Green tides have occurred in the Yellow Sea successively from 2007 to 2011. Genetic analysis of the 5-year green-tide-forming algae needs to be performed to determine the source of the biomass and understand the mechanism of the green tide blooms. In this study, free-floating green algae were collected at different sites in the Yellow Sea in 2010 and 2011. Data on 182 free-floating samples and 155 attached Ulva samples from previous studies on the Yellow Sea green tides from 2007 to 2009 were also taken into consideration. Morphology observation and molecular phylogenetic analyses indicated that the Yellow Sea green tides were dominated by a single species, Ulva prolifera, from 2007 to 2011. Considering that at least five Ulva species inhabit the north coast of China, the unialgal composition of the green tides implied that (1) there may be some special physiology and propagation pathways of U. prolifera for its rapid expansion, (2) the mechanisms of the Yellow Sea green tide formation were similar for the last five years, and (3) the intra-species genetic variation and population structure of U. prolifera need to be studied to determine the exact origin of the bloom-forming biomass.

The massive green algae (Enteromorpha prolifera) bloom in a coastal area of Qingdao between May and July 2008 prior to the sailing competition of the 29th Olympic Games has raised great concerns in local and regional communities and governmental agencies on environmental deterioration, causes and mitigation of such massive algal blooms. Results from field and laboratory studies during the bloom and post-bloom periods revealed that the bloom was originated from an offshore area 150 km south of Qingdao, and caused by complex coupled land-ocean and natural-human processes associated with increases in nutrient and organic matter loadings to the coastal ocean due to rapid expansions of agriculture, aquaculture and urbanization, behavior of algae, seasonal wind and coastal currents, and transport of nutrients and biota. This environmental disaster has brought us emerging challenges on how to integrate monitoring, analysis and prediction of complex coupled natural and human processes, environmental regulations, and management in land-ocean uses for the sustainable socioeconomic development.

Summary: Brown algae have convergently evolved plant‐like body plans and reproductive cycles, which in plants are controlled by differential DNA methylation. This contribution provides the first single‐base methylome profiles of haploid gametophytes and diploid sporophytes of a multicellular alga. Although only c. 1.4% of cytosines in Saccharina japonica were methylated mainly at CHH sites and characterized by 5‐methylcytosine (5mC), there were significant differences between life‐cycle stages. DNA methyltransferase 2 (DNMT2), known to efficiently catalyze tRNA methylation, is assumed to methylate the genome of S. japonica in the structural context of tRNAs as the genome does not encode any other DNA methyltransferases. Circular and long noncoding RNA genes were the most strongly methylated regulatory elements in S. japonica. Differential expression of genes was negatively correlated with DNA methylation with the highest methylation levels measured in both haploid gametophytes. Hypomethylated and highly expressed genes in diploid sporophytes included genes involved in morphogenesis and halogen metabolism. The data herein provide evidence that cytosine methylation, although occurring at a low level, is significantly contributing to the formation of different life‐cycle stages, tissue differentiation and metabolism in brown algae.