Nannochloris Bacillaris Research Articles

The role and composition of winter picoeukaryotic assemblages in shallow Central European great lakes

Abstract Studies on autotrophic picoplankton (APP; 3 μm) to low-temperature and low-light winter environment was confirmed by their higher maximum photosynthetic rate and light utilization parameter. Maximum photosynthetic rate and light saturation parameter increased significantly with both temperature and available light, but with different impact on the two size groups. Twenty-two picoeukaryotic strains were isolated and identified based on 18S rRNA gene sequence analysis. Taxonomic composition of the picoeukaryotic community in the studied shallow lakes was similar to other freshwater lakes in the temperate zone: members of genera Choricystis and Mychonastes were dominant, however, in Lake Balaton, common freshwater taxa such as Stichococcus bacillaris and Nannochloris bacillaris were also found.

Ruminal in vitro gas production, dry matter digestibility, methane abatement potential, and fatty acid biohydrogenation of six species of microalgae

This study evaluated the composition, digestibility [dry matter digestibility (DMD)], CH4 abatement potential, and fatty acid biohydrogenation of six species of microalgae. Lipid content ranged from 115 g kg−1 dry matter (DM) (Scenedesmus sp. AMDD) to 361 g kg−1 DM (Tetracystis sp.), while Scenedesmus sp. AMDD had the highest carbohydrate (364 g kg−1 DM) and fibre content (277 g kg−1 DM). Gas production was highest (P < 0.001) for Micractinium reisseri and Chlorella vulgaris. In vitro DMD ranged from 654 g kg−1 for Scenedesmus sp. AMDD to 797 g kg−1 for Nannochloris bacillaris. Total CH4 differed (P < 0.001) among microalgae, ranging from 1.76 mL g−1 DM for Tetracystis sp. to 4.07 mL g−1 DM for M. reisseri. Nannochloropsis granulata (marine) had higher myristic, palmitoleic, and eicosapentaenoic acid levels than freshwater microalgae. Levels of α-linolenic acid were higher in Scenedesmus sp. AMDD than all other microalgae. CH4 production negatively correlated (P < 0.05) with levels of total carbohydrate, oleic, and α-linolenic acid. Despite having a lower lipid content, CH4 reductions with Scenedesmus sp. AMDD were comparable to Tetracystis sp. and N. bacillaris. Reductions in CH4 with Tetracystis sp. and N. bacillaris occurred without a decline in DMD, suggesting that overall microbial activity was not inhibited.

Biochemical characterization of microalgal biomass from freshwater species isolated in Alberta, Canada for animal feed applications

Biochemical composition of freshwater microalgae isolates from Alberta, Canada was determined. Growth rate (0.98±0.07d−1), biomass production (0.35±0.03gDWL−1) and daily productivity (0.14±0.01gDWL−1d−1) were the same for Chlorella vulgaris (AB02-C-U-BBM), Nannochloris bacillaris (AB03-C-F-PLM), Tetracystis sp. (AB04-C-F-PLM02) and Micractinium reisseri (AB05-C-U-BBM02). Whole algal biomass (WAB) contained low ash (~2g100gDW−1) and protein (13–15g100gDW−1), high esterifiable lipid (32–36g100gDW−1), carbohydrate (27–30g100gDW−1) and energy (26–28MJkgDW−1). Oil fractionation was relatively ineffective for C. vulgaris and M. reisseri while oil was effectively extracted from N. bacillaris and Tetracystis sp. Accordingly, lipid-extracted biomass (LEB) from N. bacillaris and Tetracystis sp. contained higher protein (22g100gDW−1) and carbohydrate (43–44g100gDW−1) and lower residual esterifiable lipid (6–9g100gDW−1) than C. vulgaris and M. reisseri at 17–18g100gDW−1 (protein), 34–36g100g−1DW (carbohydrate) and 28–32g100gDW−1 (residual esterifiable lipid). Biomass had favorable essential amino acid (EAA) profiles with high EAA indices (0.9–1.1); rich in first-limiting EAA lysine (0.9–1.5g100gDW−1). Fatty acids (% of total) were predominantly monounsaturated fatty acids (MUFA; 40–53%), high in polyunsaturated fatty acids (PUFA; 27–40%) and low in saturated fatty acids (14–24%). N. bacillaris and Tetracystis sp. were particularly rich (9–12%) in α-linolenic acid (18:3n−3), had attractive n−3:n−6 ratios (0.5–0.7:1), were rich in iron (800–1616mgkgDW−1), had attractively low Ca:P ratios (0.6–0.9:1) and were virtually absent of contaminating heavy metals.

