IntroductionMicroalgae and macroalgae are of great value both as organisms for basic biological research and as resources for bio-industry. That is why algae are now considered to be very promising organisms for economical and industrial applications and are the target of genetic transformation [1,2]. Recently, such transformation has been performed successfully in microalgae, and stably transformed microalgae have been employed to produce useful materials and to analyze the gene functions required for engineering these materials [3][4][5]. However, it is still hard to establish transgenic macroalgae, which is hindering our understanding of gene functions in various physiological regulations and also slowing our utilization of macroalgae in biotechnological applications.The red macroalga Porphyra yezoensis is the most popular sea crop in Asia [6] and has recently received a great deal of attention as a model macrophyte for physiological and molecular biological studies in marine red algae [7,8]. To date, several studies have described the establishment of a transient gene expression system in P. yezoensis [9]; for example, we have succeeded in eliciting the efficient expression of the codon-modified β-glucuronidase (PyGUS) and humanized cyan and green fluorescent protein (AmCFP and ZsGFP) genes by transient transformation of P. yezoensis gametophytes using particle bombardment [10][11][12][13], which was then adapted to other Bangiophycean algae [14,15]. Despite the development of transient gene expression systems in P. yezoensis, a stable transformation system, which is needed for the fine analysis of gene function, has not been established, because of a lack of techniques to select and isolate stably transformed cells from gametophytes.The availability of a useful selection protocol, which depends on the choice of a selection agent such as antibiotics and herbicides, is generally a prerequisite for the development of a stable transformation system [16]. Thus, the identification of selection agents such as antibiotics and herbicides was carried out in P. yezoensis [17], and it resulted in the indication of the non-sensitivity of P. yezoensis gametophytes to many kinds of antibiotics at concentrations up to 5 mg ml-1 in culture medium; however, the reproducibility of the test results was not examined. Therefore, we here identified a selection agent for a stable transformation system in P. yezoensis gametophytes by reexamining the sensitivity to 4 antibiotics used in the previous study, ampicillin, kanamycin, hygromycin B and geneticin [17], as well as that of chloramphenicol and paromomycin. In addition, since the high salt conditions often reduced the action of antibiotics in many marine algae [18] and high pH levels of the medium affected the toxicity of the antibiotic trimethoprim (TMP) for the willow tree Salix viminalis [19], we also investigated using an artificial synthetic medium to determine whether modification of the medium conditions in terms of pH value and salt concentration would affect the antibi...