Diatom Genus Research Articles

Planoplatessa gen. nov.-A New, Neglected Monoraphid Diatom Genus with a Cavum.

A new monoraphid diatom genus, Planoplatessa gen. nov., is described on the basis of a detailed morphological investigation using light and scanning electron microscopy. We transferred Achnanthes joursacensis Héribaud, a species described from fossil deposits in France, to our new genus. We investigated material from Mongolia from Recent populations. This taxon is known from fossils to Recent across the Holarctic. Achnanthes joursacensis was previously transferred to the genera Planothidium and Platessa, but the morphology of A. joursacensis does not share all morphological features with these two genera. We discuss important morphological features for the delimitation of monoraphid genera based on careful morphological and molecular investigations we presented previously. Planoplatessa gen. nov. as a genus is characterized by having uniseriate striae on both the raphe and the rapheless valves, a cavum in rapheless valves only, and straight distal raphe ends on the valve face.

Effects of nutrients on the water quality and the algal community of river Nyakomisaro-Riana, Kisii, Kenya

Water quality changes affect the ecological functioning of riverine ecosystems as well as their ability to support organisms and sustain environmental values. The study eighteen months study was conducted from May 2013 to October 2014 to determine the changes in water quality and algal community. Water samples for nutrient and algae composition were collected in triplicates monthly at five sampling sites along the Nyakomisaro-Riana River and laboratory analyses were conducted by following the standard methods described by the American Public Health Association (APHA). Data was oorganized, described and analysed with One-Way Analysis of Variances (ANOVA) and bivariate regression, using the Microsoft Excel 2010 and Minitab software’s and the significance of ddifferences considered at the p-value of  0.05. Tukeys pairwise multiple comparisons were used for post hoc analysis. The level of Total Phosphorous (TP) exhibited a general increasing upstream - downstream trend with an overall mean of 250.99±25.31 µgL-1 and a range of 2.59 - 1281.40 µgL-1. The Soluble Reactive Phosphorous (SRP) concentrations ranged between 1.66 - 632.00 µgL-1 with a spatial mean of 57.24±8.59 µgL-1 within a range of ., while Total Nitrogen (TN) concentration had an overall mean of 996.49±128.60 µgL-1 and ranged between 4.76 - 4332.33 µgL-1. The lowest mean TN concentration (664.87±90.49 µgL-1) was recorded at site NK1 while the highest mean (1349.27±146.48 µgL-1) was recorded at site R1. Nitrate -Nitrogen [NO3-N] concentration ranged between 0.18 - 742.90 µgL-1 with an overall mean of µgL-1 142.28±16.40 µgL-1. Regression analysis showed a positive correlation between Nitrate nitrogen and distance downstream (b = 15.05, a = 112.12, R 2 = 0.40). Out of 135 algal species recorded, 113 had a low percentage occurrence of less than 30%. The algal community comprised of seven genera of diatoms, six genera of chlorophyceae and cyanophyceae and three genera of euglenophyceae. The percentage algal abundance indicated the existence of a climax community,the blue green algae (Microcystis aeruginosa) with a dominance of 34%. This suggested that nutrient loading into Nyakomisaro-Riana River is high. The study recommends that regular monitoring of water quality parameters and algal community changes should be conducted to control the nutrient discharges into the river.

Resurrection of the diatom genus Stephanocyclus (Coscinodiscophyceae: Stephanodiscaceae) on the basis of an integrated molecular and morphological approach

The history of the description and subsequent consideration of the diatom genus Stephanocyclus Skabitschevsky (Thalassiosirales, Bacillariophyta) is discussed. The new genus was described on the basis of a new species, S. planum Skabitschevsky, originally collected from the River Ob. However, the genus has been more or less forgotten since its initial description in 1975. Type material from Skabitschevsky's collection has subsequently been lost. An epitype for this species is recognized from the same region as the original collection, and typified with a population from Lake Baikal and using molecular data. The population from Lake Baikal fully corresponds in terms of morphological features with the population originally investigated by Skabitschevsky. Morphology of the genus Stephanocyclus is discussed and, on the basis of an integrated molecular and morphological investigation, we propose to recognize the genus, including four species within it. Three taxa are transferred to the genus, including Cyclotella cryptica, C. gamma and C. meneghiniana. A comparison of Stephanocyclus with other cyclotelloid genera is given.

