Abstract
The euryhaline bay barnacle Balanus improvisus has one of the broadest salinity tolerances of any barnacle species. It is able to complete its life cycle in salinities close to freshwater (3 PSU) up to fully marine conditions (35 PSU) and is regarded as one of few truly brackish-water species. Na+/K+ ATPase (NAK) has been shown to be important for osmoregulation when marine organisms are challenged by changing salinities, and we therefore cloned and examined the expression of different NAKs from B. improvisus. We found two main gene variants, NAK1 and NAK2, which were approximately 70% identical at the protein level. The NAK1 mRNA existed in a long and short variant with the encoded proteins differing only by 27 N-terminal amino acids. This N-terminal stretch was coded for by a separate exon, and the two variants of NAK1 mRNAs appeared to be created by alternative splicing. We furthermore showed that the two NAK1 isoforms were differentially expressed in different life stages and in various tissues of adult barnacle, i.e the long isoform was predominant in cyprids and in adult cirri. In barnacle cyprid larvae that were exposed to a combination of different salinities and pCO2 levels, the expression of the long NAK1 mRNA increased relative to the short in low salinities. We suggest that the alternatively spliced long variant of the Nak1 protein might be of importance for osmoregulation in B. improvisus in low salinity conditions.
Highlights
Organisms living in fluctuating marine environments, such as estuaries and coastal waters, require special physiological adaptations
We identified two main gene variants of the Na+/K+ ATPase, which we name NAK1 and NAK2
The encoded Na+/K+ ATPase 1 (Nak1) and Na+/K+ ATPase 2 (Nak2) proteins are 70% identical with the greatest sequence divergence between the two paralogs in the very Nterminal part where the Nak2 protein is completely unrelated to Nak1 (Figure 4)
Summary
Organisms living in fluctuating marine environments, such as estuaries and coastal waters, require special physiological adaptations. Barnacles (subphylum Crustacea; infraclass Cirripedia) are among the most obvious examples of tolerant species in such environments: they are sessile macro-invertebrates, commonly found along rocky shores in coastal areas worldwide, where they are exposed to a broad range of environmental conditions including substantial changes in salinity, temperature, desiccation and wave action. Balanus improvisus Darwin, 1854 (=Amphibalanus improvisus) is an extreme performer with regard to salinity-tolerance: both adults and larvae can thrive in close to freshwater (3 PSU) up to fully marine conditions (35 PSU) [2,3,4] and can survive fluctuating salinities [5]. Much is known about the ecology of B. improvisus, the physiological/molecular mechanisms responsible for its exceptional tolerance to salinity changes throughout the life cycle are not well understood
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