ROLE OF MULTIPLE FTSZ RINGS IN CHLOROPLAST DIVISION UNDER OLIGOTROPHIC AND EUTROPHIC CONDITIONS IN THE UNICELLULAR GREEN ALGA NANNOCHLORIS BACILLARIS (CHLOROPHYTA, TREBOUXIOPHYCEAE)(1).

Chloroplasts of the unicellular green alga Nannochloris bacillaris Naumann cultured under nutrient-enriched conditions have multiple rings of FtsZ, a prokaryote-derived chloroplast division protein. We previously reported that synthesis of excess chloroplast DNA and formation of multiple FtsZ rings occur simultaneously. To clarify the role of multiple FtsZ rings in chloroplast division, we investigated chloroplast DNA synthesis and ring formation in cells cultured under various culture conditions. Cells transferred from a nutrient-enriched medium to an inorganic medium in the light showed a drop in cell division rate, a reduction in chloroplast DNA content, and changes in the shape of chloroplast nucleoids as cells divided. We then examined DNA synthesis by immunodetecting BrdU incorporated into DNA strands using the anti-BrdU antibody. BrdU-labeled nuclei were clearly observed in cells 48 h after transfer into the inorganic medium, while only weak punctate signals were visible in the chloroplasts. In parallel, the number of FtsZ rings decreased from 6 to only 1. When the cells were transferred from an inorganic medium to a nutrient-enriched medium, the number of cells increased only slightly in the first 12 h after transfer; after this time, however, they started to divide more quickly and increased exponentially. Chloroplast nucleoids changed from punctate to rod-like structures, and active chloroplast DNA synthesis and FtsZ ring formation were observed. On the basis of our results, we conclude that multiple FtsZ ring assembly and chloroplast DNA duplication under nutrient-rich conditions facilitate chloroplast division after transfer to oligotrophic conditions without further duplication of chloroplast DNA and formation of new FtsZ rings.

DNA Content of Stichococcus bacillaris (Trebouxiophyceae, Chlorophyta) Nuclei Determined with Laser Scanning Cytometry

The unicellular green microalgal genus Stichococcus Nageli propagates by binary fission. In the present study, we measured the DNA content of Stichococcus bacillaris Nageli in propidium iodide stained nuclei using laser scanning cytometry (LSC). Using the nuclei of Ulva compressa Linnaeus and Saccharomyces cerevisiae Hansen as references, the nuclear genome size of S. bacillaris was calculated as 80 Mbp. This value is approximately 4 times larger than that of Nannochloris bacillaris (20.3 Mbp), another member of the Trebouxiophyceae that propagates by binary fission.

MULTIPLE FtsZ RING FORMATION AND REDUPLICATED CHLOROPLAST DNA IN NANNOCHLORIS BACILLARIS (CHLOROPHYTA, TREBOUXIOPHYCEAE) UNDER PHOSPHATE-ENRICHED CULTURE(1).

