We present a method for detecting rapid changes in coral gene expression at the messenger ribonucleic acid (mRNA) level. The staghorn coral Acropora cervicornis was exposed to 1 and 10 microg/L permethrin and 25 and 50 microg/L copper for 4 h. Using differential display polymerase chain reaction (PCR), mRNA associated with each toxicant exposure were reverse transcribed into complementary DNA (cDNA) fragments that were subsequently amplified and isolated. Six differentially expressed cDNA fragments were further developed into molecular probes that were used in Northern dot blots to determine the change in transcription levels of target transcripts. Changes in mRNA abundance were quantified by densitometry of chemiluminescence of digoxigenin-labeled probes hybridizing to target mRNA transcripts. The six gene probes showed varying degrees of sensitivity to the toxicants as well as specificity between toxicants. These probes were hybridized in Southern blots to genomic DNA from A. formosa sperm, which lacks zooxanthellae, to demonstrate that the genes coding for the mRNA transcripts produced are found within the coral genome. The gene probes developed in this study provide coral biologists with a new tool for coral assessment. Gene probes are sensitive, toxicant-specific biomarkers of coral stress responses with which gene sequence information can be obtained, providing a mechanism for identifying the stressor altering the gene expression.

In order to identify physiological components that contribute to salinity tolerance, we compared the effects of Na+, Mg2+ and K+ salts (NaCl, Na2SO4, MgCl2, MgSO4, KCl and K2SO4), Ca2+ (CaSO4), mannitol and melibiose on the wild type and the single‐gene NaCl‐tolerant mutants stl1 and stl2 of Ceratopteris richardii. Compared with gametophytic growth of the wild type, stl2 showed a low level of tolerance that was restricted to Na+ salts and osmotic stress. stl2 exhibited high tolerance to both Na+ and Mg2+ salts, as well as to osmotic stress. In response to short‐term exposure (3 d) to NaCl, accumulation of K+ and Na+ was similar in the wild type and stl1. In contrast, stl2 accumulated higher levels of K+ and lower levels of Na+. Ca2+ supplementation (1.0 mol m−3) ameliorated growth inhibition by Na+ and Mg2+ stress in wild type and stll, but not in stl2. In addition, under Na+ stress (175 mol m−3) wild‐type, stll and stl2 gametopbytes maintained higher tissue levels of K+ and lower levels of Na+ when supplemented with Ca2+ (1.0 mol m−3). stl2 gametophytes were extremely sensitive to K+ supplementation. Growth of stl2 was greater than or equal to that of the wild type at trace concentrations of K+ but decreased substantially with increasing K+ concentration. Supplementation with K+ from 0 to 1.85 mol m−3 alleviated some of the inhibition by 75 mol m−3 NaCl in the wild type and in stl1. In stl2, growth at 75 mol m−3 NaCl was similar at 0 and 1.85 mol m−3 K+ supplementation. Although K+ supplementation above 1.85 mol m−3 did not alleviate inhibition of growth by Na+ in any genotype, stl2 maintained greater relative tolerance to NaCl at all K+ concentrations tested.

The roles of K+ uptake and loss in the salinity response of the wild type and the salt‐tolerant mutant stl2 of Ceratopteris richardii were studied by measuring Rb+ influx and loss and the effects of Na+, Mg2+, Ca2+ and K+‐transport inhibitors. In addition, electrophysiological responses were measured for both K+ and Rb+ and for the effects of Na+ and NH4+ on subsequent K+‐induced depolarizations. stl2 had a 26–40% higher uptake rate for Rb+ than the wild type at 0.5–10 mol m−3 RbCl. Similarly, membrane depolarizations induced by both RbCl and KCl were consistently greater in stl2. In the presence of 0–180 mol m−3 NaCl, stl2 maintained a consistently greater Rb+ influx than the wild type. stl2 retained a greater capacity for subsequent KCl‐induced depolarization following exposure to NaCl. Five mol m−3 Mg2+ decreased Rb+ uptake in stl2; however, additional Mg2+ up to 40 mol m−3 did not affect Rb+ uptake further. Ca2+ supplementation resulted in a very minor decrease of Rb+ uptake that was similar in the two genotypes. Tetraethylammonium chloride and CsCl gave similar inhibition of Rb+ uptake in both genotypes, but NH4Cl gave substantially greater inhibition in the wild type than in stl2. NH4Cl resulted in a greater membrane depolarization in the wild type and the capacity for subsequent depolarization by KCl was markedly reduced. stl2 exhibited a higher Independent loss of Rb+ than the wild type, but, in the absence of external K+, loss of Rb+ was equivalent in the two genotypes. Since constitutive K+ contents are nearly identical, we conclude that high K+ influx and loss exact a metabolic cost that is reflected in the inhibition of gametophytic growth. Growth inhibition can be alleviated by reduced supplemental K+ or by treatments that slightly reduce K+ influx, such as moderate concentrations of Na+ or Mg2+. We propose that high throughput of K+ allows maintenance of cytosolic K+ under salt stress and that a high uptake rate for K+ results in a reduced capacity for the entrance and accumulation of alternative cations such as Na+ in the cytosol.

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Spores from a weakly salt tolerant strain of Ceratopteris richardii containing the mutation stl1 were irradiated and sown on nutrient medium supplemented with 200 mM NaCl. A single highly salt tolerant gametophyte was recovered and selfed to generate a homozygous sporophyte. Spores from this strain, 10α23, were used to document the sexual transmission of the trait and to monitor the inheritance of tolerance in crosses to both the wild type and to the parental salt tolerant strain. Genetic analysis showed the 10α23 strain to possess both the original stl1 mutation and an additional semi-dominant nuclear mutation, stl2, that individually conferred a high level of tolerance to gametophytes. In combination, both mutations had additive effects. Tolerance was also evident in sporophytes, but at a lower level than in gametophytes.