Phylogenetic composition and distribution of picoeukaryotes in the hypoxic northwestern coast of the Gulf of Mexico

Abstract

Coastal marine hypoxic, or low-oxygen, episodes are an increasing worldwide phenomenon, but its effect on the microbial community is virtually unknown by far. In this study, the community structure and phylogeny of picoeukaryotes in the Gulf of Mexico, which are exposed to severe hypoxia in these areas was explored through a clone library approach. Both oxic surface waters and suboxic bottom waters were collected in August 2010 from three representative stations on the inner Louisiana shelf near the Atchafalaya and Mississippi River plumes. The bottom waters of the two more western stations were much more hypoxic in comparison to those of the station closest to the Mississippi River plume, which were only moderately hypoxic. A phylogenetic analysis of a total 175 sequences, generated from six 18S rDNA clone libraries, demonstrated a clear dominance of parasitic dinoflagellates from Marine alveolate clades I and II in all hypoxic waters as well as in the surface layer at the more western station closest to the Atchafalaya River plume. Species diversity was significantly higher at the most hypoxic sites, and many novel species were present among the dinoflagellate and stramenopile clades. We concluded that hypoxia in the Gulf of Mexico causes a significant shift in picoeukaryote communities, and that hypoxia may cause a shift in microbial food webs from grazing to parasitism.

Figure 1

Geographic location of the three stations from the inner Louisiana Shelf in the northwestern Gulf of Mexico. Station AB5: 29°02′N, 90°33′W; Station 10B: 29°05′N, 89°56′W; Station 8C: 28°59′N, 19°58′W.

Figure 2

Depth profiles of dissolved oxygen (O₂), salinity, temperature, and chlorophyll a for all three stations sampled.

Figure 3

Picoeukaryote community compositions at the class taxonomic level for the six samples collected from the three stations based on the 18S rDNA sequence libraries. (a) Surface waters; (b) bottom waters.

Figure 4

Community composition of Dinoflagellates by genera at the surface and bottom waters at each station. (a) Surface waters; (b) bottom waters.

Figure 5

PCoA plot of all the 18S rDNA gene sequences retrieved from this study at the 98% sequence-identity threshold. Arrows indicate Eigenvectors with its length indicating Spearman correlation between environmental variables and phylogenetic composition of each clone library. Clone libraries are represented in the biplot by different symbols. Axis 1 and 2 represent 21.25% and 20.11% of the total variance in operational taxonomic unit (OTU) distribution, respectively.

Figure 6

Phylogenetic relationships of picoeukaryotes based on maximum likelihood analysis of partial 18S rDNA gene sequences from all six clone libraries from the Gulf of Mexico. The scale bar indicates 0.02 nucleotide changes per position. Bootstrap values above 50 were indicated on the nodes. The outgroup is Euglena gracilis. Red icons represent clones collected under hypoxic conditions. (a) Gymodiniales and MALV II dinoflagellate clades; (b) MALV I dinoflagellate clade; (c) stramenopile and cercozoan clades; (d) Prasinophyte, cryptophyte, and ciliate clades.