Symbiodinium microadriaticum

Description and significance
Symbiodinium microadriaticum is an aquatic species of single-celled algal dinoflagellates - commonly referred to as zooxanthellae - which thrive on symbiotic relationships with larger organisms, primarily in the gastroderms of coral. Endosymbiotically hosted within the polyps of coral, S. microadriaticum and other related zooxanthellae serve as extensive sources of energy for coral, without which the host organisms would lack the energetic means to grow their skeletons and cope with the harsh surrounding environment. This symbiotic relationship consists of S. microadriaticum receiving protection and vital nutrients from the host coral, while the host receives the immense energetic benefits of the photosynthesis conducted by S. microadriaticum. A similar relationship exists between S. microadriaticum and the jellyfish species Cassiopeia xamachana: in fact, this symbiotic relationship was recognized by scientists before the relationship between the algae and coral.

The importance of S. microadriaticum must not be evaluated solely upon its vitality to the formation of coral reefs; surely that is a large impact in and of itself, but the subsequent effect of the formation of these reefs must be acknowledged, too, when attempting to assess the significance of S. microadriaticum. Coral reefs themselves host a vast array of different organisms, allowing organisms - both micro- and macroscopic - to thrive based upon food sources, protection, and other such environmental conditions that the presence of reefs provide. The diversity that springs from coral reefs is immense, as are the potential applications to medicine and such that in-depth research on reefs may yield.

Genome structure
Symbiodinium spp. is a very diverse group - S. microadriaticum is one of countless (most thought to be as yet unknown) species - and one of the ways in which Symbiodinium is thought to have evolved this widespread diversity is through “cycles, or pulses, of diversification [which] occur involving host generalists giving rise to mostly host-specialized forms. Of there, a select few become widely distributed, develop a greater host-range capacity, and, in turn, undergo diversification” (576). This kind of diversification makes sense because - as previously mentioned - these organisms are extremely dependent upon their hosts: they reside in the cytoplasm of their host cells, which exposes them to intense selection pressures from both inside and outside the host. Thus, it makes sense that host and symbiont would be intimately connected genetically over the course of evolution. However, there are many examples of genetically close Symbiodinium being found to be associated with very different hosts: more extensive population studies of the organisms will expedite the dissolution of this mystery.

Cell structure and metabolism
Using freeze-substitution as the method for fixation, much about the structure of S. microadriaticum was revealed in 2000 (there had previously been many misconceptions about the cellular structure due to less sophisticated methods of cellular fixation). Pyrenoids - which are specialized areas of the plastids found in many algae - are the sites of carbon dioxide fixation, and are typically located around the periphery of the cell, presumably to ensure easy access to sources of CO2. It is not uncommon for a single, large pyrenoid to be observed emerging from the chloroplast of the cell, with which is has a continuous membrane with “a relatively thick starch coat [surrounding] the entire structure”(78). Regarding the cell wall, it is uncertain whether it contains cellulose or not: while cellulase has been reported to destroy the cell walls of isolated cells (indicating the presence of cellulose), when tested with cellulase while in the associations found in nature, no significant changes were reported in cellular structure, indicating that there may be tough structural components (such as sporopollenin) surrounding the cellulose in nature. Whatever the specific components of the cell wall may be, it has been made clear that when Symbiodinium undergoes division (its means of reproduction via cellular splitting into two new daughter cells), the old cell wall is disintegrated by cytoplasmic enzymes, and new walls are formed. These walls must be somewhat permeable or have a variety of receptors on them, as membranous exchange of gases and other such products must be facilitated in order for endosymbiosis to be successful. How do Symbiodinium reach their hosts in the first place? Some are born already associated within a host, and it has also been reported that some Symbiodinium have developed temporary flagella from there sub-surface thecal vesicles. It has been observed that “after a relatively short motile stage the cells spontaneously undergo ecdysis, a process by which they shed their thecae and flagella, again by some unknown mechanism, and settle to the substratum, where they enter the longer vegetative phase of their life cycle” (81).

Ecology
The phenomenon of coral bleaching is characterized by coral’s loss of color due to a dramatic decrease in the amount of symbiotic zooxanthellae associated with the coral. This occurrence has long been thought (correctly) to be the result of sea temperatures rising above 30 degree Celsius, but it had previously also been assumed that the rise in temperature had more of an effect on the coral itself, not on the associated organism. Based upon studies conducted in the early 1990s, however, we now know that coral bleaching has such detrimental effects for the host not because of any damage done directly to the coral; rather, the rise in temperature inhibits and eventually completely retards the photosynthetic processes conducted by the zooxanthellae. The experiments that uncovered this fact were conducted with Symbiodinium microadriaticum.