We examined the effects of phosphate enrichment on chloroplasts of the unicellular green alga Nannochloris bacillaris Naumann. The doubling time of cells was similar in phosphate-limited (no β-glycerophosphate) and phosphate-enriched (2 mM β-glycerophosphate) media. The lengths of cells and chloroplasts were similar, regardless of phosphate concentration. The relationship between the ring formation of the prokaryote-derived chloroplast division protein FtsZ and phosphate concentration was examined using indirect fluorescent antibody staining. The number of FtsZ rings increased as the phosphate concentration of the medium increased. Multiple FtsZ rings were formed in cells in phosphate-enriched medium; up to six FtsZ rings per chloroplast were observed. The number of FtsZ rings increased as the chloroplast grew. The FtsZ ring located near the center of the chloroplast had the strongest fluorescence. The FtsZ ring at the relative center of all FtsZ rings was used for division. Plastid division rings did not multiply in phosphate-enriched culture. The chloroplast DNA content was 2.3 times greater in phosphate-enriched than in phosphate-limited culture and decreased in cells cultured in phosphate-enriched medium containing 5-fluorodeoxyuridine (FdUr). In the presence of FdUr, only one FtsZ ring formed, even under phosphate enrichment. This finding suggests that excessive chloroplast DNA replication induces multiple FtsZ ring formation in phosphate-enriched culture. We propose a multiple FtsZ ring formation model under phosphate enrichment.

Patterns of asexual reproduction in Nannochloris bacillaris and Marvania geminata (Chlorophyta, Trebouxiophyceae)

Non-flagellated vegetative green algae of the Trebouxiophyceae propagate mainly by autosporulation. In this manner, the mother cell wall is shed following division of the protoplast in each round of cell division. Binary fission type Nannochloris and budding type Marvania are also included in the Trebouxiophyceae. Phylogenetic trees based on the actin sequences of Trebouxiophyceae members revealed that the binary fission type Nannochloris bacillaris and the budding type Marvania geminata are closely related in a distal monophyletic group. Our results suggest that autosporulation is the ancestral mode of cell division in Trebouxiophyceae. To elucidate how non-autosporulative mechanisms such as binary fission and budding evolved, we focused on the cleavage of the mother cell wall. Cell wall development was analyzed using a cell wall-specific fluorescent dye, Fluostain I. Exfoliation of the mother cell wall was not observed in either N. bacillaris or M. geminata. We then compared the two algae by transmission electron microscopy with rapid freeze fixation and freeze substitution; in both algae, the mother cell wall was cleaved at the site of cell division, but remained adhered to the daughter cell wall. In N. bacillaris, the cleaved mother cell wall gradually degenerated and was not observed in the next cell cycle. In contrast, M. geminata daughter cells entered the growth phase of the next cell cycle bearing the mother and grandmother cell walls, causing the uncovered portion of the plane of division to bulge outward. Such a delay in the degeneration and shedding of the mother cell wall probably led to the development of binary fission and budding.

Isolation and molecular characterization of rbcS in the unicellular green alga Nannochloris bacillaris (Chlorophyta, Trebouxiophyceae)

SUMMARY The small subunit of the chloroplast enzyme ribulose-1,5-bisphosphate carboxyase/oxygenase (Rubisco) is encoded by rbcS. We isolated and characterized three rbcS genes (NbrbcS1-1, NbrbcS1-2, and NbrbcS2) from the unicellular green alga Nannochloris bacillaris (Chlorophyta, Trebouxiophyceae). In the haploid N. bacillaris genome, each is a single-copy gene located on different chromosomes. Each mature peptide contains 140 amino acid residues. NbrbcS1-1 and NbrbcS1-2 are identical and share 80% identity with NbrbcS2, while the respective transit sequences share only approximately 48% identity with that of NbrbcS2. NbrbcS1-1 transcription was suppressed in the dark and recovered drastically after transfer to light. In contrast, NbrbcS1-2 and NbrbcS2 expression were not reduced after transfer from light to dark. In chlorophyllic tobacco cells containing green fluorescent protein fusion proteins of the transit sequences of each peptide, green fluorescent protein signals were localized on particles matching chloroplasts. The first introns of NbrbcS1-1 and NbrbcS1-2 are identical to the corresponding introns of 37 rbcS genes in eight embryophyte species. While the second intron is conserved in the green algae, the NbrbcS2 intron appears to involve sliding by one base pair. The NbrbcS1-1 and NbrbcS1-2 intron conserved in green algae and embryophytes might be an ancestral intron from nuclear-encoded rbcS.

