Relationship between growth irradiance and the xanthophyll cycle pool in the diatom Nitzschia palea

The size and de-epoxidation state of the xanthophyll cycle pool was measured in cultures of Nitzschia palea grown at six fluence rates in continuous light or with a 12 h photoperiod. In both series the size of the pool increased with increasing irradiance. The de-epoxidized form, diatoxanthin, was only present at fluence rates saturating for growth. The portion of diadinoxanthin, which was not readily de-epoxidized in saturating light, was constant and not related to the size of the pool. In the culture grown in a light-dark cycle at 300 μmol photons m^-2 s^-1 (PAR) increasing de-epoxidation took place in the latter half of the photoperiod, when the rate of photosynthesis was decreasing. A rapid, spectrophotometric method for measuring the extent of de-epoxidation of the xanthophyll cycle pool in a culture of diatoms is described. Upon addition of a small volume of hydrochloric acid to an extract of pigments in 90% acetone, the absorbance at 480 nm is reduced. The size of the reduction is a measure of the state of the xanthophyll cycle pool, since the absorbance of diatoxanthin is reduced by 5%, but the absorbance of diadinoxanthin by 87% due to an epoxide-furanoid rearrangement, which causes the absorption spectrum to be shifted by ca 20 nm towards shorter wavelengths.     

CARBONIC ANHYDRASE IN THE MARINE DIATOM THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)1

The zinc metalloenzyme carbonic anhydrase plays a critical role in inorganic carbon acquisition in marine diatoms, thus conferring on zinc a key role in oceanic carbon cycling. As a first step in determining the location and function of carbonic anhydrase (CA) in Bacillariophyceae, we purified and partially sequenced CA from T. weissflogii (Gru) Fryxell et Hasle (TWCA1) and cloned the corresponding cDNA (twca1). The twca1 sequence is different from other known algal carbonic anhydrase genes, and encodes a protein of roughly 34 kDa. The amino terminal amino acids sequenced from purified TWCA1 are 72 residues downstream of the putative starting methionine predicted by twca1. This difference may be due to the presence of a short-lived signal sequence designed to guide the enzyme to the correct cellular location. The absence of any homology between TWCA1 and previously sequenced CAs from Chlorophyceae may indicate either convergent evolution or that carbon acquisition represents a fundamental physiological difference among algal phyla.     

The topography of soft,adhesive diatom ‘trails’ as observed by Atomic Force Microscopy

Gliding diatoms foul surfaces by leaving behind ‘trails’ of secreted mucilage. Atomic force microscopy (AFM) used in ‘fluid tapping’ mode enabled the topography of the soft, adhesive trails in the natural hydrated state to be imaged, and without the artefacts resulting from fixation and/or dehydration. Diatom trails consist of a continuous, swollen ridge of material that dominates the trail, as well as a diffuse hydrated mucilage coating observed on either side of the main trail. The main trail material is evenly attached to the coverslip along its entire length, and appears to cure, or become less soft/adhesive, over time. Diatom trails observed with the scanning electron microscope were severely damaged by dehydration, while trails imaged by the AFM in ‘contact’ mode were damaged and/or removed by the action of the cantilever. The AFM used in ‘fluid tapping’ mode is an excellent tool for topographical studies of soft/adhesive biological molecules in the hydrated state, and will have great value for measuring their physical and mechanical properties when operated in ‘force modulation’ mode.     

Adhesion and motility of fouling diatoms on a silicone elastomer

Recent demands for non-toxic antifouling technologies have led to increased interest in coatings based on silicone elastomers that ‘release’ macrofouling organisms when hydrodynamic conditions are sufficiently robust. However, these types of coatings accumulate diatom slimes, which are not released even from vessels operating at high speeds ( > 30 knots). In this study, adhesion strength and motility of three common fouling diatoms (Amphora coffeaeformis var. perpusilla (Grunow) Cleve, Craspedostauros australis Cox and Navicula perminuta Grunow) were measured on a polydimethylsiloxane elastomer (PDMSE) and acid-washed glass. Adhesion of the three species was stronger to PDMSE than to glass but the adhesion strengths varied. The wall shear stress required to remove 50% of cells from PDMSE was 17 Pa for Craspedostauros, 24 Pa for Amphora and >> 53 Pa for Navicula; the corresponding values for glass were 3, 10 and 25 Pa. In contrast, the motility of the three species showed little or no correlation between the two surfaces. Craspedostauros moved equally well on glass and PDMSE, Amphora moved more on glass initially before movement ceased and Navicula moved more on PDMSE before movement ceased. The results show that fouling diatoms adhere more strongly to a hydrophobic PDMSE surface, and this feature may contribute to their successful colonization of low surface energy, foul-release coatings. The results also indicate that diatom motility is not related to adhesion strength, and motility does not appear to be a useful indicator of surface preference by diatoms.     

