Samples from two coastal experimental ecosystems were incubated in vitro and sampled over 24 h. Production rates were measured by the 14C method, the O2 and CO, light-dark bottle methods, and the I80 method. 0, production in the experimental enclosures (volume -1.3 x lo4 liters) was also measured directly.Photosynthetic and respiratory quotients were close to 1 .O. Gross production values determined by 0, light-dark experiments, CO2 light-dark experiments, and I80 were similar. 14C production ranged from 60 to 100% of gross production measured in CO, light-dark experiments, indicating that 14C uptake is not precisely fixed with respect to other measures of community metabolism. There was no evidence that 14C or any other method underestimated the rate of primary production in vitro by more than 40%. Productivities in vitro ranged from 35 to 100% of those in the mesocosm at similar light intensities.In samples from one of the ecosystems, the rate of respiration in the light (calculated from I80 data) was an order of magnitude greater than the rate in the dark. This difference may be ascribed to either photorespiration or light enhancement of mitochondrial respiration.Turnover of microplankton populations in the ocean occurs on time scales of hours to days. Measurements of community turnover rates must be carried out with in vitro incubations, presenting two problems. First, it can never be claimed that processes oc-I

Pelagic plant life draws its principal supply of dissolved or undissolved nitrogen either from the coasts or from localities where warm and cold currents meet." J. Hjort "Where cold and warm currents meet at the surface of the ocean there is a rise of temperature for the animals of the cold current and a fall of temperature for the animals of the warm current, which results in a plentiful destruction of organisms." Sir John Murray "We are well acquainted with the stream in our pursuit of whales, which keep to the sides of it but are not met within it."

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BackgroundThe role of coastal nutrient sources in the persistence of Karenia brevis red tides in coastal waters of Florida is a contentious issue that warrants investigation into the regulation of nutrient responses in this dinoflagellate. In other phytoplankton studied, nutrient status is reflected by the expression levels of N- and P-responsive gene transcripts. In dinoflagellates, however, many processes are regulated post-transcriptionally. All nuclear encoded gene transcripts studied to date possess a 5' trans-spliced leader (SL) sequence suggestive, based on the trypanosome model, of post-transcriptional regulation. The current study therefore sought to determine if the transcriptome of K. brevis is responsive to nitrogen and phosphorus and is informative of nutrient status.ResultsMicroarray analysis of N-depleted K. brevis cultures revealed an increase in the expression of transcripts involved in N-assimilation (nitrate and ammonium transporters, glutamine synthetases) relative to nutrient replete cells. In contrast, a transcriptional signal of P-starvation was not apparent despite evidence of P-starvation based on their rapid growth response to P-addition. To study transcriptome responses to nutrient addition, the limiting nutrient was added to depleted cells and changes in global gene expression were assessed over the first 48 hours following nutrient addition. Both N- and P-addition resulted in significant changes in approximately 4% of genes on the microarray, using a significance cutoff of 1.7-fold and p ≤ 10-4. By far, the earliest responding genes were dominated in both nutrient treatments by pentatricopeptide repeat (PPR) proteins, which increased in expression up to 3-fold by 1 h following nutrient addition. PPR proteins are nuclear encoded proteins involved in chloroplast and mitochondria RNA processing. Correspondingly, other functions enriched in response to both nutrients were photosystem and ribosomal genes.ConclusionsMicroarray analysis provided transcriptomic evidence for N- but not P-limitation in K. brevis. Transcriptomic responses to the addition of either N or P suggest a concerted program leading to the reactivation of chloroplast functions. Even the earliest responding PPR protein transcripts possess a 5' SL sequence that suggests post-transcriptional control. Given the current state of knowledge of dinoflagellate gene regulation, it is currently unclear how these rapid changes in such transcript levels are achieved.