MOLECULAR DIVERGENCE AND CHARACTERIZATION OF TWO CHLOROPLAST DIVISION GENES, FTSZ1 AND FTSZ2, IN THE UNICELLULAR GREEN ALGA NANNOCHLORIS BACILLARIS (CHLOROPHYTA)1

Two FtsZ paralogues (NbFtsZ1 and NbFtsZ2) were isolated from the unicellular green alga Nannochloris bacillaris Naumann. These sequences encoded proteins of 435 and 439 amino acids with tubulin signature motifs (GGGTG[T/S]G), which are important for GTP binding activity. NbFtsZ1 and NbFtsZ2 had four and three introns, respectively, and two different putative core promoters; a TATA box (TATAAAA) and an initiator element (CCCAGG) were located 40 bp and 80 bp upstream of the coding regions of NbFtsZ1 and NbFtsZ2, respectively. Southern blot hybridization and contour‐clamped homogeneous electric field electrophoresis showed that N. bacillaris contained at least one copy of each gene and that NbFtsZ1 was located on chromosome 5 and NbFtsZ2 on chromosome 3 or 4. Phylogenetically, NbFtsZ1 and NbFtsZ2 belong to the vascular plant protein families FtsZ1 and FtsZ2, respectively. The FtsZ1 proteins do not contain carboxy‐terminal consensus sequences, whereas all FtsZ2 proteins possess the consensus sequence (I/V)PxFL(R/K)(K/R)(K/R). Our study has shown that NbFtsZ2 possesses a similar consensus sequence (VPDFLRRK), whereas NbFtsZ1 does not, further supporting their classification as FtsZ2 and FtsZ1. Escherichia coli ftsZ mutants transformed with cloned NbFtsZ1, and NbFtsZ2 cDNAs were restored for the capacity to divide by binary fission, suggesting that the proteins retained the ability to function in the bacterium. An anti‐NbFtsZ2 antibody specifically recognized a single protein band of approximately 51 kDa on an immunoblot of N. bacillaris cellular proteins. Immunostaining of the algal cells with this antibody produced an intense fluorescent signal as a ring near the middle of the cell, which corresponded to the chloroplast division site.

RELATIONSHIP BETWEEN PRESENCE OF A MOTHER CELL WALL AND SPECIATION IN THE UNICELLULAR MICROALGA NANNOCHLORIS (CHLOROPHYTA)1

The cell division mechanisms of seven strains from six species of Nannochloris Naumann were analyzed and compared with those of three species of Chlorella Beijerinck and Trebouxia erici Ahmadjian using differential interference microscopy and fluorescence microscopy. Nannochloris bacillaris Naumann divides by binary fission and N. coccoides Naumann divides by budding. Distinct triangular spaces or mother cell walls were found in the dividing autosporangia of the other five strains from four species of Nannochloris, three species of Chlorella, and T. erici. In an attempt to infer an evolutionary relationship between nonautosporic and autosporic species of Nannochloris, we constructed a phylogenetic tree of the actin genes using seven strains from six species of Nannochloris, three species of Chlorella, and T. erici. Nannochloris species were polyphyletic in the Trebouxiophyceae group. Two nonautosporic species of N. bacillaris and N. coccoides were monophyletic and positioned distally. Moreover, to determine their phylogenetic position within the Trebouxiophyceae, we constructed phylogenetic tree of 18S rRNA genes adding other species of Trebouxiophyceae. Nannochloris species were polyphyletic in the Trebouxiophyceae and appeared in two different lineages, a Chlorella–Nannochloris group and a Trebouxia–Choricystis group. The nonautosporic species, N. bacillaris and N. coccoides, and three autosporic species of Nannochloris belonged to the Chlorella–Nannochloris group. Nannochloris bacillaris and N. coccoides were also monophyletic and positioned distally in the phylogenetic tree of 18S rRNA genes. These results suggest that autosporulation is the ancestral mode of cell division in Nannochloris and that nonautosporulative mechanisms, such as binary fission and budding, evolved secondarily.