Review – Interactions between diatoms and stainless steel: focus on biofouling and biocorrosion

There is a considerable body of information regarding bacterially enhanced corrosion, however, this review focuses on diatoms (unicellular algae) whose contribution to biocorrosion is less well studied. The reasons why diatoms have been neglected in studies of biocorrosion in natural waters are discussed and the question whether diatoms should be considered as inert with respect of electrochemical processes is considered. A particular focus is given to the case of stainless steels (SS), which are widely used in variety of applications in natural waters. Basic information on the cell biology of diatoms is included in the review, particularly with respect to their ability to ‘sense’ and adhere to surfaces. Investigations at the nanoscale are reviewed as these studies provide information about the behavior of cells at interfaces. Recent advances include the use of atomic force microscopy (AFM), although only a few studies have been applied to diatoms. Regarding the electrochemical behavior of SS, the mechanisms by which diatoms influence the potential ennoblement process is discussed. Such studies reveal the association of diatoms, in addition to bacteria, with biocorrosion processes.     

Microfluidic detachment assay to probe the adhesion strength of diatoms

Fouling release (FR) coatings are increasingly applied as an environmentally benign alternative for controlling marine biofouling. As the technology relies on removing fouling by water currents created by the motion of ships, weakening of adhesion of adherent organisms is the key design goal for improved coatings. In this paper, a microfluidic shear force assay is used to quantify how easily diatoms can be removed from surfaces. The experimental setup and the optimization of the experimental parameters to study the adhesion of the diatom Navicula perminuta are described. As examples of how varying the physico-chemical surface properties affects the ability of diatoms to bind to surfaces, a range of hydrophilic and hydrophobic self-assembled monolayers was compared. While the number of cells that attached (adhered) was barely affected by the coatings, the critical shear stress required for their removal from the surface varied significantly.     

Hormonal interaction in sexual reproduction of Desmarestia aculeata (Phaeophyceae)

Male and female gametophytes of Desmarestia aculeata have been grown as clonal cultures. Red light permits vegetative growth only. Differentiation of oogonia and antheridia takes place under illumination with blue light as has been previously reported for several species of Laminaria. Morphological details for differentiation of oogonia and antheridia are presented, stressing the strong resemblance with the respective processes in Laminaria. Experimental evidence indicates the presence of a hormonal factor secreted by the eggs, which is responsible for discharge and subsequent attraction of spermatozoids to the egg.     

Diatoms as indicators: The influences of experimental nitrogen enrichment on diatom assemblages in sub-Arctic streams

Indices using diatoms are widely used to assess water quality, but are usually constructed from field correlations and not tested through rigorous experimentation. We tested experimentally the performance of the Sørensen and the Shannon indices, and the trophic diatom index (TDI). Nitrogen was naturally limiting in the eight remote sub-Arctic streams used and we measured the effects of experimental nitrogen enrichment on diatom assemblages. Diatom densities increased significantly in the enriched reaches but there was no significant difference in invertebrate density between control and treatment reaches. Grazing effects were thus controlled for. Diversity within streams (Shannon index) was significantly reduced by nutrient addition but the Sørensen index did not change. The trophic diatom index (TDI), which is presumed to reflect nutrient concentration, was not influenced by nutrient addition and generally the values were low in both control and treatment reaches. Densities of the diatom genera Achnanthes and Gomphonema increased significantly with enrichment while those of Nitzschia and Fragilaria decreased significantly. Less abundant diatom species, which collectively constituted around 40% in relative abundance in the control reaches, were around 15–18% in treatment reaches. Growth forms were altered by the nutrients. Diatoms attached by mucilage pads were more abundant in treated reaches compared with control reaches. Motile diatoms became scarcer. The size of diatom species was unaffected by nutrient enrichment. This study showed that it is important to test experimentally indices that are developed for particular habitats before using them elsewhere.     

The complete chloroplast and mitochondrial genomes of the diatom Nitzschia palea (Bacillariophyceae) demonstrate high sequence similarity to the endosymbiont organelles of the dinotom Durinskia baltica

Nitzschia palea is a common freshwater diatom used as a bioindicator because of its tolerance of polluted waterways. There is also evidence it may be the tertiary endosymbiont within the “dinotom” dinoflagellate Durinskia baltica. A putative strain of N. palea was collected from a pond on the University of Virginia's College at Wise campus and cultured. For initial identification, three markers were sequenced—nuclear 18S rDNA, the chloroplast 23S rDNA, and rbcL. Morphological characteristics were determined using light and scanning electron microscopy; based on these observations the cells were identified as N. palea and named strain “Wise.” DNA from N. palea was deep sequenced and the chloroplast and mitochondrial genomes assembled. Single gene phylogenies grouped N. palea—Wise within a clearly defined N. palea clade and showed it was most closely related to the strain “SpainA3.” The chloroplast genome of N. palea is 119,447 bp with a quadripartite structure, 135 protein‐coding, 28 tRNA, and 3 rRNA genes. The mitochondrial genome is 37,754 bp with a single repeat region as found in other diatom chondriomes, 37 protein‐coding, 23 tRNA, and 2 rRNA genes. The chloroplast genomes of N. palea and D. baltica have identical gene content, synteny, and a 92.7% pair‐wise sequence similarity with most differences occurring in intergenic regions. The N. palea mitochondrial genome and D. baltica's endosymbiont mitochondrial genome also have identical gene content and order with a sequence similarity of 90.7%. Genome‐based phylogenies demonstrated that D. baltica is more similar to N. palea than any other diatom sequence currently available. These data provide the genome sequences of two organelles for a widespread diatom and show they are very similar to those of Durinskia baltica's endosymbiont.