Molecular and morphological analyses of solitary forms of brackish Thalassiosiroid diatoms (Coscinodiscophyceae), with emphasis on their phenotypic plasticity

<p>Blooms of centric diatoms are a common feature in the Bilbao estuary during summer when river flow is at its lowest and water temperature is above 20ºC. To gain insight into the specific composition of these diatom blooms, net samples and cultures of estuarine isolates were analysed under the scanning electron microscope (SEM) and by molecular analyses of the Internal Transcribed Spacers 1 and 2 plus the coding region 5.8S (ITS region) and the 28S rRNA gene. Seven species of solitary centric diatoms belonging to four genera were found in the estuary including: <i>Conticribra weissflogii, Cyclotella atomus</i> var. <i>atomus, Cyclotella cryptica, Cyclotella marina, Cyclotella meneghiniana, Discostella pseudostelligera</i> and <i>Thalassiosira pseudonana</i>. Dominant species during blooms were <i>C. meneghiniana</i> and <i>Co. weissflogii</i> in the upper estuary and <i>D. pseudostelligera</i> and <i>T. pseudonana</i> in the middle estuary. The morphological traits used to differentiate between species pairs of similar morphology (<i>C. meneghiniana</i>/<i>C. cryptica</i> or <i>D. pseudostelligera</i>/<i>D. woltereckii</i>) were observed to vary with environmental conditions, denoting a great deal of phenotypic plasticity which would hinder accurate identification of the species when using morphological approaches alone.