EVOLUTIONARY RELATIONSHIPS AMONG MULTIPLE MODES OF CELL DIVISION IN THE GENUS NANNOCHLORIS (CHLOROPHYTA) REVEALED BY GENOME SIZE, ACTIN GENE MULTIPLICITY, AND PHYLOGENY

A single cell divides to multiply, but not all cells follow the same pattern of division. We studied cell division in seven strains from six species belonging to the genus Nannochloris Naumann and classified their modes of cell division into three types: binary fission (N. bacillaris Naumann), budding (N. coccoides Naumann), and autosporulation resulting in the formation of two to four daughter cells (N. maculata Butcher, N. sp. SAG 251‐2, N. atomus Butcher CCAP 251/7 and SAG 14.87, and N. eucaryotum[Wilhelm et al.] Menzel and Wild). To determine the evolutionary relationships among these multiple modes of cell division, we investigated the strains' genome sizes, copy number of actin genes, and phylogeny. The genome sizes were determined by counter‐clamped homogeneous electric fields electrophoresis and fluorimetry. The genomes are very small and range from 12.6 Mbp (N. maculata) to 47.4 Mbp (N. atomus SAG 14.87). The genomes of Nannochloris species seem to be among the smallest for free‐living eukaryotes. Nannochloris bacillaris (binary fission), N. coccoides (budding), Nannochloris sp. (two‐cell type of autosporulation), and N. eucaryotum (multicell type of autosporulation) contain a single actin gene, whereas N. maculata (two‐cell type of autosporulation) and two strains of N. atomus (two‐cell type of autosporulation) contain two actin genes. This suggests that the actin gene was duplicated in this eukaryote, which has a very small genome. Phylogenetic analyses of partial actin gene sequences suggest that autosporulation is the ancestral mode of cell division.

ISOLATION, CHARACTERIZATION, AND CHROMOSOME MAPPING OF AN ACTIN GENE FROM THE PRIMITIVE GREEN ALGA, NANNOCHLORIS BACILLARIS (CHLOROPHYCEAE)

Historically, the genus Nannochloris has been classified using the morphology of cell division, although the mechanics of division remain relatively poorly understood. Nannochloris bacillaris reproduces by binary fission. Microscopic observation with fluorescein isothiocyanate‐phalloidin showed that actin filaments localized near the nucleus and appeared as a ring‐ or beltlike structure in the septum‐forming area in the middle of the cell during cell division. In primitive unicellular Chlorophyta such as N. bacillaris, actin is also thought to play important roles in nuclear migration and cell division. The N. bacillaris actin gene has three exons and two introns defined by two exon–intron junctions with splice site consensus sequences. The two introns are located at codons specifying amino acids 3/4 and 47/48. One of these, intron position 3/4, is conserved in the actin gene of Saccharomyces cerevisiae. The actin gene product was predicted to be 378 amino acids long with an estimated molecular weight of 42 kDa. There is only one copy of the actin gene in the N. bacillaris genome. Nannochloris bacillaris has 14 chromosomes that range in size from 230 kb to 3000 kb, and the total size of the genome was estimated to be 20.3 Mb. The actin gene is on either chromosome XI or XII. In a phylogenetic tree based on the actin gene sequence, N. bacillaris diverged before the divergence of Volvox, Chlamydomonas, and higher plants, and very shortly after the radiation of the Rhodophyta.