Valve Ultrastructure and Description of a new Luticola D.G.Mann (Bacillariophyceae) species from Colorado, USA

A new species of the diatom genus Luticola D.G.Mann is described from a wet meadow environment from Gunnison County, Colorado, based on light and scanning electron microscopy. The new species has rugose valves, with longitudinal interruptions in the striae. Unlike other species of the genus, L. coloradiana sp. nov. has a simple structure of the internal stigma, lacking the complex opening seen in all other species in the genus. The new species resembles L. plausibilis (Hustedt) D.G.Mann, described from Franz Josef Land in the Arctic, but differs in valve shape and density of the striae. Luticola coloradiana sp. nov. also resembles L. plausibiloides Metzeltin et al., a species described from subtropical Uruguay, but differs in striae density and the rugose nature of the valves. Further research is needed to assess if the similar species from the Arctic and subtropical South America also possess the unique stigma structure seen in L. coloradiana sp. nov.

Fallacia fawensis sp. nov., a new brackish water diatom (Bacillariophyceae) from Southern Iraq

The diatom genus Fallacia includes species having a conopeum which is a perforated thin sheath of silica lying along the apical axis on the external valve face and a hyaline lateral area in the internal valve face. In surveying the benthic diatoms of Basra, a new small brackish water species, Fallacia fawensis was found associated with fine-grained substrata on the western bank of Shatt Al–Arab River, Southern Iraq. This epipelic species is described based on light and scanning electron microscopy and characterized by having a porous conopeum covering the area between raphe sterna and mantle, narrow elongated marginal striae, and a structure similar to lateral hyaline areas in the valve internal side. The terminal raphe endings on the external valve face, below valve apex, the raphe sternum inner margins come close to each other, blocking raphe canal but leaving a lacuna-like thin groove for connection with the deflected upper part of the open raphe canal. These features separated this species from allied taxa of the genus and also from closely related genera, Pseudofallacia and Germaniella. Notes on the ecology and distribution of the new species as well as the associated diatom taxa are provided.

Identification of Tropical Marine Diatom Chaetoceros dayaensis CBO from Bokor Island through Morphology and Genetic Marker Analysis

Microalgae are well-recognized by many researchers working on bioenergy resources due to its capability to produce high amounts of lipids as biodiesel precursors. Diatoms as a group of microalgae that specifically categorized by their silica valve could be more attractive to the researchers because in addition to high-lipid content, their silica features can be also utilized for wider applications, or used for diatoms bioproduct purification. Identification of some diatom species, such as the diatom genus Chaetoceros, as an initial step in diatom studies were difficult to accurately perform because of the unique morphology of silica frustules. Meanwhile, some candidate universal genetic markers of microalgae could also be used for the identification of diatoms with brittle frustules. In this research, we aimed to identify isolate CBO of tropical marine Chaetoceros CBO strain using morphological and genetic approaches. The isolate CBO of Chaetoceros sp. CBO was successfully obtained from sea region of Bokor Island, Kepulauan Seribu, Jakarta, and cultivated in our laboratory as culture stock for further use. Better resolution of the Chaetoceros CBO identification until species level was shown by genetic analysis toward two proposed gene markers for microalgae (rbcL-3P and V4 region of 18S rDNA) rather than by morphological. Specific DNA region for this specimen was found in V4 region of 18S rDNA genetic marker that could be recognized by MspA1I restriction enzyme. Hence, the RFLP (restriction fragment length polymorphism) method could be also used as an initial diagnostic tool for identification of this tropical marine Chaetoceros dayaensis CBO and for stock culture labelling purposes.

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

Dynamics of phytoplankton community in scallop farming waters of the Bohai Sea and North Yellow Sea in China

