THE ECOLOGY, BIOGEOCHEMISTRY AND PHYLOGENY OF METHANE SEEP AND NON-SEEP BENTHIC FORAMINIFERA FROM THE PACIFIC MARGIN

Abstract

In an effort to understand the relationships between active methane seep and adjacent non-seep (inactive) populations of the deep-sea foraminifera Cibicidoides wuellerstorfi, a common paleo-indicator species, from methane seeps in the Pacific were analyzed and compared to one another for genetic similarities of small subunit rDNA (SSU rDNA) sequences. Pacific Ocean C. wuellerstorfi were also compared to those collected from other localities around the world (based on 18S gene available on Genbank, e.g., Schweizer et al., 2009). Results from this study revealed that C. wuellerstorfi living in seeps near Costa Rica and Hydrate Ridge are genetically similar to one another at the species level. Individuals collected from the same location that display opposite coiling directions (dextral and sinistral) had no species level genetic differences. Comparisons of specimens with genetic information available from Genbank (SSU rDNA) showed that Pacific individuals, collected for this study, are genetically similar to those previously analyzed from the North Atlantic and Antarctic. These observations provide strong evidence for the true cosmopolitan nature of C. wuellerstorfi and highlight the importance of understanding how these microscopic organisms are able to maintain sufficient genetic exchange to remain within the same species between seep and nonseep habitats and over global distances. Although organic matter degradation rates have been studied for some time, in situ rates of protoplasm degradation in deep-sea foraminiferal tests have been estimated based on laboratory experiments and sediment distribution patterns. Information regarding degradation rates of foraminiferal protoplasm is essential in the use of non-vital stains in identifying the amount and character of protoplasm in tests which remains the most commonly used method to assess living populations of benthic foraminifera (e.g., Murray and Bowser, 2000). In an effort to examine the retention potential of foraminiferal protoplasm on the deep seafloor 36 frozen, protoplasm filled Cibicidoides wuelllerstorfi were placed in natural sediments inside experimental containers and deployed on for 390 to 396 days. Despite oxygen-poor conditions (0.24 mL/L to 0.37 mL/L) that would be expected to promote preservation of organic matter, and experimental container protection from macro- and megafauna, 72% of deployed tests containing protoplasm were destroyed beyond recognition within 396 days. Of the 10 specimens (28%) recovered, 9 retained at least some protoplasm, but only 1 individual had the potential to be identified as living based on Rose Bengal staining techniques. However, in this specimen, protoplasm was clearly altered or missing in some chambers. The results of this study suggest that it is unlikely that many, if any, benthic foraminiferal specimens containing protoplasm terminated by freezing would be conservatively considered as recently living using Rose Bengal as an indicator of the extent and character of protoplasm within the test after 396 days or less exposed to in situ conditions in deep-seafloor habitats. After 390 to 396 days on the seafloor at Hydrate Ridge in the Pacific, eight artificial substrate experiments (hereafter referred to as SEA3 for Seafloor Epibenthic Attachment Cubes) were colonized by 1058 Cibicidoides wuellerstorfi. The presence of this species has been inferred as an indicator of well-oxygenated conditions, and recruitment of such large numbers in bottom-waters with low dissolved oxygen availability (0.24 to 0.37 mL/L) indicates that this taxon is not as limited by oxygen as previously thought. Clues about substrate preferences were also evident from the distribution of individuals on SEA3. For example, the wooden rod attached directly to the plastic mesh that was heavily colonized was devoid of any epibenthic foraminifera. Few studies have examined foraminiferal colonization of hard substrates in the deep-sea (e.g., Mullineaux, 1987), and to our knowledge no previous study has compared foraminiferal colonization of seep with non-seep substrates. Comparisons of abundance, size distribution, and isotopic biogeochemistry of living foraminifera colonizing experimental substrates revealed differences between seep and non-seep environments. SEA3 within active methane seep habitats at Hydrate Ridge contained significantly fewer (406 on four SEA3s a density of 44 #individuals/100 cm2) individuals compared with those in adjacent off-seep sites (594 on three SEA3s a density of 86 #individuals/100 cm2). An additional 58 individuals were on a SEA3 22, which may have experienced seep conditions despite being deployed as a nonseep experiment (density of 25 #individuals/100 cm2). This difference in abundance may be due to active seepage conditions, however, reduced foraminiferal abundances on SEA3s located at seeps resulted from increased predation and displacement by higher abundances of macro- and meiofauna observed at active seep locations. Stable carbon isotope values of benthic foraminifera from seep substrates ranged from 0.26‰ to -0.56‰ with an average of 0.03‰ while δ13C from off-seep substrates range from 0.39‰ to -0.26‰ with an average of 0.15‰. Statistical analyses indicate a significant difference between seep and non-seep δ13C. Stable oxygen isotopes of foraminiferal carbonate from seep substrates range from 2.70‰ to 2.03‰ with an average of 2.41‰ and 2.65‰ to 1.99‰ with an average of 2.39‰ at adjacent off-seep sites. These results provide some of the first information about epifaunal foraminiferal colonization potential at methane seeps and highlight the biogeochemical and ecological influences of seep habitats on C. wuellerstorfi.