DIVISION APPARATUS OF THE CHLOROPLAST IN NANNOCHLORIS BACILLARIS (CHLOROPHYTA)1

ABSTRACT Chloroplast division in Nannochloris bacillaris Naumann (Chlorophyta) was examined by electron microscopy after preparation of samples by freeze‐substitution. A pair of belts appeared on the surface of the outer and inner envelope membranes at the middle of the chloroplast. These belts seemed to be constructed of thin fibrils that run parallel to the longitudinal direction of the belts. The outer fibrillar belt increased in width as the constriction of the chloroplast advanced. It appears that the fibrillar belt is the division apparatus of the chloroplast. It encircles the chloroplast and finally divides the chloroplast in two as the diameter of the belt decreases.

Proline overproduction in cells of the green alga Nannochloris bacillaris resistant to azetidine‐2‐carboxylic acid

Abstract Cells of N. bacillaris have been selected that are resistant to the toxic proline analogue azetidine‐2‐carboxylic acid (A2C) in 7% artificial seawater (ASW). This phenotype is stable in the absence of selection pressure. A2C resistance at low salinity was demonstrated to be due to an overproduction of proline in these cells, while levels of other amino acids were unaffected. Both wild‐type and A2C‐resistant cells respond to growth in high salinity media (100% ASW, 200% ASW) by accumulation of proline, but proline levels at all salinities are higher in the A2C‐resistant cells than in the wild‐type. Proline overproduction in the A2C‐resislant cells did not affect fluctuations in the levels of other salinity‐dependent solutes, such as homarine. A mutant with this level of specificity over a wide range of water potentials has not been reported for other plants and algae. Both the wild‐type and A2C‐resistant cells were able to grow over the entire salinity range tested (7%‐300% ASW). However, the A2C‐resistant cells showed a lower division rate than the wild‐type in 300% ASW, and yield of A2C‐resislant cells was lower than yield of wild‐type cells at the salinity extremes (7% ASW, 300% ASW). The response or wild‐type and A2C‐resistant cells to rapid increases in salinity were similar for both growth and photosynthesis. The presence of a constitutive high level of proline in the A2C‐resistant cell line did not confer any obvious increased tolerance to salinity shocks, indicating that there are other important factors in the biochemical adaptation to salinity in these cells.

Proline overproduction in cells of the green alga Nannochloris bacillaris resistant to azetidine-2-carboxylic acid

Proline overproduction in cells of the green alga Nannochloris bacillaris resistant to azetidine-2-carboxylic acid

Structure-activity relations for inhibition of photosynthesis by calmodulin antagonists

The known calmodulin antagonists (trifluoperazine, thioridizine, chlorpromazine, propranolol and dibucaine) were found to be potent inhibitors of photosynthesis in both the green alga Nannochloris bacillaris isolated mesophyll cells from Asparagus sprengeri. Concentrations producing 50% inhibition were within the range of 2–180 μM, and were correlated with the concentrations reported to inhibit calmodulin-stimulated mammalian phosphodiesterase activity. The photosynthetic inhibition was characterized as largely reversible, independent of pH and bicarbonate to CO 2 ratio, and mediated through photosynthesis electron transport and possibly other mechanisms. Their action as potent inhibitors of photosynthesis limits the usefulness of these antagonists as probes of calmodulin function in both lower and higher plant. systems (intact cells) incubated in the light, and suggests a reevaluation of some previously published work.