BackgroundAs the major suppliers of food for higher consumers, phytoplankton are closely related to the yield, nutritional ingredients and even toxin contents of mariculture animals, potentially influencing the human health when they are consumed. With the increase of shellfish culture density, phytoplankton in the coastal waters have been excessively consumed in recent years, and the nutrients they depend on are becoming more and more limited, which severely restrict the shellfish mariculture and threaten the coastal ecosystems.ResultsIn the present study, nutrient concentrations, phytoplankton assemblages and scallop growth status were investigated in the main bay scallop farming waters of the Bohai Sea (Qinhuangdao site) and North Yellow Sea (Zhuanghe site) in 2018. Both phosphate and silicate limitations were observed at the two sites, with the major determinant of phytoplankton assemblages being silicate in Qinhuangdao and phosphate in Zhuanghe, respectively. The phytoplankton assemblages at the two sites displayed different community structures and succession patterns. The phytoplankton community was dominated by dinoflagellates and diatoms in Qinhuangdao, while dinoflagellates were the most abundant group in Zhuanghe, which accounted for 41.9% of the total phytoplankton abundance. The dominant genera of diatoms in Qinhuangdao were Skeletonema, Thalassiosira and Leptocylindrus, while those in Zhuanghe were Thalassiosira and Cyclotella. Greater biomass and more appropriate structure of phytoplankton contributed to higher growth rate and glycogen content of cultured bay scallops.ConclusionsOur study characterized the relationship between nutrient concentration, phytoplankton community and scallop mariculture in the main bay scallop farming waters in northern China. The results suggest that, as nutrient limitation intensified, dinoflagellates are becoming the dominant phytoplankton species in the scallop farming waters of the Bohai Sea and the North Yellow Sea, which is harmful to the coastal mariculture.

Rainfall-driven nutrient loading affects coastal phytoplankton in the southwestern Indian Ocean: a lagoon at Mauritius Island

This study aimed to test the effects of rainfall-driven nutrient loading on the microphytoplankton dynamics in the shallow water at Trou aux Biches lagoon on the northwest coast of Mauritius. Changes in density, diversity and estimated productivity of microphytoplankton were monitored in three zones—coast, lagoon and reef—during two rainfall periods of 4 and 3 days’ duration, respectively, in March–April 2017. The average nutrient loading from terrestrial runoff was 15.5% and the salinity decreased by 2.5%. Following the two rainfall events, a 3-fold increase in total microphytoplankton density (TMPD), with a dominance of diatoms, was noted within a week at the coast and lagoon, while it remained relatively unchanged at the reef. These changes in microphytoplankton densities were concomitant with adequate levels of silicate (>2 µM) and high nitrate:silicate ratios (2.84–6.93). During this study, the numbers of genera of diatoms, dinoflagellates and cyanobacteria observed were 28, 12 and 3, respectively. The Shannon–Wiener, equitability and evenness indices showed high diversity values for diatoms and cyanobacteria prior to the rainfall events and for dinoflagellates after the events. The chlorophyll a content, relative electron transport rate, and estimated productivity of microphytoplankton increased during and after the rainy periods. TMPD and chlorophyll a were correlated significantly and positively with nutrients, salinity and temperature. Toxic algal species such as the dinoflagellates Alexandrium, Dinophysis and Tripos were recorded, although not in bloom densities. These findings indicate that rainfall-driven nutrient loading led to an increase in microphytoplankton density and estimated productivity, and a change in its diversity.

The type species of the diatom genus Chaetoceros

Chaetoceros is one of the most abundant and diverse diatom genera in the phytoplankton of the world’s oceans. The genus was erected by Ehrenberg in 1844 for two species from the Southern Ocean: C. dichaeta and C. tetrachaeta. According to his description, the former possesses two setae per valve and the latter possesses four. Chaetoceros tetrachaeta, or any species with four setae per valve, has never been reported again. In spite of that, Boyer, in 1927, designated it as the lectotype of the genus. In order to comprehend what Ehrenberg meant with C. dichaeta and C. tetrachaeta we re-examined the type material preserved on mica slides and compared specimens therein with the original descriptions, the drawings published by Ehrenberg in 1872 and notes accompanying the slides. Since no types have ever been designated, we here lectotypify specimens for the two species, guided by Ehrenberg’s coding system explaining their location on the slides. Of the two species, C. dichaeta is well-studied and commonly reported. Its lectotype links unambiguously with those of specimens for which ultrastructural and molecular information has been published. Regarding C. tetrachaeta, our observations confirmed that each valve possesses just two setae, which are robust and thick, typical for the section Chaetoceros. A possible synonym, C. adelianus, occurs in the same region and exhibits the same overall morphology.

