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 rbcL transcription 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.

FIG. 1

Nucleotide sequence alignment of rbcL gene fragments from marine diatom and prymnesiophyte phytoplankton. Nucleotides identical to the first sequence in the alignment are indicated by dashes. The diatom sequences are shaded. The GenBank accession numbers are as follows: Umbilicosphaera sibogae D45843; Calyptrosphaera sphaeodea, D45842; Chrysochromulina hirta, D45846; Emiliania huxleyi, D45845; Pleurochrysis carterae, D11140; Coccolithus pelagicus, AF196307; Cylindrotheca sp. strain N1, M59080; Thalassiosira pseudonana, AF109210; Skeletonema costatum, AF015569; and Rhizosolenia setigera, AF015568.

FIG. 2

Northern slot blots of in vitro transcription products derived from various species of marine phytoplankton probed with either the diatom-specific or prymnesiophyte-specific rbcL gene probes. Each slot was loaded with either 50 or 10 ng of target sense strand rbcL RNA, whereas the last row of both blots contained 1 or 0.1 μg of total RNA from the enteric bacterium E. coli as a nonspecific negative control. The hybridization and posthybridization conditions employed are described in the text.

FIG. 3

(a and b) Temporal variation in the inorganic nutrient concentrations in the N/P/Si-supplemented enclosure (a) and the N/P-supplemented enclosure (b) during the PRIME mesocosm experiment. Symbols: □, silicate; ○, nitrate; ◊, phosphate. The insets show the dissolved nutrient concentrations (micromolar) in the enclosures from day 5 onward on an expanded scale (phosphate, ×10). (c) Molar ratio of N assimilation to P assimilation in the two mesocosms. The horizontal dotted line indicates the ratio (15:1) of N and P supplied to the mesocosms.

FIG. 4

Temporal variation in the abundance of diatom rbcL mRNA (a), the abundance of prymnesiophyte rbcL mRNA (b), depth-integrated (0 to 4.5 m) primary production (c), and phytoplankton biomass (d) (○, diatoms; ●, E. huxleyi) in the N/P/Si-supplemented mesocosm. d, day.

FIG. 5

Temporal variation in the abundance of diatom rbcL mRNA (a), the abundance of prymnesiophyte rbcL mRNA (b), (c) depth-integrated (0 to 4.5 m) primary production (c), and phytoplankton biomass (d) (○, diatoms; ●, E. huxleyi) in the N/P-supplemented mesocosm. d, day.