Srimathe Ramanujaya Namaha,
All the countless athma in this Material world (BhaddhAthmas) always has a shareeram. It’s called the Sookshma Shareeram (subtle body) which comprises of the mind (Manas), intellect (Buddhi), ego (Ahamkara), and consciousness (Chittam). This body cannot be seen by the physical sense organ eyes. This body carries the karmic imprints. This Sookshma shareeram is ONLY ONE FOR EACH ATHMA and never gets destroyed in this material, until it gets shredded in Viraja Nadhi on the way to Moksha through Acharya Krupa.
This Sookshma shareeram inside a Gross body is called the Sthoola shareeram. This is our physical visible body made of Pancha bhootams, which we say as birth and death when the Athma in Sookshma shareeram enters or leaves the Sthoola shareeram due to karmas. There are 84 Lakh types (species) of Sthoola Shareeram as per Padma Purana, categories into 6 groups inside one andam. 9 lakh acquatic bodies, 20 lakh tree bodies, 11 lakh insect bodies, 10 lakh bird bodies, 30 lakh animal bodies and four hundred thousand types of human bodies, like manushya shareeram in bhoolokam, deva shareeram in indra loka, Yatna shareeram in Hell for Athmas to withstand punishment, Gandharva shareeram etc… These types of bodies are not just in bhoolokam but spread throughout the 14 worlds of one andam.
Micro organism is one of the body from the 11lakh types of insect body category. Each Micro organism is a Sthoola body and has one athma with its Sookshma shareeram entered.
Each Athma with its own Sookshma shareeram though without any Sthoola shareeram always travels. Only thing is that, an Athma in Sookshma shareeram without Sthoola shareeram cannot relinquish its karmas by itself. It needs some Sthoola shareeram.
The Athma with Sookshma shareeram in the 5 groups of Sthoola shareeram doesn’t accumulate any karma. It only relinquishes its karma from its prarabdha karma. Only athmas in Sookshma shareeram inside Sthoola shareeram human body accumualates Punya and Paapa karmas.
The Athma that has done violent crimes in the past, if it has maxed up its paapa karma, gets lower birth bodies as insects (microorganisms) etc…
So if an Athma gets a human body as Swami explained through the Panchagni Vidya from cloud, through rain to farm land to crop to rice to transportation and finally as food for purusha, stays in purusha for 2 months, then transferred to his wife for 10 months, then finally that particular athma becomes the Kartha (doer) for that human body when born as a human baby. The remaining countless athma gets different types of bodies (Sthoola shareeram) as per their karma. They may stay in that primary karthas human body as microorganisms or somewhere outside with different Sthoola sharareem or without Sthoola shareeram but remain in their Sookshma shareeram and travel.
There is also a story in Srimadh Bhagavatam about two children Gojarna (Scholar child) and Dhundukari (cruel child) of Rishi Atmadev. The cruel child accumulated so much papa karma and when died (left Sthoola shareeram), he was wandering as preta. When Gokarna was having discourse on Srimadh Bhagavatam for 7 days, Dhundukari as preta sat on a bamboo tree and on 7th day, he attained Moksha. So it’s not always that an Athma need Sthoola shareeram to relinquish its karma.
Like, during your ThuruAradhana to Perumal, when you recite Guruparampara, rahasya trayam, the Athmas in your tongue (with Sookshma shareeram) or as microorganisms with Sthoola shareeram and those outside will reach Moksha with Acharye Ramanuja Krupa.
But anyway, BhagavAn Sriman Narayana keeps a track of all HIS properties (count less Athmas).
Adiyen (ElayaAlwar) Srinivasa (DhoddayacharyAr) Dasan.