Response of Diatoms to the Changing Water Quality in the Myristica Swamps of the Western Ghats, India

Myristica swamps are one of the rarest wetland ecosystems within the sub-tropical evergreen forests of the Western Ghats, India. As their name indicates, they harbor trees belonging to the ancient family Myristicaceae. Due to the waterlogged conditions and high humic decomposition, these swamps are acidic, harbor rare and endemic biotas, and provide ecosystem services to humans. Monitoring this rare ecosystem is crucial because the swamps that once formed a large hydrological network across the Western Ghats are now confined to isolated patches due to human disturbance such as agricultural interventions, roads, and dam construction. Due to the change in land use, there is also a drastic change in water chemistry and associated biodiversity. Biomonitoring is more precise than physical and chemical monitoring. So, the current study aimed to undertake a comprehensive analysis of the physical, chemical, and biological assessment of these swamps. The diatom assemblages are strongly affected by water chemistry and serve as a powerful indicator of environmental changes in the freshwater aquatic systems. However, there is no information on diatom assemblages in these swamps, and the present study aimed to determine the diatom assemblage structure in the Myristica swamps and their response to changing water quality. Diatom samples were taken at 17 different swamps across the central Western Ghats, and a set of environmental parameters was evaluated. Analysis revealed a total of 91 species of diatoms belonging to 27 genera across the 17 sites, from which 44 diatom species showed restricted distribution to this unique environment. Overall, the dominant diatom genera inside the swamps included, Navicula (19.8%), Gomphonema (16%), Eunotia (13.3%), Ulnaria (9.4%), Achnanthidium (8%), Frustulia (6.2%), Planothidium (5.2%), and Brachysira (2.8%). High diatom species richness was observed in the swamps having less anthropogenic disturbance, and diatom assemblage composition was primarily determined by dissolved oxygen, pH, and conductivity. The significant number of geographically restricted taxa in this study points towards our limited understanding of this tropical biome and calls for a dire need for more studies from here, not only to improve our knowledge concerning the diversity, ecology, and biogeography of these diatoms but to further encourage their use in applied (paleo) environmental sciences. Our results indicate that diatoms can prove useful environmental indicators even in harsh environments like swamps and can be a potential tool for assessing ecological and climatic change.

Temporal changes of genetic structure and diversity in a marine diatom genus discovered via metabarcoding

AbstractDiatoms’ bloom dynamics, with seasonal cycling of waxes and wanes of cell abundance, imply frequent bottleneck and expansion events that can leave signatures in the genetic structure and diversity of populations. We explored changes in genetic diversity and structure within multiple species of the planktonic marine diatom genus Pseudo‐nitzschia living in sympatry in the Gulf of Naples (Tyrrhenian Sea, Italy) over 48 sampling dates (from 2011 to 2013) by means of metabarcode data. The genus Pseudo‐nitzschia includes complexes of cryptic species along with morphologically distinguishable ones. We assessed the resolution power of the hypervariable V4 region of the 18S rDNA in delimiting species using different algorithms and a set of reference sequences. We measured intra‐specific genetic diversity and differentiation among species over three annual cycles. Between‐species divergence was higher in the P. pseudodelicatissima complex, while signs of incomplete lineage sorting and/or introgression were detected in the P. delicatissima complex. Most of the species showed significant population differentiation over time, due to changes in haplotypic composition over the three years. Some haplotypes were lost and new ones appeared, with high polymorphism in species of the P. delicatissima complex. We hypothesize that the observed temporal changes in genetic structure result from frequent founder events linked to a population dynamic characterized by cyclical alternation of clonal expansions and bottlenecks and discuss possible factors affecting the microevolutionary pathways in these planktonic diatoms.

Erratum to: Chinia gen. nov.—the second diatom genus simonsenioid raphe from mangroves in Fujian, China

Erratum to: Chinia gen. nov.—the second diatom genus simonsenioid raphe from mangroves in Fujian, China

Biogeography of six species in the planktonic diatom genus Bacteriastrum (Bacillariophyta)

