Molecular and Physiological Responses of Two Classes of Marine Chromophytic Phytoplankton (Diatoms and Prymnesiophytes) during the Development of Nutrient-Stimulated Blooms

ABSTRACT Generic taxon-specific DNA probes that target an internal region of the gene (rbcL) encoding the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) were developed for two groups of marine phytoplankton (diatoms and prymnesiophytes). The specificity and utility of the probes were evaluated in the laboratory and also during a 1-month mesocosm experiment in which we investigated the temporal variability in RubisCO gene expression and primary production in response to inorganic nutrient enrichment. We found that the onset of successive bloom events dominated by each of the two classes of chromophyte algae was associated with marked taxon-specific increases in rbcLtranscription rates. These observations suggest that measurements of RubisCO gene expression can provide an early indicator of the development of phytoplankton blooms and may also be useful in predicting which taxa are likely to dominate a bloom.

Thraustochytrids are chromists, not Fungi: 18s rRNA signatures of Heterokonta

Thraustochytrids are a neglected group of non-photosynthetic marine protists characterized by the presence of a sagenogenetosome, an ectoplasmic net and a cell wall composed of non-cellulosic scales. Though originally classified as fungi, they are ultrastructurally closest to the labyrinthulids, which have often been treated as protozoa, but are now usually considered to be more closely related to the heterokont algae. In order to clarify their controversial taxonomic position and evolutionary relationships, we have cloned and sequenced the 18s rRNA genes of the thraustochytrids Thraustochytrium kinnei and Ulkenia profunda . Phylogenetic analysis by several methods confirms earlier suggestions based on ultrastructure that thraustochytrids constitute a deeply divergent branch of the phylum Heterokonta, which is currently classified in the kingdom Chromista. The phylum Heterokonta also includes numerous chromophyte algae (e.g. brown algae and diatoms) as well as the oomycetes. Heterokont 18s rRNA sequences can be distinguished from those of all other eukaryotes by having an AU not a UA base pair at the very base of helix 47 in the V9 region. We also tabulate signature sequences that can be used to characterize 18s rRNA sequences from each of the major heterokont taxa.