The Biodiversity and Distribution of Abyssal Benthic Foraminifera and Their Possible Ecological Roles: A Synthesis Across the Clarion-Clipperton Zone

Abstract

Benthic foraminiferal research in the North Pacific has a long history, with works published over a century ago providing important information about the taxonomy and distribution of morphospecies. These studies focused mainly on areas outside the Clarion-Clipperton Zone (CCZ). Our knowledge of foraminiferal faunas within the CCZ originates largely from recent baseline investigations related to likely future seabed mining of the polymetallic nodule deposits. These have revealed highly diverse assemblages of sediment-dwelling morphospecies among the meiofauna and macrofauna, as well as megafaunal xenophyophores and nodule-attached fauna. Morphological analyses have been complemented by metabarcoding studies that yielded even higher numbers of molecular species (Operational Taxonomic Units - OTUs). Monothalamids, the vast majority undescribed, constitute a substantial proportion of both morphological and molecular datasets, with multichambered agglutinated and calcareous foraminifera being less common. Their importance in this abyssal (>4,000 m depth) habitat likely reflects food limitation combined with carbonate dissolution close to and below the carbonate compensation depth. Literature records, supported in a few cases by genetic data, suggest that many morphospecies found in the CCZ have wide geographical distributions across the Pacific abyss and in other oceans. At smaller spatial scales (several 100s of kilometers) there is a general uniformity in assemblage composition. Nevertheless, many morphospecies are too rare to conclude anything about their geographical distributions. Similarly, the part played by benthic foraminifera in CCZ ecosystems is largely a matter of speculation, although their abundance across different size classes suggests that it is significant. Meiofauna-sized taxa that consume freshly-deposited organic detritus may be important in carbon cycling, particularly at the shallower, more eutrophic eastern end of the CCZ. Megafaunal xenophyophores can provide habitat structure for other organisms, potentially enhancing benthic biodiversity. Foraminifera of all sizes could be among the earliest recolonisers of disturbed or redeposited sediments. Their potential contributions in terms of both ecology and biodiversity make these protists significant members of benthic communities in the CCZ.

Macrofaunal foraminifera from a former benthic impact experiment site (IOM contract area) in the abyssal eastern Clarion-Clipperton Zone

“…These include, among others, Ammobaculites agglutinans, Cribrostomoides subglobosus, Cystammina galatea, Reticulophragmium trullissatum, 'Reophax' (= Hormosinella) distans, 'Reophax' (= Hormosinella) guttifera, and the different morphotypes (likely separate species) of Thurammina papillata. This is consistent with the wide distribution of many of the common foraminiferal morphospecies across the abyssal Pacific, as discussed further by Gooday et al (2021).…”

“…The baseline studies have included research on benthic foraminifera, focussed largely in the United Kingdom 1 (UK-1) and Ocean Minerals Singapore (OMS) contract areas at the eastern end of the CCZ (Goineau and Gooday, 2015Gooday et al, 2015Gooday et al, , 2017Gooday and Goineau, 2019). In both areas, these protists are a common and diverse component of benthic assemblages in the meiofaunal and macrofaunal size ranges, as well as being dominant members of the megafauna across the wider CCZ (reviewed in Gooday et al, 2021).…”

“…Our goal is to characterise the taxonomic composition and diversity of the larger foraminifera in these samples, and thereby to address the question of whether there was any evidence for a lasting impact of the 1995 experimental disturbance on macrofaunal foraminifera. This is an important issue given the major role that foraminifera are believed to play in the functioning of abyssal ecosystems in the CCZ (Gooday et al, 2021).…”

Scientific and budgetary trade‐offs between morphological and molecular methods for deep‐sea biodiversity assessment

“…However, the additional knowledge gained from MBT in areas that are severely undersampled provides a great benefit to science and research, especially if commercial mining is to begin and baseline data are required. MBT may also be the only way to evaluate the diversity of certain foraminifera, notably members of the Komokiacea and other taxa that accumulate stercomata (Gooday et al, 2021). These groups are very common in abyssal samples, but it has not yet been possible to amplify their DNA (Lecroq et al, 2009).…”

Restoration experiments in polymetallic nodule areas

“…According to Gooday et al ( 2015 ), foraminiferal communities on nodules are characterized by a high number of morphospecies (they found 75 morphospecies on seven nodules), that are widely distributed at regional scales in the abyssal Pacific but not necessarily at global scales. Nevertheless, many morphospecies are too rare to provide information on their geographical distributions (Gooday et al, 2021 ). Furthermore, polymetallic nodules harbor metazoan communities including, for example, corals or sponges that can only live on the hard nodule substrate, but not on the soft abyssal sediments (Amon et al, 2016 ).…”