A Comparison of Morphological and Molecular-Based Surveys to Estimate the Species Richness of <i>Chaetoceros</i> and <i>Thalassiosira</i> (Bacillariophyta), in the Bay of Fundy

<div><p>The goal of this study was to compare the ability of morphology and molecular-based surveys to estimate species richness for two species-rich diatom genera, <i>Chaetoceros</i> Ehrenb. and <i>Thalassiosira</i> Cleve, in the Bay of Fundy. Phytoplankton tows were collected from two sites at intervals over two years and subsampled for morphology-based surveys (2010, 2011), a culture-based DNA reference library (DRL; 2010), and a molecular-based survey (2011). The DRL and molecular-based survey utilized the 3′ end of the RUBISCO large subunit (<i>rbc</i>L-3P) to identify genetic species groups (based on 0.1% divergence in <i>rbc</i>L-3P), which were subsequently identified morphologically to allow comparisons to the morphology-based survey. Comparisons were compiled for the year (2011) by site (n = 2) and by season (n = 3). Of the 34 taxa included in the comparisons, 50% of taxa were common to both methods, 35% were unique to the molecular-based survey, and 12% were unique to the morphology-based survey, while the remaining 3% of taxa were unidentified genetic species groups. The morphology-based survey excelled at identifying rare taxa in individual tow subsamples, which were occasionally missed with the molecular approach used here, while the molecular methods (the DRL and molecular-based survey), uncovered nine cryptic species pairs and four previously overlooked species. The last mentioned were typically difficult to identify and were generically assigned to <i>Thalassiosira</i> spp. during the morphology-based survey. Therefore, for now we suggest a combined approach encompassing routine morphology-based surveys accompanied by periodic molecular-based surveys to monitor for cryptic and difficult to identify taxa. As sequencing technologies improve, molecular-based surveys should become routine, leading to a more accurate representation of species composition and richness in monitoring programs.</p></div

Example of a morphological species-discovery curve used to determine when colony isolation should discontinue.

<p>This curve was generated for colonies isolated from the Passamaquoddy Bay plankton tow subsamples on June 1, 2010 for development of the culture-based DNA reference library (DRL). Isolation was terminated after 60 mins (n = 42 colonies of 26 morphological species isolated) because 30 mins had elapsed with no new morphological types found.</p

Comparisons between the number of species found during the morphology (morph) and molecular-based (mol) surveys.

<p>Data were summarized for the 2011-monitoring year (<b>A</b>), and then by site (<b>B</b>), and by season (<b>C</b>). In the year summary (<b>A</b>), taxa were placed into five main categories in bold font (two of these were divided further into three subcategories each) as indicated on the figure and described in the text. In the by site (<b>B</b>) and by season (<b>C</b>) summaries, the taxa are placed in only three categories including taxa that were identified: in common to the morphology (morph) and molecular (mol) -based surveys (black); or in the morphology-based survey only (white); in the molecular-based survey only as unique (gray) or cryptic (vertical lines) species. Sites were Passamaquoddy Bay (PB) and The Wolves (WV). Seasons were Jan, Apr, May (winter), June–Sept (summer), and Oct–Dec (fall).</p

<i>Chaetoceros</i> and <i>Thalassiosira</i> diversity and abundance as recorded during the morphology-based surveys in 2010 and 2011.

<p>PB = Passamaquoddy Bay; WV = The Wolves; J = January; F = February; M = March; A = April; My = May; Jun = June; Jul = July; Aug = August; S = September; O = October; N = November; and D = December.</p>a<p>n indicates the number of tow subsamples in which each taxon was present (n = 59 for 2010 and n = 46 for 2011).</p>b<p>Months are the months each taxon was present.</p>c<p>n>1 are the number of tow subsamples with a HELCOM rating>1.</p>d<p>Months n>1 are the months in which the higher abundance occurred.</p