The diatom <i>Chaetoceros socialis</i>: spore formation and preservation

Planktonic diatoms thrive in the water column, and several species can transform into resting stages – spores or resting cells – that sink to the bottom of the sea. Resting stages are generally produced when environmental conditions are not optimal for growth and can remain viable in sediments for a long time. We tested different aspects related to the formation of spores in Chaetoceros socialis, one of the dominant diatoms in the Gulf of Naples (Mediterranean Sea, Italy). Observations in time lapse and confocal laser scanning microscopy allowed illustration of the formation of endogenous spores, in which valve deposition is accompanied by an acytokinetic nuclear division. The complete transition from vegetative cell to spore takes about 8–10 h and, if exposed to the light, spores can germinate again after a few days. The depletion of nitrogen source in the culture medium induced the formation of spores, with very similar dynamics among different sympatric strains; extremely high percentages of spores (up to >95%) were produced after 4–5 days. Once formed, spores can remain viable for up to nine months, and anoxic conditions favour their preservation. Identical dynamics of spore formation were detected in freshly established cultures and in cultures produced by the germination of spores kept dormant for different lengths of time. Our results suggest that spores of C. socialis can, in principle, rapidly shift between vegetative cells and resting stages that may explain the success of this species in coastal water. Our results also demonstrate that storage of resting spores may represent an alternative to cryopreservation.</p

Cell length of <i>Pseudo-nitzschia multistriata</i> strains assigned to A) POP_A (green), B) POP_B (red), and C) putative hybrids (black), as determined by Structure.

<p>The grey dots in the background represent the cell size classes recorded in the natural environment.</p

Placement of the 22 samples along the first two PCA axes (GEnALex); samples of <i>P. multistriata</i> gathered at LTER_MC in 2008 are indicated with grey squares; those in 2009 with grey diamonds, those in 2010 with black squares, and the one in 2011 with a white diamond.

<p>Samples 11 (2009), 19, 20, and 21 (2010), which showed a higher abundance of POP_B strains, but contained a minority of strains assigned to POP_A and others assigned as putative hybrids, are enclosed within an ellipse (see text). The two PCA axes capture 73.2% of the variability.</p

Genetic structure of the strains of <i>Pseudo-nitzschia multistriata</i> within each sample, as defined by Structure.

<p>Samples are presented in ranking order (1–22) along the x-axis. Each vertical bar represents one strain. The y-axis indicates the proportion of a strain’s genotype assigned by Structure to a population (i.e., K-cluster); the two populations identified by Structure are indicated POP_A (green), POP_B (red). Strains in each sample are ranked along the x-axis based on their assignment probabilities. Strains not assignable to any of the two populations above the 0.9-probability-level, are indicated to the right side of each sample. Samples have been grouped by year of collection: samples 1–7 in 2008, samples 8–15 in 2009, samples 16–21 in 2010 and sample 22 in 2011.</p

The Gulf of Naples (GoN) and the location of the Long Term Ecological Research station MareChiara (LTER-MC; 40°48.5′N, 14°15′E).

<p>The Gulf of Naples (GoN) and the location of the Long Term Ecological Research station MareChiara (LTER-MC; 40°48.5′N, 14°15′E).</p

Genetic diversity of <i>Pseudo-nitzschia multistriata</i> per year of sampling (2008–2011).

<p>For sampling dates see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114984#pone-0114984-t001" target="_blank">Table 1</a>.</p><p>Included in the table are the number of strains analyzed (N), the number of genotypes (G), the genotypic diversity (G/N, in %), the average number of alleles per locus (A/l), the total number of alleles (Na), the average number of alleles when resampled 1000 times for N = 157 as smallest sample size (Na157) and associated Standard Error (SE), the number of private alleles (PA), the expected (He) and observed (Ho) heterozygosity, and the fixation index (F_IS ± SE) over the seven microsatellite loci. Resampling for the smallest sampling size on the samples grouped per year was not performed for the 2011 sample (*).</p><p>Genetic diversity of <i>Pseudo-nitzschia multistriata</i> per year of sampling (2008–2011).</p

Placement of the individual <i>Pseudo-nitzschia multistriata</i> strains along the first two FCA axes (Genetix); specimens assigned by Structure to POP_A are indicated with grey squares, those to POP_B with black squares and those classified as putative hybrids (HYBR) with white squares.

<p>Placement of the individual <i>Pseudo-nitzschia multistriata</i> strains along the first two FCA axes (Genetix); specimens assigned by Structure to POP_A are indicated with grey squares, those to POP_B with black squares and those classified as putative hybrids (HYBR) with white squares.</p

Genotypic diversity of the inferred <i>Pseudo-nitzschia multistriata</i> populations POP_A, POP_B and the putative hybrids as identified by Structure over the four years of study.

<p>Included in the table are the number of strains (N), the number of genotypes (G) and the genotypic diversity (G/N).</p><p>Genotypic diversity of the inferred <i>Pseudo-nitzschia multistriata</i> populations POP_A, POP_B and the putative hybrids as identified by Structure over the four years of study.</p

Cell concentration of <i>Pseudo-nitzschia multistriata</i> in the surface samples collected at LTER-MC from January 2008 to December 2011.

<p>All LTER-MC samples are indicated with dots along the x-axis. The LTER-MC samples from which <i>P. multistriata</i> cells have been isolated for genotyping are indicated with small vertical bars under the sample dots (for sample numbers see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114984#pone-0114984-t001" target="_blank">Table 1</a>).</p

Genetic diversity of the strains of <i>Pseudo-nitzschia multistriata</i> sampled within the 22 field samples collected at the Long Term Ecological Research station MareChiara.

<p>Sampling dates are expressed as DDMMYY. Included in the table are the number of strains analyzed (N), the number of genotypes (G), the genotypic diversity (G/N, in %), the average number of alleles per locus (A/l), the total number of alleles (Na), the average number of alleles when resampled 1000 times for N = 7 as smallest sample size (Na7) and associated Standard Error (SE), the number of private alleles (PA), the expected (He) and observed (Ho) heterozygosity, and the fixation index (F_IS ± SE) over the seven microsatellite loci.</p><p>Genetic diversity of the strains of <i>Pseudo-nitzschia multistriata</i> sampled within the 22 field samples collected at the Long Term Ecological Research station MareChiara.</p