Stepwise adaptation to salinity in the green alga Nannochloris bacillaris

A severe inhibitory decrease in water potential owing to salinity increase (ψ = −0.2 to −5.5 MPa) was tolerated in Nannochloris bacillaris Naumann if an intermediate salinity step at ψ = −2.6 MPa was provided. Photosynthesis, respiration, and growth were all enhanced by the provision of an intermediate step. The length of time of the application of the intermediate salinity step was critical to the resulting abilities of the cells to exhibit substantial adaptation. Increasing the length of the intermediate step from 10 to 120 min resulted in a 168-fold increase in the rate of photoassimilation of bicarbonate. Adaptation of other functions (e.g., growth) took much longer. The effects of pulse shocks, downward then upward water potentials, indicated that when there was an inhibition of photosynthesis and it was irreversible over the short term. Pulse shocks to noninhibitory water potentials did not cause inhibition. These results were correlated with measurements of volume changes, rates of excretion of solutes, and ultrastructural changes.

Phytoplankton ecology in an unusually stable environment (Montezuma Well, Arizona, U.S.A.)

Relatively minor annual amplitudes of change in certain major nutrients, and especially pH and water temperature were measured in the spring-fed system of Montezuma Well, Arizona during a four year study. phytoplankton diversity was low but for the most part, composition was spatially and temporally constant; total seasonal phytoplankton density was significantly correlated with regional incident light. Phytoplankton species composition changed briefly during and for a short period following the summer monsoon. Ultraplankton (<5 µm diam.) numerically comprised nearly 80% of the phytoplankton community throughout most of the year. The limited residence time of water in the Well may have provided a competitive advantage for cells with high surface area:volume ratios and correspondingly rapid division rates. Nannochloris bacillaris Naum. and Coccomyxa minor Skuja were perennial dominants. Diatom populations did not increase with annual increases in vernal solar radiation. Low pH, high dissolved CO2, and limited residence time for metabolic inhibitors are considered to be largely responsible for the reduced blue-green populations in the Well. The only flagellated photosynthetic group present in Montezuma Well was the Cryptophyta. Desmid populations were minimal, even though pH was consistently below circumneutral (6.5) and free CO2 concentrations high. The role of grazing by an amphipod, Hyalella montezuma, on annual phytoplankton abundance is examined.

Is autosporulation a feature of Nannochloris?

The green algal genus Nannochloris Naumann was originally described as having a simple binary division mechanism and lacking autosporulation and a parental cell wall. An initial observation of autosporulation in one isolate ascribed to this genus encouraged a broader investigation to determine whether or not autosporulation is restricted to only certain forms or is the reproductive mechanism used in the genus as a whole. Five cultures were examined; all had typically chlorophycean ultrastructure and both chlorophylls a and b. Nannochloris atomus differed from the other forms (two Nannochloris spp., Nannochloris bacillaris, and Nannochloris maculants) in having a pyrenoid and different wall structure and other ultrastructural features. However, reproduction in all five was by autosporulation with a thin, yet clearly defined, parental wall surrounding the two–four daughter cells in electron micrographs. These and related findings of other workers point to the need for a redefinition of the genus.

PHOTOSYNTHETIC AND GROWTH RESPONSES TO SALINITY IN A MARINE ISOLATE OF NANNOCHLORIS BACILLARIS (CHLOROPHYCEAE)1

ABSTRACTA marine unicellular alga, Nannochloris bacillaris Naumann, was studied with respect to growth, viability and photosynthesis during the steady‐state and also subsequent to changes in the concentration of artificial seawater medium. Cells grew exponentially over the range of 2% to 300% artificial seawater, but more rapidly at lower salinities. In contrast to growth, photosynthesis as measured by both oxygen evolution and bicarbonate photoassimilation was not obviously inhibited for cells adapted within the range of 7% to 200% artificial seawater. In 300% artificial seawater, photosynthesis, especially bicarbonate photoassimilation, was inhibited. Osmotic shocks caused by transferring cells from 200% to 7% artificial seawater had little if any effect on growth, viability or photosynthesis. However, equal shocks in the upward direction (from 7% to 200% artificial seawater) caused long lag phases in growth, totally inhibited photosynthesis and very often led to cell death. Intermediate upward shocks were less deleterious, but did result in lags in growth.