically driven differences between the hosts on the other hand. Our study thus shows that even between closely related diatom strains, host selection affects microbiome assembly, superimposing the predominantly stochastically driven recruitment process.

Assessment of marine benthic diatom communities: insights from a combined morphological–metabarcoding approach in Mediterranean shallow coastal waters

We investigated the advantages and disadvantages of light microscope (LM)-based identifications and DNA metabarcoding, based on a 312-bp rbcL marker, for examining benthic diatom communities from Mediterranean shallow coastal environments. For this, we used biofilm samples collected from different substrata in the Ebro delta bays. We show that 1) Ebro delta bays harbour high-diversity diatom communities [LM identified 249 taxa] and 2) DNA metabarcoding effectively reflects this diversity at genus- but not species level, because of the incompleteness of the DNA reference library. Nevertheless, DNA metabarcoding offers new opportunities for detecting small, delicate and rare diatom species missed by LM and diatoms that lack silica frustules. The primers used, though designed for diatoms, successfully amplified rarely reported members of other stramenopile groups. Combining LM and DNA approaches offers stronger support for ecological studies of benthic microalgal communities in shallow coastal environments than using either approach on its own.

A First Estimate of Species Diversity for Benthic Diatom Assemblages from the Revillagigedo Archipelago, Mexico

Recent investigations at previously unexplored localities on the Mexican coast have confirmed the high taxonomic potential of benthic marine diatom assemblages (BMDA) in the region. An exploratory study of epiphytic diatoms of macroalgae in the Revillagigedo Archipelago (RA) suggested that further studies would yield many more taxa, prompting the hypothesis that diversity measurements, based on ecological indices, would be among the highest in pristine environments. Thus, the aim of this research was to enrich the record of epiphytic diatom floristics of the RA, and to estimate species diversity based on information theory (H′). Floristically, 167 identified taxa are added here to the BMDA species list of the RA, bringing the total to 397 taxa overall, including 52 taxa that are potentially new records for the Mexican Pacific coast. Among the most conspicuous genera are Mastogloia with five new taxa and it remains the most diverse genus with 55 taxa overall, followed by Cocconeis (27), Nitzschia (24), Amphora (23), Navicula (19), Diploneis (17) and Grammatophora (15). As expected for a pristine environment, the computed species diversity values for the BMDA were high, ranging from H′ = 3.92–5.2, depicting stability. Future surveys are expected to further increase the species richness of BMDA for the RA.

Light intensity and spectral composition drive reproductive success in the marine benthic diatom Seminavis robusta

The properties of incident light play a crucial role in the mating process of diatoms, a group of ecologically important microalgae. While species-specific requirements for light intensity and photoperiod have been observed in several diatom species, little is known about the light spectrum that allows sexual reproduction. Here, we study the effects of spectral properties and light intensity on the initiation and progression of sexual reproduction in the model benthic diatom Seminavis robusta. We found that distinct stages of the mating process have different requirements for light. Vigorous mating pair formation occurred under a broad range of light intensities, ranging from 10 to 81 µE m−2 s−1, while gametogenesis and subsequent stages were strongly affected by moderate light intensities of 27 µE m−2 s−1 and up. In addition, light of blue or blue–green wavelengths was required for the formation of mating pairs. Combining flow cytometric analysis with expression profiling of the diatom-specific cyclin dsCyc2 suggests that progression through a blue light-dependent checkpoint in the G1 cell cycle phase is essential for induction of sexual reproduction. Taken together, we expand the current model of mating in benthic pennate diatoms, which relies on the interplay between light, cell cycle and sex pheromone signaling.

Indonesian coral reef habitats reveal exceptionally high species richness and biodiversity of diatom assemblages

We present the results of the first ever research on biodiversity and biogeography of marine benthic diatoms from coral reefs of the tropical islands of Indonesia, including West Borneo, East Java, South Celebes, Komodo, Rinca and Flores. The sampled islands offer a broad range of coral reef microhabitats and host diatom assemblages from seaweeds, sea grass, dead corals, scrapes from other biological surfaces, sand, rocks and other solid surfaces. The results of this study are based on the light (LM) and scanning electron microscopy (SEM) identification of diatoms only. Our taxonomic analysis revealed the presence of 911 diatom taxa representing 188 genera. Biodiversity indices such as Shannon, Simpson and Evenness as well as species richness and diatom relative abundance were calculated for each sample examined. We find that these indices were higher for Indonesia than those published for tropical oceanic islands or regions from both hemispheres, e.g. Galapagos and Martinique, but comparable to Madagascar. The non-metric multidimensional analysis showed that the type of habitat from which a particular sample was taken was of greater significance in the taxonomic composition than the geographical aspect. In terms of biogeography, cosmopolitan taxa were dominant in terms of relative abundance, but species richness was higher within tropical zonal and local forms. Many taxa were identified only at the genus level, and may turn out to be new to science.

Morphology of Australoneis gen. nov. and A. frenguelliae comb. nov. (Achnanthales, Bacillariophyta) from the Antarctic Peninsula

Based on marine benthic diatoms collected at Potter Cove (Western Antarctic Peninsula), we present a detailed study, using both light and electron microscopy, of the morphology of Campyloneis frenguelliae, a species recently transferred from Cocconeis. Ultrastructural observations revealed a combination of new and unusual features within the Achnanthales: a raphe valve with different areolation patterns, a hollow valvocopula with no apparent fimbriae, and a sternum valve externally ornamented by an embossed structure, areolae occluded by hymenes mostly supported by one short peg, and multiple marginal pores that open internally into shallow valve depressions. In view of these unique set of features, we describe Australoneis gen. nov. to include Australoneis frenguelliae comb. nov. and designate a sample from Potter Cove as epitype of the latter. The main characteristics of the new genus are compared to those of its closely allied Cocconeis, Campyloneis, Xenococconeis and Amphicocconeis.