ABSTRACT Marine planktonic diatom species can exhibit contrasting distribution patterns, from endemic to cosmopolitan. Endemicity is counter-intuitive for planktonic species given their potentially large population sizes and ample migration opportunities by means of ocean currents. Here, we analyse the biogeography of six species of the diatom genus Bacteriastrum with apparently contrasting distribution patterns. Occurrence data obtained from metabarcode reads in samples from Ocean Sampling Day (OSD) sites and from Tara Oceans stations as well as from their observational records included in biodiversity data repositories of GBIF and OBIS were plotted in occurrence maps. According to metabarcoding data, the six species examined here occur all over the temperate and tropical parts of the oceans. Observational records corroborate this finding for B. elegans, B. furcatum, B. hyalinum and B. mediterraneum. Instead, atypical colony former B. jadranum is encountered at just a few distant sites, and solitary B. parallelum only in the Gulf of Naples. The metabarcoding data reveal that the latter two species are far more widely distributed than their actual sightings according to GBIF and OBIS, and that B. hyalinum also occurs in the Arctic. The most likely reasons for the discrepancies among the different data sources are the relatively recent description of B. jadranum and B. parallelum, their diminutive and inconspicuous habitus, and their possible misidentification in routine plankton counting. HIGHLIGHTS • Metabarcoding data of Tara Oceans and OSD discern Bacteriastrum species.• Pan-oceanic distribution of Bacteriastrum species according to global metabarcoding data.• Distribution patterns of phytoplankton species can be inferred from global metabarcode datasets.

Investigations into the relationship between domoic acid and copepods in Scottish waters

Abstract This study investigated impacts of the algal toxin domoic acid (DA) on copepods in Scottish waters. Inspection of seasonal patterns revealed that several common copepods (Acartia spp. Dana, 1846, Calanus spp. Leach, 1816, Centropages spp. Krøyer, 1849, Pseudocalanus spp. Boeck, 1872, and Temora longicornis (Müller O.F., 1785)) regularly coexist with potentially toxic species from the diatom genus Pseudo-nitzschia H. Peragallo in H. Peragallo and M Peragallo, 1900. A short field study investigating the DA content of Calanus spp. at the Scottish Coastal Observatory site at Stonehaven recorded DA during every sampling event. The highest DA levels were associated with a July bloom (∼135000 cells L−1) of Pseudo-nitzschia cf. plurisecta Orive & Pérez-Aicua 2013. Several studies have previously investigated effects of ingested DA on copepods but information on effects of dissolved DA is lacking, therefore, simple exposure experiments were carried out to measure mortality of copepod species at ecologically relevant concentrations of dissolved DA. The highest concentrations tested (≥ 50 ng DA mL−1) decreased survival in Temora longicornis only; survival of other copepod species was unaffected. However, T. longicornis feeding on non-toxic algae in the presence of dissolved DA did not accumulate DA in their tissue. This study provides evidence of the potential for Calanus spp. to act as vectors for DA to higher trophic levels in Scottish waters.

Abundance, Diversity and Distribution of Phytoplankton in Coastal Water Adjacent to St. Martin’s Island, Bangladesh

The purpose of this study was to ascertain the composition, abundance, and distribution of phytoplankton in the coastal waters of the Bay of Bengal, Bangladesh (BoB) adjacent to the St. Martin’s Island.Samples were collected from 14 stations on low tide during the day time from March 13 - 19, 2020. In the present investigation, a total of 62 diatoms and 6 dinoflagellates were identified. Among diatoms, most dominant species were contributed by the genera Coscinodiscus followed by Chaetoceros, Thalassiosira, and Thalassionema. Among dinoflagellates, four species of Ceratiumand one species from each of Dinophysisand Gonyaulax were identified. The cell count of total phytoplankton varied from 87,500-437,500 in d/m3 of sea water and the highest phytoplankton was found at station 4. Coscinodiscus was the most dominant genus of the centric diatoms in all the stations and its abundance ranged from 25,000-100,000 in d/m3. The highest number of Coscinodiscus was found at station 4. The result of this study showed that phytoplankton composition and their abundance varied from one station to another. But not much difference was observed among the ranges of physicochemical factors. Water temperature, salinity, pH and dissolved oxygen ranged from 26.5-27.17°C, 32.09-33.76 ppt, 8.16-8.20, and 5.85-6.12 mg/l, respectively. The species richness factor and the diversity index factor as determined with the help of generic abundance which revealed the ranges from 1.51-4.93 and 1.52-2.22, respectively. The ranges of salinity and temperature recorded in the present investigation are quite consistent with the optimum ranges reported for marine habitats. The qualitative structure of phytoplankton shows similarities with other studies carried out in the BoB at different times. However, variability in the population density of phytoplankton in different studied stations may not be responsible for different water quality parameters rather than oceanic currents and tides. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 10(2), 2021, P 21-34

Toxicity of the Diatom Genus Pseudo-nitzschia (Bacillariophyceae): Insights from Toxicity Tests and Genetic Screening in the Northern Adriatic Sea.