S. microadriaticum is only one species of zooxanthellae, however: there are many more which form associations with coral: it has been observed that not all species will dissociate from the coral at the same temperature. From this observation has emerged a hypothesis called the Adaptive Bleaching Hypothesis (ABH), which suggests that upon dissociation with one species of zooxanthellae, the coral is able to adapt by forming new associations, replacing the poorly-adapted species with another species, better adapted to the new (higher) temperature. Should this hypothesis prove to be accurate, it may have widespread implications for the ability of coral reefs to survive raising sea temperatures: however, if the water near the surface (where coral and their associated zooxanthellae must live in order to procure sunlight) rises to temperatures previously not experienced by surface-dwelling organisms, there may be (eventually) no species of zooxanthellae that are genetically equipped to deal with the rising temperatures. This caution should not completely retract from the compelling nature of the ABH, however, because this hypothesis emerges largely out of the discovery (a) of 4-8 (depending on whom you ask) different clades of zooxanthellae, each presumably with a good number of as-yet unstudied species and (b) of the notion that symbiotic relationships between zooxanthellae and coral probably date back to the Jurassic period, indicating the great genetic diversity has had the chance to evolve for this group (and indeed, it has, given that closely-related dinoflagellates have been found to associate with a variety of different species in a generally flexible manner). Rising temperatures are not the only potential cause of photosynthesis inhibition that lead to algal and host death, however; bacteria such as Vibrio shiloi are also known to inhibit the photosynthetic processes of S. microadriaticum. This is accomplished by the bacterial secretion of a substance commonly known as Toxin P, which has a high affinity for binding with zooxanthellae. Once bound, it introduces an extra NH4Cl to the zooxanthellae, which is thought to somehow allow the transport of NH3 into the zooxanthellae. Previous studies have shown that NH3 contributes to the destruction of the pH gradient and hence, the organism’s ability to conduct photosynthesis. Without this process, both the S. microadriaticum and its host coral suffer and dissociate, causing coral bleaching.

Pathology
While the mutualistic relationship between S. microadriaticum and coral are generally not known to be parasitic in nature, a fascinating experiment was conducted by Sachs and Wilcox in 2005 that serves to “demonstrate the dynamic nature of this symbiosis and illustrate the potential ease with which beneficial symbionts can evolve into parasites” (425). This dynamism was demonstrated by means of treating S. microadriaticum to infect jellyfish hosts and reproduce vertically; when this was done (as it is in nature, due to the nature of natural S. microadriaticum replication), a mutually beneficial symbiotic relationship was favored by the algae because the fitness of S. microadriaticum was intimately connected with the ability of its jellyfish host to reproduce. This intimacy was limited, however, when Sachs and Wilcox treated S. microadriaticum to transmit its genetic material horizontally: this lateral-gene transfer encouraged the rapid proliferation and dispersal of algae, as their transmission had become infectious. An indirect relationship between the spread of S. microadriaticum and the successful reproduction of its jellyfish host was uncovered in this case: thus, lateral-gene transfer in this case encourages a parasitic relationship to develop where the spread of the associated organism serves instead as a detriment to the host’s fitness. In nature, however, it is in the best interest of the S. microadriaticum to encourage the success of its host; therefore, it can be seen why vertical gene transfer is the zooxanthellae’s favored mechanism under normal conditions.

Application to Biotechnology
While an unfortunately small amount (if any) research has been done regarding the potential biotechnological applications of S. microadriaticum, it should be pointed out that many would argue passionately for more extensive population studies of S. microadriaticum’s associations with coral reefs to be conducted - and quickly - in order to attempt to find a way to preserve coral reefs, as they are sources of immense biological diversity. Stemming from this idea, research into potential medical applications is often conducted at sites of such diversity; while these areas are not biotechnological per se, they would be well-worth pursuing.

Current Research
There is emerging a renewed interest in studying S. microadriaticum and related Symbiodinium of late, considering the peril which coral reefs worldwide have been put in by rising sea temperatures. The ABH referenced earlier may well have connections to rising nitric oxide levels (this was first proposed a couple of decades ago, but renewed interest has spawned renewed research). It is thought that increasing heat may increase the production of NO in Symbiodinium, which possibly contributes to the causation of the inhibition and eventual destruction of photosynthetic processes (ultimately resulting in dissociation from the host, leaving coral reefs highly vulnerable to destruction). Along a similar vein, there have been newly-invigorated efforts by marine biologists in the past few years to finally get a more coherent picture of the phylogeny of zooxanthellae in general and Symbiodinium in particular, such that the effects of environmental changes might be able to be foreseen and possibly altered using knowledge gained from natural processes and genetic differences (ie, different levels of organismal adaptability to rising temperatures). A more firm grasp on the phylogenetic relationship between zooxanthellae living throughout the oceans of the world will provided biologists with a better idea of how biological reactions to environmental changes might proceed.