Diatoms of the genus Pseudo-nitzschia H.Peragallo are known to produce domoic acid (DA), a toxin involved in amnesic shellfish poisoning (ASP). Strains of the same species are often classified as both toxic and nontoxic, and it is largely unknown whether this difference is also genetic. In the Northern Adriatic Sea, there are virtually no cases of ASP, but DA occasionally occurs in shellfish samples. So far, three species—P. delicatissima (Cleve) Heiden, P. multistriata (H. Takano) H. Takano, and P. calliantha Lundholm, Moestrup, & Hasle—have been identified as producers of DA in the Adriatic Sea. By means of enzme-linked immunosorbent assay (ELISA), high-performance liquid chromatography with UV and visible spectrum detection (HPLC-UV/VIS), and liquid chromatography with tandem mass spectrometry (LC-MS/MS), we reconfirmed the presence of DA in P. multistriata and P. delicatissima and detect for the first time in the Adriatic Sea DA in P. galaxiae Lundholm, & Moestrup. Furthermore, we attempted to answer the question of the distribution of DA production among Pseudo-nitzschia species and strains by sequencing the internal transcribed spacer (ITS) phylogenetic marker and the dabA DA biosynthesis gene and coupling this with toxicity data. Results show that all subclades of the Pseudo-nitzschia genus contain toxic species and that toxicity appears to be strain dependent, often with geographic partitioning. Amplification of dabA was successful only in toxic strains of P. multistriata and the presence of the genetic architecture for DA production in non-toxic strains was thus not confirmed.

Taxonomy and valve ultrastructure of new and interesting freshwater fossil diatoms (Bacillariophyta) of Miocene age from the Espanola Formation of New Mexico, U.S.A. II. Description of a New Grunowia species with comments on the genus

We describe a new species of the diatom genus Grunowia, G. mannii Kociolek & A.Danz, sp. nov., from a Miocene fossil deposit in New Mexico, USA. The species has wide fibulae, a keel that is off-center externally, with distinct proximal raphe ends, both internally and externally. The species is differentiated from others in the genus in the narrow valves that are swollen and rounded in the center. We formally transfer two species from Nitzschia to Grunowia, making the new combinations, Grunowia constricta (Chen & Zhu) Kociolek & A.Danz, comb. nov. and Grunowia pseudosinuata (Hamilton & Laird) Kociolek & A.Danz, comb. nov. We discuss the distribution of the genus over time and space. This report from the Miocene represents the oldest known member of the genus Grunowia in the fossil record.

Fontina Gen. nov. (Bacillariophyta): a new diatom genus from a thermo-mineral spring of the French Massif Central (France)

Light and scanning electron microscope observations on the genus Fontina gen. nov. from a thermo-mineral spring of the Massif Central (France) are presented. Based on morphology, the new genus shows features that have not been described previously, such as the stria pattern, interrupted at the valve face/mantle junction and composed of one long, slit-like areola on both valve face and mantle. Externally, areolae are closed by hymenes. Fontina arverna sp. nov. is described as the typus generis of Fontina. This new genus was compared with some similar freshwater genera such as Adlafia, Chamaepinnularia, Genkalia, Germainiella, Microfissurata and Nupela. So far, we are unable to establish an evolutionary link or cladistic association with currently known genera. The new genus is described from a sampling site characterized by high conductivity and high carbonate concentration, brackish conditions and whose water originates deep in the crust (high lithium concentration).

Four new Orthoseira Thwaites species from Maui, Hawaii, and the Columbia River Gorge, Oregon, with comments on frustular morphology in the genus

Light, scanning electron and focused ion beam microscope observations are presented for four new species of the diatom genus Orthoseira Thwaites. Three species, O. mauiana sp. nov., O. loweana sp. nov. and O. hawaiiensis are described from Maui (Hawaiian Islands), and one species, O. oregoniana sp. nov., is described from Oregon, USA. These four species all possess carinoportulae, the feature that diagnoses the genus, but differ in many other characters, including presence and types of spines, caverns, and pore fields. We compare and contrast these species with each other, and other species currently assigned to the genus. We also compare the different morphologies with other genera proposed, but not currently recognized, such as Liparogyra Ehrenberg, Stephanosira Ehrenberg, and Porocyclia Ehrenberg, and consider the systematic relationships of these groups within Orthoseira.